PRIN
No.
I).---ALIMENTARY
BY
CANAL
MILLETT
T.
OF
THE
MOSQUITO.
•
THOMPSON.
INTRODUCTION.
L•TER•TV•E on the mosquitoesis rather extensive,especiallythat
which dealswith the genusAnopheles. Comparativelyfew articles,
however,discussthe internal anatomyof these fliesand in many of
those papers where details of internal structureare given, these
are not cited from morphologicalmotive. The structure of the
salivaryglandsor the alimentary canal may be described,but it is
to make clear the life historyof the malarial Plasmodiumor as a
guide in dissectingwhen studying these parasites,and not from
interestiu the anatomyitself. ]n view of the secondarypurposeof
such descriptionsit has not seemedbest in this 1)al)erto make a
completesurveyof articlesof this classor to commentat length on
the interpretationsof s(ructurethat are presented. Criticism and
discussion will be limited
to the few researches
which
deal exten-
sivelywith the internalanatomyor which discussthis from a morphologicalview-point.
Of the studiesthat fall into this last group, severalare found
which give accountsof Anopheles. Nuttall and Shipicy's,"The
structm'eand biologyof Anopheles"and Chfistophers'"The anatomy and histologyof the adultfemalemosquito"arecomprehensive
and very valuable. Annett and Dutton's"Report" is excellentbut
more limited in scope. As far as anatomicaldetails are concerned
at least, Giles' "IIandbook of the gnats or mosquitoes"is unre-
liable. I have been unableto obtain Grandl)r• and Charmoy's
"Les moustiques." With respectto Culex on the other hand, I
know of no single comprehensive
work, but scattereddescriptions
of the various internal organs exist. The external form of the
larvaeand pupaeof mosquitoes
has beenstudiedconsiderably,
but
the internal anatomyof thesestageshas beenneglected. Raschke
givesthe bestaccountfor the larvain "Die larve von C'ulex•e•orosus." Ore'knowledgeof the metamorphosis
of the internalorgans
• From the Biological laboratory, Clark university, and the Laboratory of
the U.S. bureau of fisheries, Woods Hole, Mass.
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HISTORY.
is summedup in two excellent papersby Hurst, "The pupal stage
of Culex" and "The post-embryonic
developmentof a gnat,"and in
detailsgiven by Miall and Hammond ('9•-) from Hm'st'sunpublished notes.
Three species
of Culexas they occurredat differentseasons
furnishedthe materialon which this paper is based. C. stimul•msand
C.pipicons
were usedfor the studyof the perfectinsect,and a small
midsummer
form that was not identifiedgaveme larvaeand pupae.
Comparativenoteswere madeat all stages,but failed to bring out
appreciable
differences
betweenthe three formsat anytime. The
genusCulex wasusedinsteadof the more importantAnopheles,
becauseearly in the studyit becameimpossible
to obtaina sufticient number of specimensof the latter, either larvae or adults.
However,enoughmaterial was availableto enableme to compare
the two generawith respectto many structuresand to checkthe
descriptions
given in the literaturedealing exclusivelywith Anopheles. The specimens
of Anophelesat my disposalwere representativesof the commonest
New Englandspecies,A. p•enctipenuis.
TECHNIQUE.
The chitin of the imagoof the mosquitois not heavyenoughto
offerespecialobstacles
to research,
exceptas it interfereswith the
penetration
of thekillingfluids. Thiswasovercome
withoutinjury
to importantstructuresby cutting off the dotsumof the thorax
while the insectwasimmersedin the reagent. Of all killing fluids
that were tried, Gilson'smercuricnitrateprovedmostvaluable,and
in the end wasexclusivelyused. In order to removeair from the
scales,
the mosquito
wasdippedfor a momentin alcoholbeforeit
wasplungedintothe warmfluid. I •nadeGilson'sfluidaccording
to the followingformula:-70 % alcohol
10 parts.
Distilled
86
water
"
Corrosive sublimate (crystals)
2
"
Glacial acetic acid
•
"
Nitric acid (80%)
1•
"
Dissection
is of 1)rimary
importance
in a studyof the thoracicand
abdominal
portionsof the alimentary
canalof themosquito.But
THOMPSON: ANATOMY OF MOSQUITO.
147
it must be constantlycontrolled and supplementedby sections.
Knowledgeof the structureswithin the headof the insecton the
other hand, has to be derived almost wholly from sections. Reconstructionproved of slight value. I used serial sectionscut in the
three planesusually employedand dependedon careful study of
these,addingto and controllingthe interpretationsby studyof dissections and of material
cleared
with
caustic.
Excellent
thick sec-
tions- 30 • or over -- for controland demonstrationwere prepared
by killing in Flemming'sfluid and allowingthe specimens
to blacken
somewhat. Material treated thus may be imbeddedin paraffin,secrioned,and the sectionsmounted without staining. The discoloration producedby the killing fluid gives sufficientcharacterto the
structures.
With the larva of the mosquito,study of the living animalor of
fresh dissections
is of paramountimportance. Sectionsof the head
especiallyare not readily interpretabletaken alone. Fortunately
for research,the wriggler of Culex hasa large,transparenthead, so
that the containedstructurescan easilybe made out in the living
animalor in wholemountsof the head. In the light of knowledge
obtainedin this way, sectionsbecomeinterpretableand through
these in turn we are enabled
to understand
sections of the
Ano-
pheleswriggler,wherethe headis smalland opaque. An excellent
methodfor mountingthe whole headis to stainwith picro-carmine
and then clear through Weigert's fluid. Fine preparationsmay
also be obtained by staining with haematoxylin,but this method
is slowerand lessuniformly successfulthan the picro-carminestain.
The 1)upastage can only be studiedfrom serial sectionsand to
work to bestadvantagea seriesof specimens
the agesof which are
approximatelyknown, is needed. Such a serieswas obtained for
Culex by segregatingmaturelarvaein a dish and eachhour removing all pupae to separatecontainers,in which they couldbe reared
for any desirednumberof hours. A very completeseriesoughtto
be obtained. In the summerour speciesof Culex spendfrom 48 to
70 hoursaspupae,and I did not find that a set of specimens
representing'in agesnearlyevery hour up to the thirtieth of pupal life
and then more distant intervals, was too extensive.
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PROCEEDINGS: BOSTON SOCIETY NATUltAL HISTORY.
IMAGO.
1tead (t•d mo,t/,pctrts.--An extendeddescriptionof the external anatomyof the mosquitodoesnot comewithin the scopeof this
paper. A few points with respectto the head,however,m,st
noted. The part of the head that lies in front of the largecompoundeyes is inflated aboveto form a rectangularbox,which is
called the clypeus. This seemsto corresl)ondto the "face" of
other flies. A vertical furrow is impressedinto the anteriorface
of this box.
At the foot of the furrow a boss of chitin for muscle
insertionprojectsinto the cavity of the clypeus,while near the roof
of the 1)oxon eithersidea shortchitinousala alsoentersthe cavity.
Ventrally and at the sides, the pre-ocularregion of the head is
rounded and formsas it were an imperfectcylinder,at the summit
.of which the mouthpartsarise,nearly at one level. The anterior
wall of the clypcuslies forward of this point by one half of the
length of the box in the femaleinsectand nearlytwo thirdsof the
lengthin the male. The postocular
or epicranialregionof the head
is extensivein the female but truncatein the male mosquito.
•Vithin, the head is stre,gthcned by a roesialcrest which continues from the level of the ante,nae along the roof of th(• head to
the border of the occipitalforamen. It is also l•raccdl•y two
hollow cylindrical struts which pass froIn the ventral border of
the occipitalforamento the cheeks. I shallrefer to thesestrutsas
te•toria, without intendi•g to imply any necessary
morphol(•gical
connectionbetweenthem and the analogous
internal head bracesof
other i•sects. Among the flies,hollow tcntoria similarto thoseof
the mosquitoare found i• Chironomus,
A,opheles,and 8imuli,m.
They appearto be wantingin manyfamilies,as,for cxaml•lC
, the
Tipulidae,Asilidae,Dolichopodidae,
and the Muscidca.The Ta})a•i(l flies have solid tentoria with somewhat the same relations as
the hollow struts of the mosquito. In the female of Oulox the tentoria arisein front of the borderof the occipitalforamenttndt•scend
at an angleof twenty-fivedegrees
with the floorof the head. Each
strut hasa shortspurnearthe lower end,to whichno ol)vi(•usfunction canbe assigned,
and aboveas the cheeksare approached,
flares
out i,to an irregt•larfunnel-shaped
"head." In the male mosquito
the struts arise from the border of the occipitalforamen,and the
THOMPSON: ANATOMY OF MOSQUITO.
149
differentlyshapedhead makesthe angle with the floor measure
nearlythirty-five degrees. The strutsare providedwith the seeminglyuseless
spurnearthe foot,but aboveexpandsuddenly
to form
the "head" and then the tube narrowsagainas it meetsthe cheeks.
The mouthparts(pl. 16, fig. 46-47) of the mosquitoneed no
elaboratedescriptionhere. The usuallyacceptednomenclature
will
be employedfor the various stylers. The labrum is horseshoeshapedin sectionand formsthe wholeor the major part of the tube
(pc) throughwhichthe insectsucksbloodor otherliquid. There is
no good reasonto retain the namelabrum-epipharynx
for this dorsal styler,with the consequent
implicationthat the organ is compound. Becherlong ago('89.)pointedout that the separation
into
two partsundermanipulationwasan artifact. Morphologically
for
the imagoof Culex(pl. 16, fig. 46-47) andfor the imagoesof other
flies(I(r/tpelin,'89.,'83) the labrumis a unitarystructure. During
the metan•orphosis
of Culex also,the labrumis formedas a simple
tube. The canal(pc) on its ventral faceis mouldedby the infolding of the ventralwall. The labrumreceivesa singlemusclewhich
probablyservesfor a retractor and depressor. This muscleis inserted on the boss of chitin at the base of the labrum and its fibers
arisefrom the dorsalandposteriorwallsof the clypeus. It maybe
called the labral muscle(pl. 12, fig. 2). This musclehasbeendescribedas "labral muscle"(Annerr and Dutton,: ox), "elevatorof
lal•rum-epiphar•.nx"
(Nuttall andShipley,:ox-:o3),"retractorof labrum" (Giles,:o2), and "•'etractorpartesproductae"(Meinert,'81).
It correspoudsin part to the "pharyngeal muscle" of Dimmock
The mandiblesare wanting in the male mosquitoand are slender,
delicatelancetsin the female. Dimmock('SX) has figured these
organsin the crosssectionof the mouthpartsas lying beneaththe
hypopharynx. Giles(: 02) gives a similararrangementin one figure while in anotherhe showsthe mandiblesabovethe hypopharynx
at the sidesof the labrum. This last is the properpositionfor the
stylerswhen at rest. The mandiblesare morphologicallydorsalto
the hypopharynx. The other positionis due to ntisplacement
during the l•rocesses
of sectioning. Each mandibleis retracted by a
mandibularmuscle(,•nd m) which ariseson the "head" of the
tentoriumand is insertedon the baseof the stylet. As thesemuscles approachtheir insertionsshort fibers connectthem with the
]150 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY.
adjacentwalls of the head, but I am not certainwhetherthe muscle
is augmentedby thesefibersor insteadpartiallyinsertson the wall
of the head.
The maxillaeof the malemosquitoare no strongerthan the ma•diblesof the female,but the maxillaeof the latter sex are powerful
cuttinglancets,armed near the al•exwith recurredteeth alongthe
outer border of the blade.
cle is delicate.
This border is thin and its chitinous cuti-
The inner border of the blade is broad and its chitin
is thick (pl. 16, fig. 47). Proximally,the inner borderterminates
with an articular spur. From the poi•t where the palp •d the
outerborderof the maxillabladeu•ite, an apodemcentersthe head
andextendsalmostto the occipitalforamen(pl. 12, fig. 2, etpo).
The maxillaehave a complexmusculaturewhichmust ensurea considerablefreedomof movement,althoughit is possibleto assign
functionsto the differe•t musclesonly in a generalway (pl. 12,
fig. 2; pl. 13). The most proreinvent
musclein sectionsliesparallel
to and outsideof the maxillaryapodeme. It arisesfromthe under
side of the lower end of the tentoriumand the adjacentwallsof
the head and is insertedon the base of the palp and apodeme.
XVhile the general action of the musclemustbe the retractionof
the maxilla, it would seem that it will also tend to divaricate
maxillaand pall•uS.To adol)tDimmock's('81) and Nuttall and
Shipley's(: Ol-: 03) name,thismuscleis the ret•'actor
of the maxilla. Another iml•ortant musclearisesfrom the "head" of the
tentoriumand is insertedon the articularspurof the maxilla. As
it also sendsfibersto the dorsal a•gles of the baseof the lablure,I
call this musclethe double retractor. From its positio• it is prol•ably the chief retractorof the maxilla blade, which cutson the uI)
stroke. It has an extensivetrachealsupply.
Two pairsof musclesare found which probablyserveto protract
the maxillae. The larger of these,the protractorsof the maxillae,
areinsertedon the free endsof the apodemes
(prot m•tx)andexte•d
forward, inward, and upward to the tentoria. The origin is from
the under surfaceof the strut just below the "head." The smaller
pair of muscles,
the maxillo-labials(•etx-li), are insertedtowards
the ends of the apodemes,
and run forward alongthe inner sideof
the apodemesto the ventral borderof the labium. In additio• to
theselarger muscles,each maxilla receiveson the articular spur a
short band of fibersfrom the adjacentwall of the pharynx. Per-
THOMPSON: ANATOMY OF MOSQUITO.
151
hapsthese maxillarymusclescounteractthe divergentpull of the
retractors
of the
maxilla.
The
retractors
of the
maxilla
were
described
by Annerrand Dutton (:0x) as "muscleattachingmaxillary process
to occipitalregionof the skull." Possiblythe double
retractoris to be seenin Meinert's"retractorscalpelli"('Sx,fig.15).
In the samefigurethe "protractorscalpelli"may representthe protractor of the maxilla. This protractoris described
by Annerrand
Dutton and figured in figure 1, plate 18, but is not named. The
maxillo-labialmuscleis describedby Annerr and Dutton as "muscle
to baseof labium." It is less readily traced in other studiesand
may be eitherthe "protractorscalpelli"or the "retractorscutiventralis" of Meinert('Sx,fig. 16), while it is possiblethat the muscle
representedin figure1, plate 8, of Nuttall and Shipley'saccount
under the name "protractormaxilla" represents
it. I have not
foundanymusclethatwill answerto Meinert's"retractorscalpelli"
as shownin his figure 16.
The hypopharynxhas a central"body," the mid-dorsalline of
whichis traversed
by the salivarygutter,and thin lateral"wings."
The hypopharynx
is continuous
with the labiumat its first appeal'ancein the pupa(IIurst, '90) and neverbecomes
a separatestyler
in the male insect,so that the salivarygutter traversesthe dorsal
faceof the labium. At the baseof the hypopharynx,
thesalivary
gutterbecomes
continuous
with the salivarypump,or asit is more
oftentermed,the "receptacle
of the salivaryduct." This pump
(p1.12,fig. 1) consists
of a chitin-linedcup,openaboveandin front
into the salivarygutter,and closedbehindwith a plateof thin
chitinoverlaidby a massof cells. This plate furnishes
insertion
for two muscles
andis piercedby the orificeof thesalivary
duct.
•Vhen the muscles
attachedto the platecontract,
they drawthe
plateback,outof thecup,andsalivacanflowin fromthesalivary
duct. On their relaxation,the elasticityof the chitincauses
the
plate to re-enterthe cup,forcingthe contained
saliv•alongthe
salivary
gutterintothewound.NuttallandShipley
(:0x-:03)and
AnnettandDutton(:ol) havecarefully
described
thesimilar
pump
of Anopheles.In A. punctipe•,.•is
thepumpappears
to beof the
samesizeas that of Culexin specimens
of similarlength. The
hypopharynxreceivesmusclefibers from the mandibularmuscles.
The muscles
whichoperate
thesalivary
pump,
thehypopharyngeal
muscles,
will bedescribed
in connection
withthepharynx.
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HISTORY.
The labium, the largestof the mouthparts,is cylindrical,with a
deep furrow along the dorsalsurface. It terminatesin a median
ligula and two lateral labellae. The labellae are moved by small
muscles,labellar muscles(Annerr and Dutton, :O:t),from the walls
of the distal part of the labium itself. The organ as a whole is not
well provided with muscles. Proximally, the dorsalangleswhere
it meets the head, receive fibers from the double retractor muscles
and on the ventral
border
the maxillo-labial
muscles arise.
It is
probable that the return of this organ to positionafter the displacementduring blood sucking,is accomplished
withoutthe activity of muscles. The lumen of the labium is traversedby the two
labial tracheae and by two nerves (li •) derived from the infraesophageal
ganglionof the brain.
The remaining musclesof the head fall into two groups: those
associated
with the variouspartsof the alimentarycanal,and those
supplyingotherorgans. The former classwill be discussed
in connection with the partsof the alimentarycanalwhich they supply.
The latter groupmay be brieflynoted. Each antennareceivesscattered fibersfrom the fronsand two large musclesfrom the "head"
of the tentorium. One of these, the inner antennal muscle, is
insertedon the inner angle of the baseof the antenna. On contraction it must tilt the antenna forward
and i•ward.
The other mus-
cle, the outer antennal muscle,is inserted on the outer posterior
angle of the baseof the antennaand henceon contractionmusttilt
the antennabackwardand outward. This muscleis especiallylarge
in the male mosquito. On the outer face of either tentorium is
inserteda slendermusclewhich entersthe headthro•gh the occipital foramen. The musclesof this pair may be called the te•mtorial
musclesa•d they correspondto a musclelabeled"retractor of the
maxilla" by Nuttall and Shipleyin their figure 23 on plate 8, andto
the "salivary muscle"of Christophers(: 01). The "retractor maxillae" in the remainingfiguresgiven by Nuttall and Shipleyis the
retractor of the maxilla of my series. The subocularmusclesare ,•
pair of minute musclesthat arise on the floor of the el)icranium
nearly 0.1 min. behindthe basesof the tentoria and are insertedon
the strutsnear the origin of the protractorsof the maxillae. These
muscleswere not recognizablein the specimenfrom which figures
on plate 13 were drawn, but have been inserted in two instances
(figs. 10 and 11) from anotherseriesof sections.
THOMPSON:ANATOMY OF MOSQUITO.
153
The head of the Anophelesmosquitois relatively higher and
broaderin the epicranialregion than the head of Culex and the
pre-ocularregion slopesdownward. This makesthe musclesrun
forward at different anglesfrom thosein Culexand sectionsof the
two forms appearsomewhatunlike,although,with a singleexception, the samemusclesare presentin both. The musclesof the
mouthpar[sand head, outsideof those supplyingthe alimentary
canal,(lifter from the musclesof Culex as follows. The retractors
of the maxillae arisemorelaterally and fitlly 30 t• eaudadfrom the
basesof the tentoria,sothat their originis wholly from the wallsof
the headand not partly epicranialand partly tentorial. The posterior or apodemeendsof the protractors
of the maxillaealso,lie farther back and the upperor tentorialends are in the samelevelas
the double retractormuscle. Hence both protractormaxillaeand
doubleretractormuscles
may be presentin onesection,whichis not
the casewith Culexwherethe origin of the protractorsis distinctly
eaudadfrom the origin of the doubleretractors(pl. 13, figs.7, 15).
The subocular
musclesare muchlarger,andarisingfartherbackin
the head,they are lesslikely to be mistakenfor fibersof the retractorsof the maxillae. They are insertedon the tentoriaimmediately caudadof the originof the doubleretractormuscles,
nearthe
insertionof a pair of minutetransverse
muscles,
not represented
in
Culex, which arise on the walls of the epicraniumdorsalto the
origin of the anteriormostlateral dilator of the esophagus.
T/•e fore •jut.--The stomodaeal
portion of the alimentarytract
of b}sectsis subjectto great variationswith respectto the number
and characterof the regionsthat are differentiatedin it. It may
well be considered
doubtfulif homologics
betweenregionscanlegitimately be soughtbeyondthe limits of any one order. Hence a nomenclaturefor the variousparts need only be self-consistent
within
a single order and may properlybe subjectto individualpreferences. The stomodaeumof the imagoesof the Diptera offers
especialobstaclesto the establishmentof a systemof namesthat
shall meet all conditions,for there is a considerablerange of variation within the group. For example,in Musca(pl. 14, fig. 16)
the labral or proboscis
canal(pc) is succeeded
by an intracephalic
subclypealpump (ph)which ,showsdistinct anteriorand posterior
regions. Then the esophagus
(oes)commences
andhenceextends
through the circumesophageal
nerve-collaror "l)rain." In other
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flieswith a similar succession
of proboscis
canal,subclypealpump,
and extensiveesophagus,
the differentiationof the subclypealtube
into two regionsappearsto be wanting. In contrastto the arrange-
mentsin theseflies,the Tipulidae(pl. 14, fig. 1•) have a chitinous
"pump" in the regionof the brain(p•p)and the esophagus
is
restrictedto the extreme rear of the head. The Culicidae(pl. 14,
fig. 17) resemble
this lastmentioned
familywith the exception
that
the "pump" is dilated behindthe nerve-collarinsteadof being
of nearlyuniformdiameterthroughoutits length. The Simuliidae
possess
a "pump" whichis not markedlydilatedat any point,but
it is distinctlydividedinto a preneuraland a postneuralportion.
This subdivision
foreshadows
the arrangementfoundin the Asil-
idac and Tabanidae
(pl. 14, fig. 19). liere the postneural
partof
the "pump" is vestigialwhile the preneuralpart is stronglydeveloped. If a •omenclatureis to be satisfactory
for all casesit clearly
must take into accountthree differentiatedregions of tile stomo-
daeum,in additionto the unspecialized
esophagus:tile proboscis
canalof the labrum(pc), the subclyl)eal
tube(ph), and themore
posteriorchitinized"pump." Of thesethe subclypeal
tubeand
the pump may be subdivided. Two additionalfactorsmustbe
considered.The subclypeal
tubeappears
to beanessentially
homologousstructurethroughoutthe groupof the fliesand the pump
probablycorresponds
to the anteriorendof the elongate
esophagus
of thoseflieswhereonly a subclypeal
tube (p/•) is differentiated,
e. #., Muscidae.A systemof nameswhichgroupsthe subclypeal
canaland pumpundera singleexpression
is therefore
inadvisable,
as for exampleChristophers'
and Annett and Dutton'saccounts
wheretheseregionscollectively
form the "pharynx" or ttnrst's
description
in whichthe terms"buccalcavity" and "pharynx"
are usedindifferentlyfor all the specialized
intracephalic
stomodaeu m.
Twosystems
of nomenc.lature
fortheDipteranstomodaeum
which
arefreefromthe objections
cited,exist. The olderseems
to have
comeintobeingby simpleextension
of thenames
givento theparts
in Musca,
i.e., "pharynx"for thesubtlypeal
tube,and"esophagus"
for all the remainingfore gut. ThusMeinert('81)callsthepump
of Tabanusand Asilus"pars prior oesophagi"
or "pars turnida
oesophagi."
With Culexor Tipulathisregionis simply
"oesophagus." Dimmock
('81)withrespect
to thepartsin Culex,
themost
THOMPSON:ANATOMY OF MOSQUITO.
155
complicatedtype of Dipteranalimentarycanalthat he studied,refers
to the pumpas "oesophagealbulb" and to the subclypealcanal as
"pharynx." On the other hand,Nuttall and Shipley(:0l-:03) in
their carefulstudyof Anophelesterm the subclypealcanal "buccal
cavity"andthe pump"pharynx." The olderof thesenomenclatures
hasdecidedadvantagesover the newer systemused by l•uttall and
Shipleyin that it recognizesthe probableequivalencebetweenthe
pump and the anterior end of the "esophagus" of such a fly as
Musca,and in that it givesthe expression
pharynx--which is generally employedin insect morphologyto indicate the first differentiated intracephalicregion of the stomodaeum
-- to the constantly
presentsubclypeal
canalrather than to the inconstantpump. It also
leavesthe term buccalcavityvacantand availablefor any subregion
of thesubtlypeal
canalwhichit mayseembestto delimit.
A s•udy of the larvae of the Diptera in the above connection
offersno assistance,
but rather complicates
the problem. A majority of I)ipterouslarvae have a fore gut that is practicallyundifferentiated throughout its cephalic extent. In other forms, well
markedregionsare present. But theseare not alwayscomparable,
even in closelyallied genera,e. 2.. Culex and Corethra. Further,
the larval regionsmay or may not coincidewith the regionsof the
imaginal tract. The wriggler of Culex has a buccalcavityand a
pharynx,the latter beingimperfectlydistinguished
from the esophagus below and at the sides. But the larval buccalcavity and
pharynxbothgo to form the subclypealcanal of the imagoandthe
pump of the latter stage is derivedfrom the anterior end of the
esophagus
of the larva. Suchpeculiarities
findtheir explanation
in
the extremelyadaptivecharacterof the Dipterouslarva,but thisin
turn makesit unwiseto lay stresson larval relations,in an attempt
to obtaina systemof namesfor the imaginalparts.
Employingthe nomenclature
just chosen,the stomodaeal
portion
of the alimentarycanal of the mosquitoshowssuccessively:
the
labralor proboscis
canal,the subclypeal
tube or pharynx,the pump
or antlia, and an esophagus
which extendsthroughthe occipital
forameninto the thorax. The midgut is differentiated
into cardia
andstomach
or mid-intestine.The hindgut is modifiedto forman
ileo-colonand a rectum. Valves are developedat the union of
pharynxand antlia, pharyngeo-esophageal
valve, at the posterior
end of the antlia, at the junctionof esophagus
and cardia,eso-
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phageal valve, and at the point where the stomachand ileo-colon
cometogether.
Accordingto Dimmock('8:•) both labrumand hypopharynx
take
part in the formationof the probosciscanal,the latter stylerform-
ing the floor. Kr:,ipelin('8•-) on the other hand,considers
that
this canal is wholly labral in origin. It was not possibleto
decidebetweentheseviews. The shelf formed by the produced
lower internalanglesof the labrum (pl. 16, fig. 46-47) favorsKriipelin's view, but in this casethe delicatecharacterof theseplates
wouldseemto requirethat the hypopharynxshouldbe drawnup
againstthem during blood suckingand so indirectly completethe
probosciscanalventrally.
Sincethe anterior wall of the clypeusis forwardof the basesof
the mouthpartsgenerally,the labrummeetsthe headsomedistance
in front of the point •vherethe mandibles,maxillae,hypopharynx,
and labironare inserted. In this regionthe sectionof the proboscis
canal is that of an arch, and the hypopharynxunquestionably
suppliesthe flattenedfloor (pl. 13, fig. 3). Caudadof the basesof the
mouthparts(pl. 13, fig. 4) the canalmay be consideredas intracephalicand as pharynx. At this point the thick chitin of the side
wallsof the canalthinsabruptlyand in materialclearedwith caustic
the probosciscanal appearsto terminatewith a spuron either side.
There is no real point of demarcation,however, between the probosciscanal and the pharynxas I h,•ve limited them, exceptthe
completionof the tube astl•e mouthpartsjoin the headand the formationof theseseenlingspurs. The chitin of the floor of the canal
is ah'eadythin andsoharmonizes
with thethinnedwalls. On theroof
of the ca•al the heavychitin extendsfor a considerable
spaceinto
the head,thinningout graduallyfrom the sidestowardthe midline,
and so forminga tongueof heavierchitm amidthe weakerchitin.
Nuttall and Shipley'sdescription
of thisstructure,anteriorhardpalate in Anophelesas "somewhatin the formof a trowel" is equally
vivid for (:ulex, since its arch is steeper than the arch of the
pharynx roof generally(pl. 13, fig. 5). Just beforethis palate
terminates,its surf,•ceis roughenedby a few minute conicalspines.
The proboscis
canalreceivesimmediatelyanteriorto the "spurs" at
its proximalend,a pair of musclesfrom the alae of the clypeus.
These are the "epipharyngealmuscles"(Annerra•d Dutton, :ox)
and the region of their insertioncloselycorresponds
to that
THOMPSON: ANATOMY OF MOSQUITO.
157
occupiedin the larva by an epipharynx-likestructure. Meinert
('81) figuresfor Culex musclesjoiningthe alaeof the clypeusto
tile roof of the box.
I have not been able to find these in three
speciesof Culex at my disposal,and th6y are ,lot presentin
eles2•tmctipennis.
The pharynxextendsfrom the union of the mouthpartswith the
headto the anteriorend of the esophagealantlia. It is developed
asa pumpingorganandthe characterof its walls and the form of
its crosssectionvary in different levels. At first,in the regionof
the anterior hard palate, the floor and walls have a thin intima
throughoutand this steadilyinvolvesmore and moreof the roof of
the canal as the anterior hard palate narrows. At the sametime
the floor of the pharynx becomescurved,so that by the time the
trowel-shapedhard palate has vanished,the pharynx consistsof a
curved ventro-lateral plate and a flattened dorsal plate, the two
meeting in high dorso-lateralangles (pl. 13, fig. 6). The section
of the tube is either crescenticor that of an inverted arch, accord-
ing to the positionof the dorsal plate as it curvesdown il•to the
lumen or is drawn upward by muscularaction. For a spaceafter
the hard palate vanishes,both walls and roof remain thin. Then
the dorsal plate becomesuuiformly thicker. Finally, thinning
again,it, forms the pharyngeo-esophageal
valve at the e,trance to
the antlia. Ilence the dorsal plate or roof of the pl•arynx is differentiated into four regions' an area of thin chitin with a medi:m
tongue of thicker chitin, anterior bard palate; a region with uniformly thin chitinousintima, soft palate; a region with ,niformly
denser intima, posterior hard palate; and a narrow area of thin
chitinforminga valve. The valveand posteriorhardpalateregions
are derived from the "pharynx" of the larva. The l'emainderof
the pharynxand the proximalpart of the proboscis
canalare formed
from the "buccal cavity'•'of titis earlier stage.
The chitinous
intimaof the floor and sidewall of the pharynxis
uniformlythin exceptin the region of the posteriorbard palate.
Here, beginningfirst alongthe borderof the dorso-lateralangles,
an area of thicker chitin forms on either hand and widens to involve
all the wall of the canalfor a space. Then narrowingagain,these
areasterminatewith stoutspurswhich projectpast the anteriorend
of the antlia. At the point where they are most extensive,they
meet acrossthe floor of the pharynx and in the female insect a
158 PROCEEDINGS:BOSTONSOCIETY NATURAL HISTORY.
delicate crest projectsdownwardfrom this traverse. The male
Culex has the same arrangementof thickenedareasbut lacksthe
crest. The developmentof these thickened areas in the walls of
the pharynxnear its unionwith the antlia mustmakethe ventrolateral plate rigid at this point. The antliahasalsoat its anterior
end ratherunyielding
walls. Hencelhe thin dorsalplate beyond
the posteriorhard palateformsan efficientpharyngeo-esophageal
valve, dippinginto the lumen of the canalthroughelasticityand
beingwithdrawnby muscles.
At its anteriorendthe pharynxfor a spaceis firmlyboundto the
walls of the head by an ascendingchitinousplate on either side.
Theseplatesare continuous
with the cuticleof the head and the
intima lining the pharynx,and representintracephaliccontinuations
of the platesformedby the union of the genaewith the clypeus
(p1.13,fig. 3-5). The platessoondegenerate
intolowcrestsalong
the dorso-lateralanglesof the pharynxandfade out.
At its hinderend the pharynx is held in place by two pairs of
muscles. The shorterof these,the lateralpharyngealmuscles,
run
outwardfrom the externalfaceof the spursat the rear of the phar-
ynxto the tentoria. The others,the ascending
pharyngeal
muscles,
passfromthe internalfaceof the spursto the vertexof the head.
The pharynxasa pumpingmechanism
is constructed
onthe same
principleasthe salivarypumpor antlia. The lumenis diminished
by the inspringing
of the chitinous
dorsalplate,whichis withdrawn
againby the actionof muscles.The muscles
whichlift theroofof
the pharynxin thispumpingactionmaybe calledthe elevatorsof
thepalate(NuttallandShipley,:01-:03). Whencompared
withthe
homologous
musclesof manyflies thesemusclesappearrelatively
weak. They are considerably
specialized,
however,astherearefive
distinctpairs,insteadof a singlemassor at most a larger anterior
and a smallerposteriorsection. When this latter differentiation
occursthe posteriorsectionis calledthe protractorof the pharynx
(Meinert,'81). Possibly
it findsits equivalent
in the fifth pair of
the Culicid muscles. ()f the five pairs,the anteriormostinsert on
the rear of the anteriorhard palateregion,the next on the softpal-
ate, the three posteriorpairson the hardpalate. The pharyngeoesophageal
valveis elevatedby a pairof valvularmuscles
from the
frons. These lie caudadfrom the buecalganglion. The valvular,
ascending
pharyngeal,
andlateralpharyngealmusclesare described
THOMPSON: ANATOMY OF MOSQUITO.
159
byNuttallandShipley,
but arenot named.Theelevators
of the
palateare the "musculiantliaepharyngis"
(Meinert,'sx), the
"pharyngeal
muscles"
(AnnettandDutton,:ox; Dimmock,
'8:t,in
part),andthe"protrusor
muscles
of thelabrum"(Giles). Thelast
identifichtionis basedon the figures. The accompanying
description is obscure.
The pharynxserves
asoriginfor two pairsof muscles.Nearits
anteriorend the maxillarymusclesariseand near the posteriorend
the hypopharyngeal
nmscles.Theselatter operatethe salivary
pump. •Vith the femaleof Culextheyhavean extensive
origin
from the wallsof the pharynxbetweenthe ventrallyprojectingcrest
alreadydescribed
andthe neighborhood
of the third pair of elevators of the palate(pl. 12, fig. 1,/q/p m). They arisein part also
from the lateral borders of the crest, but not from its median sur-
face. In the male the smallermuscleshave a limited originin the
level of the third pair of elevatorsof the palate. No musclewith
the positionassigned
by Gilesto his "muscleopeningthe salivary
valve" could be found in eilher Culex or An•pheles and the
accountof the salivarygland given by him is misleading
in other
respects.The hypopharyngeal
muscles
correspond
to Giles'"nmscles closingthe salivaryvalve," to Meinert's "retractoresreceptaculi," and to Annett and Duttou's "muscleto the salivary
receptacle."Nuttall and Shipleydescribe
thesemuscles
but do not
name them.
The structureof the pharynxdescribed
for Culexis closely
paralleled by that foundin the malarialmosquito.There is the same
succession
of anteriorhard, soft, and posteriorhard palatesand
valve,but for zlnophelesm,c•dipeuuisas describedby Nuttall and
Shipley,the structureson the intimain the regionof the tip of the
anterior hard palate are more complicatedthan they are in Culex.
Annett and Duttondescribefor .-t. cost,lisa complexarrangement
of hair-like processes
on the ventralplate of the pharynxbelowthe
pharyngeo-esophageal
valve. Nuttall andShipleyrecordthat they
failedto find suchstructures
in the species
of Anopheles
that they
studied,A. rnaculipe•mis,
andI do not findthemin
The bracingof the pharynxto the wallsof the headanteriorlyand
the formationof heavilychitinizedareasandterminalspursposteriorly are similarin both generaand in Anophelesalsothe ventral
crestis developedas a partial originfor the hypol)haryng.
eal mus-
160
PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY.
cles. It would appearthat the pharynx of Anopheleshas somewhat more sharplymarkedanglesthan the pharynx of Culex and
may therefore be a more effectual pump. The musclesare the
same as in Culex: hypopharyngeals
from the walls and crestof the
pharynx,five pairsof elevatorsof the palate with the samerelations
to the softand posteriorhard palates,epipharyngeals
from the alae
of the clypeus,lateral pharyngeals,ascendingpharyngeals,and
valvulars. The ascendingpharyngealsarisefrom the medianinterhal crest alone and not from crest and vertex
of the head as is the
case with Culex.
The antlia of the female mosquitois pyriform with the bull)
behindthe nerve-collar. The chitinousintima which linesthis part
of the alimentarycanalis somodifiedthat it is convenientto speak
as if it werecomposed
of three racquet-shaped
pJates,
one dorsal,
the othersventro-lateralin positions.
The bordersof the plates are reflectedand a thin band of chitin
is includedbetweeneach adjacentpair of plates. The platescan
springi•ward until they almostobliteratethe lumenof the canal
(pl. 13, fig. 12) or be drawn back1)y musclesuntil the lumen()f
the btllb has a nearly circular section. As the plates terminate
posteriorlythey are clothedby a densecoat of fi•e bristlesand
this developmentcontinuesfor 10 /• or moreon the thin intima of
the esophagus.The postneuralbulb is the activepart of the pump.
Here the dorsalplate receivestwo large dorsaldilator musclesfrom
the vertex of the head. These musclesa•'ecompressedand in an
averagefemaleof C•dcxsti•tda•s their greatestdiameterwas(;0/•,
the lesser diameter 31 /•. In a male of the same s'ze the corre-
sponding(liametcrswere 60/• and 15/•. Thesemusclesco•'resl•ond
to the "musculisuperiores
antliae" (Meinert,'81) and to the "superior pumpingmuscles"(Christophers,:
0x), "post dorsaldilators"
(Nuttall and 5hipley,:01-:03). Either lateralplate receivesfive
lateral
dilator
muscles from the sides of the head.
These muscles
trend forward and upward to their insertionsand are compresse(l.
In a•x ayeragefemale of C. sti•tlet•,s the greatestdiameterof the
first to the most posteriormuscle was 54, 56, 72, 54, and 44 /•
respectivelyand the lesserdiameter 27, 20, 37, 18, and 20/•. The
total crosssectionof the female'smusclesis nearlytwice that of the
muscles of the male.
The lateral dilators have been described as
"musculiinferioresantliae" (5[einert, '81), "lateral pumpingmus-
THOMPSON:ANATOMY OF MOSQUITO.
161
cles" (Christophers,:0x), "musclesof the pumpingorgan" (Annett
andDutton,: 0x)• "dilator muscles"(Giles•:02). Dimmockfigures
and refers to the lateral
and dorsal dilator
muscles but does not
name them. In the anterior,preneural,part of the pumpthe ventral unitingstripbetweenthe ventro-lateralplates--alwaysbroader
than the two dorso-lateralstrips--becomes wide and fiat, so that
the antlia in this region is four-sided(pl. 13, fig. 8). This must
diminish the movement of the walls and dorsal plate. At this
point the dorsalplate receivesa vestigialdorsal dilator muscle(a•t
dil). In Anopheles,
wherethe activityof thispart of the pumpis
lessdiminished,two dilator musclesreachthe dorsalplate, "anterior
dorsaldilators" (Nuttall and Shil,ley,: 0x-:03).
The epitheliumof the proboscis
canal,pharynx,and antliais inconspicuous.It is flattened over the probosciscanal and antlia
more than over the pharynx. The intrinsic muscle coats are not
evident, exceptover the hinderend of the antlia wherea powerful
sphinctermuscleis developed(pl. 12, fig. 2, s)•hm).
The male of Culex hasa more attenuatedantlia with a relatively
smallerbulbousportionthan the female. The total capacityis less
than that of the female'santlia even when the latter insectis rbpresentedby a smallerspecimen. Computedby Cote's rule, the largest male antlia foundin C. stirn•da•s had a capacityof approximately 0.0008 cu. mm. The smallestfemale antlia of the same
speciesmeasured0.002 cu. min., and the largest 0.004 cu. ram.
The time taken by a mosquitoof this speciesto draw up its meal of
blooddependson the nature of the spotpierced,averagingaccording to my observations
aboutoneminutefor a light and nearly half
as long again for a heavy meal. But under especiallyfav.orable
conditionsthe lessermealcan be takenin 50 seconds,
the greaterin
65 seconds. Sincea heavy meal consistsof nearly4.5 cu.ram. of
blood,a very rapid action of the antlia wouldseemto be required,
especially
if thispumpemptiesitselfat eachstroke. Moreprobably,
however, it acts to maintain a continuous rather than an intermittent flow.
The antlia of Anophelescorresponds
closelyto that of Culex in
structure. The most importantdifferenceis the greaterfunctional
activityof the anterioror preneuralpart asa pumpingorganwith
an accompanyingdevelopment of the anterior dorsal dilator mus-
cles. As a result,the antliaof Anopheles
is more uniformlycylin-
162
PROCEEDINGS: BOSTON SOCIETY NATURAL
HISTORY.
drical than the antlia of Culex. This point of difference is of
intei'est
in connection
with the variedspecialization
of the antliae
in the Diptera. On a basisof their pumpingmechanisms
the group
can be separatedinto two main groups, which may be termed
monantlial
and diantlial.
The
former
division
includes the Musei-
dac (pl. 14, fig. 16), the Syrphidae,and otherfamiliesthe members
of which possess
only the pharyngealpump. The diantlialgroup
have both a pharyngealpump and an antlia. Amongthese flies
ß
several forms of antlia are found. For example, Culex (pl. 14,
fig. 17) and Anopheleshave an antlia which is best developed
behind the "brain," i.e., it •naybe describedaspostneuralin type.
As alreadynoted,the pump of Anophelesis lessmarkedlypostneural than that of Culex, approachingwhat may be called the
amphineuraltype, where it is nearly cylindricalthroughout. The
Tipulidaeare perhapsthebestexampleof thistype of antlia(pl. 14,
fig. 18,2)u,2)). Simulimn,while amphineural
with respectto the
form of its antlia, has this region differentiatedinto distinctpreand postneuralsections.The antlia of the Tabanidae(pl. 14, fig.
19) and the Asilidaeis not onlysubdividedas in Si•nulium,but
the preneuralpart aloneis functional,the post-neural
sectionbeing
rudimentary. This type may be calledprenenral.
The traeheation of the head is interestingwith respectto the
supplyto certainparts of the alimentarycanaland muscles. Thus
the extensivetrachealsupplyto the elevatorsof the palateshows
that these musclesare of considerableuse, and that the pharynx,
althoughovershadowed
by the greatesophageal
pump,is yet by no
meansunimportant
asa suctorial
bulb. The thoracicportiono[ the
alimentarycanalis paralleledon either sideby a trachealtrunk.
From either trunk two traeheaeare given off into the head, the
externaland internaltraeheae(pl. 13, fig. 12). These lie closeto
eachotheruntil they have passedthe dilatormusclesandthen part
company. The externaltracheaascends
into the cleftbetweenthe
opticg•anglion
andthe "brain" (pl. 13, tlg. 10), supplies
thesenervous massesand the protractor muscleof the maxilla, and then
breaksup intobranchessomeof whichrunin the roesialplaneof the
elypeusto supplythe labralmuscleandthe elevatorsof the palate.
The internaltracheafollowsthe ascending
line of the tentoriumand
its terminaltwigsreachthe headof the strut. But beforethis point
is attained,the tracheahas spentitself in two large branches. The
THOMPSON: ANATOMY OF MOSQUITO.
163
first to arise, the labial trachea(li tra), runs forward between the
double
retractor
and retractor
of the maxilla
muscles and enters
the labinto. It givesbranchesto the doubleretractor,the retractor
of the maxilla,the salivarypump,and probablyto the mouthparts
generally. One prominenttracheolefollows the doubleretractor
muscle,so that this musclehas an extensivetrachealsupply. The
second branch of the internal trachea (pl. 13, fig. 6-9) trends
sharplyupward and dividesto supplyon the onehand the maxillary palpusand on the other the lateral pharyngealand antennal
musclesand the elevatorsof the palate,which thusreceivea double
trachealsupply. A mediantracheolereachesthe labral muscleat
its origin.
The esophagus
is mainlythoracicin the femaleCulex and almost
wholly so in the male, owing to the more truncate head which
leavesless spacebeyondthe antlia. The esophagus
is a slender
canal witIt thin walls, lined with a delicate intima which is smooth
exceptfor the area of bristlesat the anteriorend. The wails are
composed
of a singlelayer of flattenedepithelialcells,the boundaries of whichcan hardlybe distinguishedin sections. The usual
muscleco:•tsare present,but poorlydeveloped. Near the posterior
end of the esophagus
three diverticulaarise,two from the dorsolateral and onefrom the ventralwall of the gut (pl. 14, fig. 20,fres).
Then the esophagusterminateswith the esophageal
valve, a shallow curtainwhich dipsinto the first region of the midgut, the cardia. Within this valve the otherwiseinsignificantcircularismuscle
coat is specializedto form an annular muscle. The valve appears
to be without an external blood sinusand its walls have an epithelium similarto that of the esophagus. Unless the cardiais greatly
distended,the point where the epitheliumof the esophagusceases
is very noticeableat the shoulderof the cardianearthe termination
of the reflectedfaceof the valve (pl. 16, fig. 41).
The dorsalesophagealdiverticulaare small and lie closeto the
dorso-lateralwallsof the prothorax. The ventralpouchis large and
may extend into the abdomen. It does not appear to be as large
relatively as the correspondingpouch in Anopheles. Usually all
three sacsare filled with bubblesof air, and I have never found anything else in them. Nuttall and Shipley (:ox-: o•), however,have
made a carefulstudy of the possiblefunctionsof thesepouchesin
Cule.rand seemto have conclusivelydemonstratedthat they serve
164
PROCEEDINGS:
BOSTON SOCIETY NATURAL
HISTORY.
ß
as food reservoirs. Otherinvestigators
recordthe detectionof blood
in the sacsof the mosquitoand the similar pouchesdevelopedin
other flies occasionallycontainfood. In view of these factsthere
seemsto be no goodreasonfor denyingthat thesedive•icula serve
as storageplacesfor food. The name "food reservoirs"(Nuttall
and Shipley,:o:t-:o3) or "esophagealdiverticula" (Granpr• and
Charmoy,:oo) shouldreplacethe older and misleadingterms"aspiratory vesicles,""suctorialvesicles,"etc.
When
one of these diverticula
is extracted and is examined under
a microscope,
the onlystructurenotedat first is an irregularandvarying meshof delicatestriae. More carefi•lstudymakesit clearthat
these "striae"
are wrinkles
or folds of the delicate intima that lines
the pouch,varyingin sizeand positionwith the torsionor tensionof
the particular
partunderobservation.
By staining
thefreshtissue
with Bismark brown, distinct musclefibers becomevisible and the
nuclei of the epitheliumappear (pl. 16, fig. 42). With a • oilimmersionlensthe epitheliumitselfcan be distinguishedin placesas
a granularcloud. VVhetherthe musclebandshave asregulara distributionwith Culex as the similarbandsdescribedby Nuttall and
Shipleyon the ventral sacof Anopheles,
wasnotdetermined. The
"striae" sometimes
passacrossa musclewithoutalteration,but usually, of course,they are more numerousalongthe line of the muscle,
interlacingand radiatingin groupsfrom its borders. Under a lowpowerlensthis givesthem a strongresemblance
to muscularfibrillae.
The appearancevanisheswhen a higher magnificationis used,but
shouldbe noted,since,with somereagents- e.g., methylinbluethe colorfills the "st•qae,"seeminglystainingthem. If a pouchis
torn the excessivelywrinkled intima gives a fibrousappearance
to the fragments. This is probablythe basisfor Giles' assertion
that theseorgansare composedof finelybranchedtracheae. "The
fibers are neither more or less than extremely elastic and' distensible tracheae,which swell out into bubble-containingdilatations.
.... Apart from a few looseconnectiveelementsthe sacsconsist
of nothingelsethan thesecuriouslymodifiedtracheae." In connection with this statementwith respectto structuresthat are obvious
diverticula from the esophagus
and have a less extensivetracheal
supplythan any otherpart of the alimentarycanal,exceptpossibly
the pharynxand antlia, Giles alsoholds that there are two ventral
sacs,that thereis no organicconnection
betweenthe esophagus
and
THOMPSON:ANATOMY OF MOSQUITO.
165
the sacs,andthat the pouches
have an aerostatic
function. Nuttall
andShipley(: 0x-:03) havethoroughly
exposed
theinaccuracy
of this
accountof the sacsof Anopheles. It wouldbe equallyinapplicable
to Culex. The two genera show no essentialdifferenceswith
respectto the structureof the foodreservoirs.
Salivary gla•ds.--The epitheliumof the salivaryduct is thin
but columnarand the duct has a trachea-likeaspectowing to annu-
lar thickeningsof its intima. It forks within the headand in the
thorax each branch suppliesa tripartite gland. The duct within
eachof the threeaciniof the glandis (Uulex stimulans)of uniform
diameter,has a delicatesmoothintima, and is traceableto the apex
of the acinuswhere it ends blindly. The acini are elongateand as
their first describerMacloskie('88) and later writers havenoted,
the central and lateral divisionsare unlike in appearance. Maclos-
kie gives no evidenceto supporthis suggestion
that the central
acinussecretesthe poisonouselement of the saliva. This acinus
(pl. 14, fig. 23, ca) is slender,cylindrical,
and its nucleiare prominent, surroundedby granularcell contents. In sections
the larger
partofeachcellis filledwithsecretion
which,
asprecil•itated
by
the reagents,stainslightly with haematoxylin
andis coarselygranular. The nuclei are pushedover againstthe basementmembrane.
The lateral acini apically are of the samediameteras the central
acinus,but their basesare distended. Their cells are filled with a
transparentsecretionof high refractiveindex. Toward the baseof
the glandnearlyeverycellcontainsa hugeovoidvacuolefilledwith
the secretionwhich forces the nucleus against the basementmembrane. Sectionsshow clear spaceswherethesevacuoleswere situ-
ated. Nearer the apicesof the acini the vacuolesare smaller and
less numerous.
The differencesin the appearance
of the lateral aciniand the central acinus are due to the different
substances secreted and stored
in the cells. During the later pupa stage the distinctionsare want-
ing and all partsof.the glandresemble
the centralacinus
of the
imagoas viewedby transmittedlight. Sections,however,show
no secretionin the cells. The differencesmake their appearance
a
short time before the imago emerges.
T/•e midg•tt.--The part of the midgut which receivesthe eso-
phagealvalve (pl. 14, fig. 20) is bestcalledthe cardia. As with
most other flies,the region is not definitely marked off from the
166
PROCEEDINGS: BOSTON SOCIETY NATURAL
HISTORY.
stomachin either Culex or Anopheles. Sectionsshowonly a gradually increasingaffinityfor haematoxylinon the part of the cellsas
the esophageal
valve is approached. In shapeand other characters
the cells resemblethose of the stomachgenerally and there is
no alteration
in the muscle coats.
The stomach extends into the
seventhabdominalsegmentand is the longestsingleregionof the
alimentarycanal. Its thoracicend is a narrowtube,but the abdominal part is wider and capableof great distension. Here is stored
the meal of blood while it is being digested. The wall of the
stomach
hasan epithelium
composed
of smallcubical•ellswith oval
nucleiand granular cytoplasm(pl. 14, fig. 21, st). The fi'eeborder
of the cellsis moretransparentthan the remainingarea. Probably
this corresponds
to the "striatedborder" describedby Christophers
(:0:1) for Anopheles. The margins of the cells make a very sharp
line in sections,but I did not succeedin demonstratinga cuticle.
In the freshly removed stomachthe nuclei are not visible and the
cytoplasmappearsfilled with refringentgranules. Over the surface
delicate circular and longitudinal musclespassin an irregular network which cuts the surfaceinto mm'e or less rectangularareas.
The epithelium often protrudes between the musclesforming
"islands" of one or more cells. The stomachnarrowssuddenly
at the posteriorend and a low valve is formed. The trachealsupply is extensiveand the interlacingvesselsserveto retainthe canal
in position.
The physiologicaldissimilaritybetween the stomachsof Anophelesand Culexin relation to the malarial parasitemay possibly
be accompaniedby visible histologicaldifferences,althoughnone
have sofar been described. Conclusiveproof awaitsthe elaborate
processof removingstomachs
from mosquitoes
of both generawhen
in the samephysiologicalstate, and passingthem through all the
reagentssimultaneously.I have not been able to obtain a sufficient numberof Anophelesto do this.
The hind.qut.--The anterior end of the first region of the hind
gut, the ileo-colon,is modifiedto form a low valve adjacentto
the valve (pl. 14, fig. 21) whichterminatesthe midgut. The Malpighiantubulesopeninto the circularcleft betweenthesetwo annular valves. There are five Malpighian tubules, an unusualnumber
amongthe flies. They extendforward for a spaceparallelto the
stomachand then recurve,their tips reachinginto the last joint of
THOMPSON:
ANATOMY
OF MOSQUITO.
167
the abdomen. In fresh material under transmitted light the cyto-
plasmis steelgrayin color,the nucleiaretransparent,
and the cells
are filled with refringentgranules. Sectionsshowa definitestructurelessborderalongthe free edgesof the cells. This stainsdeeply
with "orangeG" but I am not certainwhetherit is to be regarded
as a cuticle.
The ileo-colon is a canal of small diameter and its walls are thin.
The cytoplasmof the epithelialcellsas seenin sectionsis often
vacuolated,
especiallytowardthe baseof the cells. A thin, distinct, cuticularlining is visible,and for a short spaceimmediately
behind the entranceof the Malpighiantubules this is toughened
by bristle-likechitinouspapillaewhichpoint caudad(pl. 14, fig. 21,
ilc). This hirsutebelt cannotbe regardedas characterizing
a
"region" in the gut of the imagoand the areadoesnot correspond
to any distinctregionin the hind gut of the larva. The longitudinal and circularmusclesof the ileo-colonare fairly well developed
and like the stomachthis part of the alimentarycanal has an extensive tracheal supply.
Posteriorly,the ileo-colonpassesimperceptiblyinto the rectum.
The depth of the epithelialcells of the rectumis variableand the
musclecoatsare less developedthan in the ileo-colon. There are
six rectal papillae,eachconsisting
of a coneof largeepithelialcells
arrangedabouta commonaxis. The trachealsupplyto the rectum
is generous,
especially
to the papillae. Tracheaederivedfrom the
mainlongitudinaltrunks that follow the alimentarycanalon either
siderun to eachpapilla,emit branchesthat courseover the baseof
the coneand adjacentwalls of the rectum, and then oneor more
branchesenter the axis of the papilla. Theseaxial tracheaeascend
to the apex of the coneand divide,their branchescurvingbackin a
somewhattortuouscourseamongthe componentcells.
THE
LARVA.
The largerand broaderheadand the elongaterespiratorysiphon
on the dorsumof the eighth joint of the abdomenare perhapsthe
mostobviousof the external differencesthat distinguishthe wrigglersof Culexfromthoseof Anopheles. Otherwisethegroundplan
is similar: the three thoracicsegments
are consolidated
'intoa single
168
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BOSTON SOCIETY
NATURAL
HISTORY.
massand there are nine abdominaljoints, of which the eighth bears
the respiratorysiphonand the ninth four anal valvesand a ventral
keel of setae. Under the scopeof this paperthe head alonerequires
detaileddescription.
The headof the Culexwriggler(pl. 15, fig. 29-32) is flattenedin
the dorso-ventralplane. Forward of the antennaeit narrows.
Behind them its cross section would resemble that of a watch, a
flattened oval. The dorsal surface is the more convex, descend-
ing rapidly in the rear, more gently at the sides,and with a
comparativelysteepslopeforward from the antennae. The anterior margin of the head is boundedby a borderline of thickened
chitin (bordl) beyond.which
a shelf-likefold projects. The shelf
bearstwo simplesetae. The investingcuticleof the top of the head
is strongandis reinforcedby two thickerscythe-shaped
areas(sca)
which extendbackwardfrom the antennae. At pupationthe cuticle
splitsalongthe innermarginof eitherarea to the occipitalforamen.
On the ventralsideof the head the transverseline (tr l) separates
the pre- from the post-antennal
region. This boundaryconsists
of a
narrow-linearband of dark, thickened ehitin laterally and a low
transversefold mesially. Iteavy cuticleextendsoverthepost-antennal ventralregionof the headand is traversedby two narrow-linear
"lines" of dark, thickened ehitin which pass from the transverse
line to the border of the occipital foramen. These divergent lines
supportthe ventralendsof the tentoria,the black,triangular,mental
selerite,and furnish a point of origin for the depressorsof the
antenna(ant m) and othermuscles. At pupationthe •entral head
cuticlesplitsin the midlinefrom the occipitalforamento the transverse line.
The relationsof the pre-antennalregion of the headare hard to
describe. As noted,the dorsal surfaceis convexand heavily chitinized. The ventral face slopes sharply inward and downward
toward the entranceto the pharynx, a point almostin the center
of the head. Thus the pre-antennalregionis in reality a broadrostrum whichoverhangsthe mouthparts,
situatednear the transverse
line below,slightlyforward of the entranceof the pharynx(pl. 14,
fig. 26). Its crosssectionwouldbe the segmentof a circle,the
arc beingrepresented
by the convexdorsalsurface. In the midventral line, about halfway betweenthe anteriorshelf-likefold and
the entranceof the pharynx,a crest is formed. This bears four
THOMPSON: ANATOMY Or MOS(•UITO.
169
stout setae and is flanked by setoseareas. Here the thin chitin
characteristicof the ventral face of the rostrum generally, is
strengthenedby a triangularsclerite. I regard this structureas an
epipharynx. It receivesa pair of slendermuscles(I•aschke,'87)
which ariseon the top of the head and probablyfunctionas retractors.
The border line and shelf-like fold alreadydescribed,mark the
boundarybetweenthe heavierchitin of the dorsal and the thinner
chitin of the ventral face of the rostrum, above and in front.
But
on either side, near .•he ante•xnae,the thicker chitin involvespart
of
the
ventral
face of
the
rostrum.
These
infiected
areas of
heavy chitin I call the black-sl•otareas, becauseeach bearsa conspicuous
patchof pigment(blks). From either areaa narrow-linear
"line"
of heavier chitin traverses the ventral face of the rostrum to
strengthen the sclerite of the epipharynx. In the bay of thin
cuticle bounded in front by the anterior shelf and border line,
and posteriorly by the black-spot areas, lie the sm,•ll, median
palatum(pal) and the larger,lateral fiabellae(j•/). These are protuberances,
denselyclothedwith fine hairs,the fiabellaehaving in
additiona peculiararrangementof long yellow setae. Black-pigmentedapodemes(pl. 15, fig. 31) that are continuous
externally
with the black-spot areasenter eachfiabellaand the two fiabellae
are united by a transverserod. To the apodemesthe fiabellalmuscles(retryq) are attached,two for each fiabella,an inner and an
outer,both retractorin functionand actingsimultaneously.When
these muscles contract the seta-bearingarea of the fiabella is
depressed
and the setaecometogetherin a brushwhosetip points
caudad. On their relaxationthe fiabellaeprotrudebeyondthe front
of the head and the setae stand out in great yellow fans. The
medianprominenceor palatum passivelyfollowsthe movementsof
the fiabellae, which are strictly synchronous,as far as I have
observed.
TAemout]q•arts.--Themouthparts(pl. 15, fig. 29, 32) of a Culex
wriggler consistof huge maxillae,moderatesized,toothed mandiblesand scleriteswhich representlablure and hypopharynx. The
maxillae concealthe greater part of the under surfaceof the rostrum.
The palp is minute. Theseappendages
are not joined to the head
by definite articulationsas are the mandibles,and do not move as
freely. Within the epipharynxabove,mandibles,maxillae at the
170 PROCEEDINGS: BOSTONSOCIETY NATURAL HISTORY.
sides,and transverseline below,lies a funnel-shaped
cavityleading to the pharynx. Into this spacethe tips of the fiabellal setae
dip whenthoseorgansare depressed.It maybe calledthe buccal
cavity,andwithinthe transverse
line fourstructures
aresuccessively
differentiatedfrom its floor: a delicate,hair-fringedcrest,a black,
triangular
mentalsclerite
(tr), a complicated
saddle-shaped'
sclerite
with a mediancrestand lateral toothedspurs(/i), and confluent
with this last, an arch of heavy chitin which is piercedby the
orificeof the salivaryduct (],yp). The mental scleriteis bracedto
the divergent lines at its lower angles. The arch sclerite,and
indirectlythroughit the saddle-shaped
sclcrite.are strengthened
by
a narrow-linear"line" of heavy chitin(pl. 15, fig. 31, /•tr) which
crosses
the thin chitinof the buccalcavit3'fromthe black-spot
area
of eitherside. Thesohypopharyngeal
traverses
give supportalsoto
the walls of the pharynx by short apodemes.
The mentalscleritewascalled"underlip" by Meinert('86) and
corresponds
to the structuresfigured by Miall and Hammondfor
larvae of Chironomus,('92, :oo), Miall ('93) for Dicranota,and
Miall and Walker ('95) for Pericomaas "submen•um." Ilaschke
identified
the fold
that
I
have
termed
the transverse line with
the menturn,"kinn." For Miall, the scleriteswhich correspond
to the saditle-shaped
scleriteof Culex are "menturn." With Culex,
however,this scleritesheathesthe bud whichformsthe labinto of
the adult and is to be regardedas the larval labinto. The mental
scleritewould seemto be either a menturnor a submentum,as its
hypodermis
duringthe metamorphosis
passes
into the floorof the
head(pl. 15, figs.34, 36,fold).
The mandibles have a powerful musculature. Each receivesa
convergermuscleon the lower internal angle and a divaricatoron
the lower externalangle. Thesemusclesariseby triple headson
the walls of the epicranium. Each maxilla hastwo muscles. One
consistsof two parallel bandsand arisesnear the originsof the
mandib•lar
muscles
(depmctx),
the otheris singleandarises
from
the divergent line (retr max). Both insert at the middle of the
base of the maxilla.
The action of these muscles was determined
with difficulty,but by watchingslow contractions
in a dyinglarva
it was found that the double band pulled the maxilla candadand a
little outward(venttad)while the singlemusclepulledthe maxilla
candadandinward (dorso-mesad).Hencethe doublemusclemay
THOMPSON:ANATOMY OF MOSQUITO.
171
well be called the depressorof the maxilla, the single musclethe
retractor of the maxilla. The labium is suppliedby a pair of musclesrunning from the divergent lines to the hinder anglesof the
sclerite. They are depressorin action. Under the hypopharyngeal scleriteare a few scatteredmusclefibers.
The ./'()regut.-- The fore gut of the Culex larva is divided into
buccal cavity, pharynx, and esophagus. The buccal cavity has
ah'eadybeen described. Its epitheliumis columnarand behindthe
epipharynxabove and the hypopharynx below there is a consider-
able area of undiffercntiatedintima (pl. 15, fig. 32, be). The
pharynx is sharply marked off from the buccalcavity by a fine
lineof denser
chitinandits flattened
epithelium
serves
as a further distinction. The pharyngeo-buccal
and pharyngeo-csophagcal
openingsare not in line, sincethe esophagus
leavesthe pharynx
rather from the floor, and in longitudinalsectionsthe rear of the
pharynxoften overhangsthe esophagus
like a blind pouch(pl. 15,
fig. 3'_)). This appearanceis an artifact,due to the contractionof
the walls of the esophagus
when the animal is killed. Actually,
pharynx and esophagusare not sharplydelimited. The general
appearance
whendissectedout of the head,is as if the esophagus
expandedto form a cupwith high-arched
sides(pl. 15, fig. 35) open
widely in front wherethe walls passto the buccalcavity and closed
aboveby a dorsalplate. The archlugborderswhere walls and roof
meetmay be termedthe crestsof the pharynx. They are approximatedat the extremerear of the pharynx (pl. 15, figs.30, 33) and
as they sweepforwardin the high curveand descendagaintoward
the buccal cavity, they diverge. The dorsal plate or roof of the
pharynxthereforeis narrowedposteriorly. When at restthe angle
betweenthe posteriorendsof the crestsmeasuresabout60 degrees.
But the sidewalls of the pharynx are mobile and may approachor
spreadapart from one another. The angle betweenthem consequently varies, and the dorsal plate must accommodateitself to
changesin the width of the spacebetween the crests. This is
accomplishedby folding the plate inward along its median line.
•Vhen the pharynx is dilated,the dorsal plate is almostplane,but
When
thewallsof thepharynx
approach
eachotherstrongly,
the
plate foldsin until it nearlytouchesthe floorbeneath. The folding
is mostmarkedposteriorlyand becomesprogressivelyweakertoward
the anterior end of the pharynx, where a transversecrescenticsclerite
172 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY.
is developed
in the dorsalplateand verypossibly
servesasa check
(ces).
The pharynx has an elaborate musculature. The two halvesof
the dorsalplate are unitedby transverse
or obliquebands(dot
t)h) which attach to a longitudinalscleritedevelopedon either
sideof the medianfold. A pair of diagonalmuscles
(pl. 15,figs.
30, 33, dia,q)reachthe pharyngealcrestsneartheir posteriorends,
fi'om origins on the roof of the head, in each case on the side
oppositeto that of the insertion. -At the summitof their curvethe
crestsreceivelateral muscles
(l(•t ,5)from the scythe-shaped
areas
on the walls of the head, while nearer the border of the buccal
cavity,severallesserlateralmuscles(lesslat) reachthe dorso-lateral
walls. At their approximatedposteriorendsthe crestsare bound
to the epicraniumby stout muscles,
the retractorsof the pharynx
(pl. 15, figs.30, 32). The hinderpartsof the dorsalplatefurnish
insertionfor two elevatorsof the dorsalplate (el dp), whichdescend
from the epicranium. The lateraland ventralwallsof the pharynx
and the walls of the esophagus
generallypossess
a well developed
circularis
muscle-coat
and one of the most characteristic
of the
pharyngeal
musclesis probablyonly a specialization
of the circularis. This musclegirdles the pharynx from the angle wherethe
crestsmeet, along under either crest and beneath the floor below.
It maybe calledthegirdle-muscle
or cingulum
(pl.15,figs.32,35,ci•).
In front of the point where the cingulumpassesunder the floor of
the pharynx,two slendermusclesariseand extendbackto the walls
of the epicraniumnearthe occipitalforamen. Theseare the ventral
retractorsof the pharynx. They run in companywith a numberof
slendermuscleswhich emerge at intervals from beneaththe circu-
laris of both pharynxand esophagus.The cmnplexthat is formed
by thesemuscles
maybe calledthe lateral dilatorof the esophagus
(1,t dil oes). There are alsotwo pairs of dilatormuscles
that
insert on the dorsal wall of the esophagus(pl. 15, fig. 32, ant dil,
(lot dil). The largerof theselie abovethe supra-esophageal
ganglion and closelyparallelthe retractorsof the pharynx; the smallel'
lie beneaththe ganglion.
When a wriggler is feeding the significance
of the peculiarphar-
ynx andmusculature
becomes
apparent.It will be notedthatthe
pharyngealcrestsrapidlyapproach
and separateso that the dorsal
plate mustbe constantly
infoldedandwithdrawn. The greatfans
THOMPSON: ANATOMY Or MOSQUITO.
173
of flabellal setaeremain spread,but sweepdownward in a small arc
with a flickeringmotion. The mandiblesand maxillae are alsoin
incessantrapid motion. At intervalsthe fiabellaeare depressed
and
presumablyany food that may be entangledon their setaeis brushed
off by the maxillae. When the pharynx is filled with food, an extreme contraction occurs,the crests approach very closeto one
another, and the food is forcedinto the esophagus.The retractor
musclesof the fiabellaedo not producethe flickeringmovements
just described,beingconcernedonly with depression
of the fiabellae.
They remain tense during the gentler movements. The retractors
of the pharynxand the elevatorsof the dorsalplate are alsopassive.
The motion appearsto be causedby a slightto and fro movement
of the entire pharyngealand buccalregions,very possiblydue to
activity of the epipharyngealand lesserlateral musclesand the lateral dilatorsof the esophagus.The changesin the pharynxitself,
as shownby the movementof the crests,can be readilyinterpreted.
I believethat the retractorsof the pharynxandthe elevatorsof the
dorsalplate remain tense. This would make the elevation of the
dorsalplatelargelythe passiveresultof the divergenceof the crests
asthe pharynxwallsseparate. Probablythe cingulumis the most
importantagentin approximating
the wallsof the pharynx. The
diagonalmusclesmay assistand from their crossedpositionshould
be able to exert considerableleverage. Perhapsthey are the chief
factor in producingthe extreme compression
observedwhen the
animalswallows,
for any very markedcontraction
of the cingulum
apparentlywouldtend to closethe pharyngeo-esophageal
opening.
The headof the Anopheleslarva is proportionally
muchsmaller
than that of Culex and, as :Nuttalland Shipley(:0x-:03) have
pointed out, is rounded. "In fact the diameterfrom abovedown-
wardsis verylittle lessthan from sideto side,exceptanteriorly,
wherethe dorsalsurfaceslopesdownwardand forward." This rostral regionis narrow and compressed
at the sides,and the fiabellae
andpalatum
arecarried
well?u.tbeyond
theapices
of themandibles
and maxillae. The fiabellaeare relativelymuch smallerthan in
Culex. Hence they recall the fiabellaeof the larva of Simulium,
but are not stalkedasare the fiabellaeof this genus. The maxillary palpiof the Anopheleslarva are very large and conspicuous.
Betweenthe maxillaethe homologue
of the mentalsclerite
is recognizable,
am1in sections
labialandhypopharyngeal
sclerites
arefound.
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BOSTON SOCIETY NATURAL
HISTORY.
The elongationof the rostrumandthe greaterrigidity of its ventral
surfacerequirea differentmethodof feedingfrom that employedby
the Culex larva. Nuttall and Shipleystate that the maxillae,mandibles, and maxillary palp form the sidesand floor of a cavity
and that the fiabellae"are suddenlybent back into this space,the
mandiblesand maxillae moving forward to meet them and at the
same time openingout; they are then as suddenlyreleasedand
fly back to their original position. This movement.... is repeated
with great rapidity, often some 180 times to the minute producing a current sweepingin convergent curvestowards the abovo
mentionedcavity. From time to time the mandiblesare approximated and the stiff curved hairs of their upper edge are run
through the brushes" (fiabellae). That this may aid in removing
food fi'o,nthe fiabellaeor "brushes" is shownby the further observation that at intervals "the brushesdisappearfar into the mouth
and are then slowly withdrawn, passingthrough the fine carding
bristleson the inner face and anterior edge of the maxillae." In
Culex, according to my observations,the food is borne into the
spaceformed by the mandiblesand maxillaeby a flickeringmotion
of the expandedfiabellasandretractionof theseorgansis infrequent.
The cardingof the fiabellalsetaeis i,'regularin occurrence
and cannot be observedin detail owing to the unfavorablepositionassumed
by the feedinglarva. This subordinater•le for the fiabellaeis the
main differencebetweenthe feedinghabit of Culexand that of Anopheles. Sectionsindicatethat Anopheleshas a pharynxof the sa•ne
general form and musculatureas Culex, but of relativelyinferior
size.
The esophagus(pl. 16, fig. 37, oes)has already been partly
described. Its epithelium is flattened, especiallytoward the posterior end of the region. The chitinousintima is strong. Beneath
the well developedcircularismuscle-coatlies a longitudinalseries
of muscles,someof which emergeto form the lateral dilatorsof the
esophagus.In the anteriorpart of th,e thoraxthe esophagus
dips
into the cardia as the esophagealvalve. This valve is a deepcm'rain, thicker at base than at the free border. At the free border
the spacebetweenthe inner or directface and the outeror reflected
face is occupiedby a bloodsinus. Above,the spaceis filled by the
circularfibersof the annularmuscle(a•n m). In younglarvaethe
cellsof the epitheliumof the valve, except for a more columnar
THOMPSON: ANATOMY OF MOSQUITO.
175
aspectat the curve at the shoulderof the cardia, upper bend, are
like those of the epithelium of the hinder end of the esophagus.
Immediately beyondthe "upper bend,"the esophageal
type of epithelium is succeededby the type characteristicof the cardia and
stomach. For convenience,the point where the changeoccurswill
be called the "break" in the epithelium. In mature larvae a band
of narrow columnar cells(ann) muchlike the adjacentcellsof the
reflectedface of the valve, but staining more deeplyand with more
vacuolated cytoplasm,is differentiated on the valve side of the
"break." This functionsin the pupal stageas a regenerativering.
Delicate longitudinal fibers pass from the esophagusacrossto
the shoulderof the cardia (fi), outsidethe valve, as is the case
with severalotherDipterouslarvae,but the largebloodsinusfound
in the larva of Simuliumat this point (Miall and Hammond,:oo)
is poorly representedin Culex and Anopheles. Miall and IIammond also describe the fibers in the larva of Simulium
as muscular.
My sectionsand dissectionsof Simuliumdo not lend themselvesto
this interpretation. The fibersare evidentlynot muscularin Culex
and those of Simulium appear to me to be similar connectiveele-
mentsfrom the sheathwhich encloses
both esophagus
and cardia.
If the similarfibersin any formproveto be muscularthey mustnot
be thought of asrepresenting
the longitudinalismuscle-coat. For
in Dicranota(Miall, '93) whichis exceptional
in havingthe longitudinal musclesdevelopedover the shoulderof the cardia, these
passbeneath the blood sinus. The fibers are in the usualposition
externalto the sinus. In Simulium,Miall hasobserveda distribution
of the intimawhollylike that observedin thefreshgut of Culex.
The esophageal
valveof Anophelescloselyresembles
the valve of
Culex, but has a band of longitudinalmuscleswithin the valve
betweenthe annularmusclesand the epithelialcellsof the upper
part of the reflectedface.
The salivaryductof the Culexlarva is an extremely
thin-walled
tube whichis not differentiatedat any point. It is lined with a
distinctchitinous
intima. It forksastheoccipital
foramen
is passed
and eitherbranchentersa cylindricalgland. Theseglandstaper
somewhat
at theirproximalor ductends. The lumenis ampleand
notinfrequently
a distension
occurs
at the distalendof thegland
whichappears
in thelivinglarvaasa spherical
bodyof highrefractive index. The epithelial
cellsof thelivingglandarealmosttrans-
176 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY.
parent. In sectionsthe cellsare clearlydefined,their cytoplasm
stainsdeeply,the moderatesizednuclei (11.5/z) arereticulate,and
nucleoliare present. There are usuallynot more than five or six
cellsin the crosssectionat any point. As the wrigglernearsthe
pupa stage numerous
vacuolesoccurin the cytoplasmof many of
the glandcells,so that it appears
spongy. This is a signof degeneration. Immediatelyprior to pupation,a belt of smallcellsforms
at the neck where the gland narrows to meet the salivaryduct.
The cellsof this belt are seeminglyderived from the oldercellsof
the neckof the glandandperhapsalsofrom the epitheliumof the
outer end of the duct. They give rise to the imaginalsalivary
glands.
T/•e mi½t#•tt.--The midgut (pl. 16, figs. 37, 45) of the larva
showsthreeregions:cardia,a ring of eight caeca,andthe stomach.
The cellsof the cardiastainmore deeplythan thoseof the caecaor
stomachepithelium,but are otherwisesimilar. As ah'eadynoted,
the fore gut intima terminatesat the "break" wherethe ca•'diacepithelium beginsnear the upperbendof the esophageal
valve. The
cardiais lined by the peritrophicmembrane. In the mosquitothe
cardiaof the larva gives rise to the cardia of the imago through
a metamorphosis
similar to that by which the stomachof the
larva gives rise to the stomachof the adult. The esophagus
and
esophageal
valve on the other hand,have a wholly differentmetamorphosis,so that in Culex the cardiawould appearto be unques-
tionablya part of the midgut. This seemsto be the casein
Nematocerousfliesgenerally,althoughthe evidenceis as yet not
completefor any oneform. The larva of Anopheles,Chironomus,
and Simulium like that of Culex has a cardia whose epithelium
resemblesthat of the stomach. The embryologicaldevelopmentof
the cardiaof the Chironomus
larvashowsit to be of roesenteric
origin
in this case(Miall and Itammond,:00). Its relationto the cardia
of the imago has not been studied. From Vaney's (:o9.) account
it would seemthat the larva of Stratiomyabelongsto this type
and the cardia,"proventricule,"of the larva of Ptychopteraas figured by van Gehuchten('90) is more like stomachthan esophagus in the histologicalstructureof its epithelium. Occasionally
no
cardiais developedand in suchinstances
the esophageal
valvehangs
directly into the stomach,e.g. Phalacrocera(Miall and Shelforal,
'97) and Dicranota(Miall, '93).
THOMPSON: ANATOMY O• MOSQUITO.
177
The cells of the epithelium of the eight caeca are transparent,
polygonal,subequalin size,and showconsiderable
intercellularsubstance. The nuclei are large and have nucleoli. Although the
surfaceof thesepouchesappearsnodulatedas if from the contraction
of musclefibers, none were found. Sectionsshow that the cells are
on the averagelarger than the cellsof the cardia or mid-intestine
and very irregular in depth. The caeca usually contain a dark
ochre-colored
fluid.
Thestomach
iscylindrical
andextends
backto thefourthorfifth
segmentof the abdomen. It is abundantlysuppliedwith t•'acheae
and the cells of its walls are polygonal,with granular, yellowish
cytoplasm,large oval nuclei (17 x 10 •), and nucleoli. The cells
vary greatly in size, but are of uniform depth. Like the cells of
the cardia and caecathey have a striated border. The walls of
the stomach are nodulated, but this seems to be due more to the
irregular size of the componentcells than to the contraction of
musclefibers, although scatteredmusclesare visible in sections.
In preparations,
the basementmembraneis very distinct. Protrusionsof cells or parts of cellsinto the lumen, either in fresh or sectioned material was not observedfor any part of the midgut.
The above descriptionwould not be wholly applicableto the
stomachand caecaat all stages. For the processes
by which the
epitheliumis destroyedand replacedduring the pupa stagebegin
beforelarval life is ended, and involve progressivelymore and more
cells as the wriggler nears the momentof pupation. Small regenerative nuclei appearin increasingnumbersbet•veenthe basesof the
cellsof the epithelium. The cytoplasmnear the free margin of the
cellsbecomesspongy. The striated borderdissolvesin placesand
occasionally
cellsprotrudeinto the lumen of the gut.
T]•ehind g•et.--In the neighborhood
of the fourth and fifth segmentsof the abdomenthe stomachnarrowsand the hindgut begins.
This portion of the alimentary canal is divided into ileum, colon,
and rectum. The ileum (pl. 16, fig. 45, il), extendsback to the
eighthjoint of the abdomenand is not separatedfrom the stomach
by a valve. Probablythis is in adaptationto the continuouspassageof bulky wastesthroughthe gut. In the imago,in which the
meal of blood is storedin the stomachwhile it is being digested,a
valveexists
between
theileumandstomach.Thefreshlyextracted
ileum appearsas if wholly composedof circular musclefibers, is
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NATURAL
HISTORY.
transparent,and has an inti,na which is thrown into longitudinal
folds. Sectionsdemonstratea very thin epithelium,the nuclei of
the flattened cells measuring12 by 6.5 tz. Longitudinalmuscles
cannot be found. For a spaceimmediatelybehind the stomachthe
circularmusclesare alsowanting(pl. 16, fig. 45, 0s), and into this
naked annulusthe five Malpighian tubulesempty. Thesetubules
in every way resemblethe Ma]pighiantubulesof the perfectinsect.
The secondregionof the hind gut, the colon(pl. 14, figs.26, 27,
co) is distinctfrom the ileum,but is not sharplymarkedoff from the
rectum. Raschke ('87) grouped colon and rectum together as
"Enddarm" and Hurst'sfiguresincludeboth as "rectum." In the
text, however,the colonwould seemto make part of his "ileum."
The colonlieswholly within the eighth abdominaljoint. Its walls
have a deep epitheliumof large polygonalcellsand appearto be
ratherunyielding. Beltsof slendercircularmuscles
occurat regular
intervals, but no longitudinalmusclescan be demonstrated. The
living cellsof the colo• are granular. Sectionsshowfaint striations
radiating from the region of the nucleus. The oval nuclei of the
cells measure12 by 16 tz.
Posteriorly,the colonpassesgraduallyinto the rectum(pl. 16,
fig. 44, rec) which is wholly within the last joint of the abdomen.
The rectum has delicate walls that tea,' easily. Sectionsshowa
very thin epithelium,resemblingin structurethe epitheliumof the
colon,and a seriesof regularly placed,slender,circularmuscles.
THE
PUPA.
The firstabsolutely
diagnostic
signof the on-coming
of the pupation moult asobservedin C•dexstim•dct•sis the al)pearance
of two
white spotsin the prothorax,as the air chambersat the base of the
pupalrespiratory
trumpets
fill withair. At th•smomentprobably
the trtxn•petsare evaginated beneath the cuticle. The larva be-
comesincreasinglyturgid in aspectand intermittentperistalsislike wavesof contractionpassover its body. The tnrgescence
is
largely due to the evagination
of the pupal thoracicappendages
under the cuticle. The chargingof the air-sacsusuallyprecedes
the moultby aboutthreeminutes,but in oneinstanceeightminutes
elapsed,duringalmostall of whichtime the larva lay passive.
THOMPSON:ANATOMY OF MOSQUITO.
179
Suddenlya crack darts acrossthe top of the head and the pale
pupal head showsin the gap. From two to fifteen seconds
later
the cuticleof the thorax rupturesalong the mid-dorsalline and the
respiratorytrumpetsspringup to the surfaceof the water. The
tear in the thoraciccuticlewidensand the integumentof the anterior
abdominalrings alsosplitsalongthe back as the puparapidlyworks
the head and thorax clear and wrigglesforward in the compressed
remainsof the old skin, until only the apicesof the mouthpartsand
legs,and the tip of the abdomenare still sheathedby it. The abdomen is straightand eachring lies forward of its counterpartin the
old cuticle. Hence the intima which is being withdrawn from the
main longitudinaltracheaestretchesback from the eighth segment
to the respiratorysiphonlike a white cordon eitherside. Finally
the tracheaeare cleared,and the liberated pupa slips forward with
flexed abdomen,restsfor a few seconds,
and darts away,perfectly
formed,but of a light maltesecolor. The moult from the moment
when the head cuticle splitsto the clearingof the trachealtrunks
has occupiedfrom one minute five seconds,to one minute fortyfive seco•ds. The whole processfrom the appearanceof the airsacsto the final darting away of the pupa takes on an averagefour
minutes,but may occurin asshorta time as two minutesor be prolongedbeyondten minutes.
The external aspectof the Culex pupawith its flattenedhead,
high-archedthorax and nine-jointedabdomenis familiar. Male
pupaecanbe distinguished
fromfemaleby the largergonapophyses.
The gray color of the new pupa changesin an hour or so to the
typical black brown. A few hoursbeforethe fix emergesthe pupa
becomesquite black,owingto the formationof the imaginalscale
pattern under the integument. Shortly after pupationa spaceis
formedbetweenthe integumentand the dermisof the pupa(Hurst)
into which the scalesand hairs project. This fills with air before
the fly emergesbut the imago doesnot becomemuchdisplacedin
relation to the parts of the pupa skin. •N'o violent movements
precede or accompanythe liberationof the fly. The abdomenof
the pupa straightensand the imagoquietly worksitself free by
slight contractions•
the emergenceon the averageoccupyingfour
or five minutes. As much more time may elapse before the
insect takes wing. I have found in Corethra and Chironomusthat
the spacebetweenthe cuticleand the dermisfills with air prior to
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the emergenceof the fly as it does in Culex. I have observed
the liberation of the imago of Chironomusonly. Here a large
amount of air is admittedunder the skin,sothat the pupa has a
glassy appearance,the preliminarymovementsare prolongedand
violent, and the imago becomesmarkedly displacedin relationto
the pupal parts. The apex of the abdomen,for example,finally
liesthree jointscephaladof its own sheath. In onespecies
studied,
the smallapple-green
fly emergessorapidlythat it takeswingwithin fifteenor twenty secondsfrom the momentwhenthe pupal cuticle first ruptures. Comparingthis with the slow emergenceof
Culexleadsme to think that a connectionexistsbetweenthe rapidity of the emergence,tile amountof air admitted beneaththe cuticle, and the relative displacement
of the imago in relationto the
pupal sheaths.
Tire mo•t/qgarts.- The mouthpartsof the pupa developdu•mg
the last larval instar. The two organsthat arise from invaginate
imaginalbuds,the labium(pl. 15, figs.32, 34, l{) andthe maxillary
palpus(pl. 15, fig. 29, maxp bd), appearfirst. The developmentof
the labial bad may be taken as typical as regardstile histological
changes. The dermisbeneaththe labial scleritethickensand proliferates,until a plate of minutecells,whichstaindeeplywith haematoxyli•l, is formed. lVhile steadilygrowing upward, this plate is
depressedand fi•ally the b•d consists
of a bipartiteeminencearising from the floor of a deep pocket. Both pouchand eminence
secretea delicatecuticle. The labial imaginalbudsof Chironomus,
Anopheles,and Corethracloselyresemblethe bud in Culex and a
similardoublestructurehas beendescribedfor the Pupil)arousfly,
5[elophagus(Pratt, '93). Later in the i•lstar than the labial and
palpbuds,the dermisof the top of the headthickens,stainsdeeply
with haematoxylinand growsout as a cylindricalpouch. This bud
forms the labrum (pl. 15, fig. 32, let bd). It projects caudad,
depresses
the adjacent tissuesand lies in a shallowroesial furrow. Of slow growth until immediatelybefore pupation,it the,•
elongatesrapidly, and reachesback to the origi• of the el•ipharyngeal muscles(epi m) before the head cuticlesplits. At the
sametime that this bud elongates,
the cellsof the hypodermis
of the
fiabellaeenlargeand showa marked aftinityfor "orangeG." The
dermis then retracts, separatil•gfrom the overlying cuticle. In
terms of their
dermis the fiabellae are converted into shallow
THOMPSON:ANATOMY OF MOSQUITO.
181
depressions
or pits in the wall of the head. The dermisof the palaturn remains in position. The mandibles and maxillae undergo
almost no metamorphosisprior to pupation. Shortly before the
moult, their dermis thickens and takes on some of the staining
reactionsof the dermisof an imaginalbud, assuminga condition
intermediate
between
the cells of the fiabellal dermis
and the cells
of the labral, labial, or palp buds.
The larvae of Anophelespassthrougha similarseriesof changes
as far as their mouthpartsare involved, so that the wrigglers of
bothCulex and Anopheleshave at maturity an evaginatelabral bud,
invaginate buds for the labium and the maxillary palpi, thickened
mandibularand maxillary dermis, and the dermis of the fiabellae
and epipharynxretractedfrom the cuticle. The changesbeyond
pupation are not known for Anopheles. With Culex, the successive alterationsduring the pupation moult are shown in figures48
-50 on l•late 16. As the head cuticleis sloughed,the labral bud
swingsover into the plane of the other mouthparts. Probably
its rapid growth just prior to the moult is an important factor in
rupturing the head cuticle, while the changeof positionduring
pupation must assistin pushing the head case away at an angle
favorable to the extraction of the ventral mouthparts (pl. 15, fig.
36). The changeof positionis partly due to the growth of the
labrum,l•artly to a contractionof the retractorsof the fiabe]lae,and
partly to a shorteningof the dorsalwall of the buccal cavity. The
moult reducesthe retractormusclesto ovoid masses(pl. 17, fig. 56,
ret•'fl), and shiftsthe fiabellal depressions
back to the entranceof
the pharynx. The mandiblesand maxillae are withdrawn as cylindrical tubes. The bud of the maxillarypalpuselongatesslowlyat
first and the labial bud has a still more tardy evagination. This is
probably to allow time for the retraction of the transversefold
formed by the hypodermisof the transverseline, mental sclerite,
andthefrontwallof the pocketof •he bud(pl. 15,figs.34, 36,
œo•) .
The mouthpartsof the pupa are closelypressed together (pl.
14, fig. 24) and form the central part of an ovoid shield which
coversthe front and sidesof the body. The antennae,legs,wings,
and halteressuccessively
build the lateral parts of this shield. The
shieldis in closecontactwith the body along its bordersposteriorly
and ventrally, and above in front the centralpart is fused to the
PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY.
thorax. Belowin front and generallyalongthe sidesof the body
an irregular spaceis left betweenbodyand shield. This spaceis
filledwith air (Hurst)andinto it the recurredapicesof the mouthpartsandthe foldedlegsproject.
When the dermisshrinksawayfrom the cuticlea few hoursafter
pupation,the varioustrophi becomeoval or cylindricaltubesand
remodelingof the imaginalformsis possible. The mandibles,
maxillae, and maxillary palpi--these last occupythe samecuticular
sheathas the maxillae--undergothe leastmodification.In male
pupaethe mandibularsheathwas emptyby the endof the first day
of pupal life. During a variableperiodof not lessthan ten hours,
the
labrum
remains
circular
in section.
Then
its ventral
face
becomesinfolded, and through differentiationof the fro'row,the
probosciscanalor so called epipharynxis formed(pl. 14, fig. 24,
_pc). The lablure retains a cylindric or oval crosssectionuntil
near the end of the first day of pupal life. Then the dermis
of the mid-dorsal
face is either elevatedasa low crestor proliferatesand thickens(pl. 14, fig. 24, hyp). In bothsexescellsappear
in the cavityof the lablurebeneaththe alteredwall and ultimately
build the salivarygutter. Thesecellsprobablyariseby invagination or migrationfrom the modifiedarea above. After they appear,
the labium of the male pupa losesthe roesialdifferentiationand
quickly mouldsitself to the imaginal form, but with tile female
pupa the ridge or thickeningfinallyseparates
from the labiumas the
hypopharynx(pl. 14, fig. 25). I did not succeedin obtainingsections which showedthe actual separation. After disassociation,
both lablure and hypopharynxquickly attain the imaginal structure.
The,/bre gt•t.-- Sincethe mouthpa•'tsof the pupa are in close
contactwith one another,it cannotbe saidthat the alimentarycanal
is lengthenedby the additionof a proboscis
canal so long as the
der,nislies againsttile cuticle of the stylets, and althoughrepresented by the intracuticular spacesbetween labfinn and lablure
after their dermis has parted from the cuticle,yet the canalis not
actuallyformeduntil the ventral wall of the labrum is invaginated.
If a new-formedpupa be examined,the fore gut will be found
empty and clean. The buccalcavity is closedby the collapseof
its walls, except at the poste,'iorend, beneaththe shallow dorsal
diverticulawhich representthe fiabellaeand epipharynx(pl. 15,
THOMPSON: ANATOMY OF MOSQUITO.
183
fig. 36, epi). Structurelessddbrisfrom the strandsof protoplasm
that formed the coresof the epipharyngealand fiabellal setaefills
the divm'ticulaand spreadsinto the buccal space. In front of the
curve where the potentialprobosciscanal entersthe buccalcavity,
the salivary duct opens. The pharynx retains somethingof the
form which characterizedit in the larva and the less markedly columnar epitheliu•nas contrastedwith the epitheliumof the buccal
cavity. The roof hasevidentlyshortened,sincethe dorsalpharyngem musclesare crowdedtoward the elevatorsof the palate, but
this and other changesin the relation of organs cannot be •accurarely formulated,owing to the lack of fixed pointsfrom which
to measure.
After the third hour of pupal life (pl. 17, fig. 56) the lumenof
the buccalcavity opensagain. It is soonfreed from detritusand
remainsclean during the rest of the metamorphosis. The fiabellal
pitssteadilydiminishin sizeand finallyfadeout. The cellsof the
epipharyngeal
pit histolyzeand beforethe tenth houris passedhave
dissolved
into
a mass of broken
elements.
This
lies
within
the
davity of the head, completelycut off from the buccalcavity (pl. 17,
fig. 57, epi), andis slowlyabsorbed
withoutthe interventionof pha-
gocytes. Seeminglythe separation
of the degeneratingepithelial
pouchfrownthe buccalcavity (pl. 16, fig. 39) is accomplished
by an
inpushing
of theadjacent
epithelium
ratherthanbya proliferation
of cellsat the margin of the pit. Mitotic figuresare absent. It is
not impossible,however,that someof the epitheliumwhich closes
beneaththe degeneratingcellsand cutsthem off from the lumenof
the buccalcavity is derived mediarelyor immediatelyfrmn the epithelium of the epipharynx as it is destroyed. If this is so, and
the epipharynxof the mosquitowriggleris a true epipharynx,part
of the anteriorhard palate of the imagois epipharyngealin origin.
And in sucha casethe small conicalsetaenear its apex mightgain
a morphologicalvalue as epipharyngealstructures This will not
affectthe characterof the labrumas a simpleand not a compound
organ.
While the changesdescribedhave beentaking place,the pharynx
has becomecylindrical and the cells of its epitheliumnow exactly
resemblethe cellsof the epitheliumof the buccalcavity. It is not
clear how this alteration is accomplished. No nfitotic figurescan
be found and no waste is given off into the lumen of the gut. The
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loss of histologicaland structuraldistinctionsbetween the buccal
cavity and pharynx leaves as landmarksfor the entranceto the
pharynx(pl. 17, fig. 56, •) only the cingulummusclebelowand the
anteriormostdorsalpharyngealmuscleabove. The dorsal mark is
opento the objectionthat this musclelies behindthe actual bound-
ary; and the relationsin this neighborhood
are obscured
alsoby
the retractionof the fiabellalhypodermis
backto the epipharynx-whichdoesnot seemto havemovedmuch- andby the shortening
of the roof of the pharynx. These criteriaenableus to judge
the morphologicalvalue of any comparison
betweenthe limits of
the larval and imaginal fore gut regions. Nevertheless,
it will be
interestingandperhapsnot unprofitable
to worktheseout (pl. 17,
fig. 56-58). Roughly speaking,that part of the pupal buccopharynxwhich is in front of the anteriormost
dorsalpharyngeal
muscleand the cingulummusclerepresents
the larval buccalcavity.
Ventrally, the boundaryearlybecomesindistinguishable
owing to
the lossof the cingulummuscle,but dorsallya slight bendcomes
in near the point where the two regionsmeet (•). This bend
coincideswith the rear border of the soft palate of the imago.
Pharyngeo-esophageal
boundariesare not developedin the larva
ventrally,but dorsallythe insertionof the retractorsof the pharynx
(retr•p]•)
markstherearof thepharynx.Thesemuscles
metamorphoseinto the ascending
pharyngealmuscles
of the imago(ascph).
The anteriordilatormuscles
immediatelycaudadbecomethe anterior
dorsaldilatormuscles
of the antliaof the imago. The epipharyngeal
pit lies at first above and then forward of the marked curvein the
gut. This curveremainsafter the epipharynxand its muscleshave
vanishedand the front wall of the clypeusformsdistadof it. So,
unlessit shifts positionmarkedly,for which there is no evidence,
the epipharyngealmusclesof the imagoinsert on the roof of the
buccalcavityvery nearthe pointoccupiedby the larval epipharynx.
The esophagus
undergoesno metamorphosis
beyond the destruction and regenerationof the muscle-coats,
the differentiationof the
antlia at its anteriorend (pl. 17, fig. 57), and certainchangeswhich
involve the esophageal
valve. The esophageal
epitheliumis handed
intact to the imago,probablywith an increasein the number of the
componentcells. The muscle-coats
histolyzeabout the eighth hour
and are replaced somewhat later by the imaginal muscles. The
processcouldnot be studied in detail becauseof the minutesize of
THOMPSON:ANATOMY OF MOSQUITO.
185
the new elements. The antlia graduallyforms during the first half
of the secondday of pupal life, but in commonwith other partsof
the alimentarycanalthe characteristic
imaginal intima with its division into platesis not developeduntil just before the fly emerges.
This intima is secretedbeneath the delicateundifferentiatedpupal
intima which lines the stomodaeum. The proctodaeumsimilarly
has a pupal intima beneathwhich the imaginalintima is secreted.
The lu•nen of the esophagus
remainscleanduring the metamorphosis,except at the posteriorend, xvheregranularrubbishcontaining
chromatinis found,seeminglydrifted in from the cardia.
All the musclesof the head continue intact until the eighth
or tenth hour and then histolysisoccurs. But although small
leucocyte-likecells can occasionallybe noted, these are rare and
there is nothingwhichdefinitelyassigns
a phagocyticrdle to them.
The mandibularmuscles,the depressorsof the antennae,the musclesof the •naxilla,the epipharyngeals,
the retractorsof the fiabellae, the diagonals, the dorsal pharyngeal muscles,the ventral
retractors of the pharynx, and the cingulum histolyze ai•d are
absorbedbetweenthe tenth and thirtieth hour of pupallife. The
first imaginalmusclesappearabout the eighth hour xvithtwo bands
of small dark myoblasts,
probablythe maxillo-labialmuscles. ])y
the seventeenth
hour the sidesand floorof the headcavityare trav-
ersedby similarbelts of small myoblasts. The ultimate sourceof
the new cellsis uncertain,owing to the minute sizeof the elements
of both the larval and the imaginalmuscles. From the moment
of their appearance
the new nuclei are closelyassociated
•vith the
dermisof the head and their cellsresemblethe small,dark cells of
this epithelium. There is, however,nothingwhich would militate
againstapplyingvan Rees' ('88) interpretationof the musclemetamorphosisof Muscato Culex. By this the new myoblastswould
1)e derived from the nuclei of the larval muscles.
The retractorsof the pharynx,the elevatorsof the dorsalplate,
the lateral muscles,and the anterior,posterior,and lateral dilators
of the esophagus
are worked over into imaginalmuscles. The
anterioresophageal
dilatorsalterfirstand theirmetamorphosis
may
be taken as typical. A majority of the large reticulatenuclei
becomesmaller,darken,and losethe reticulum;othershistolyze.
Then a largenumberof small dark nucleiappear,perhapsderived
from the older musclenuclei by amitoticdivision. The two setsof
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nuclei,older and younger,remain side by side until the twentieth
hour is reached,when the contractile substance,hitherto hostileto
haematoxylin,stainsreadily with it and all the nuclei are larger
and reticulate--tbe imaginalcondition. The final increasein the
amount of the contractilesubstanceand the completeattainment
of the imaginalstainingreactionare soonreached. The relationof
the larval to the imaginalmusclesis as follows:-Larval Series.
Retractors of the pharynx.
Lateral musclesof pharynx.
Elevators of the dorsal plate.
Anterior dilators of the esophagus.
Posterior dilators of the esophagus.
Lateral dilators of the esophagus.
Imaginal Series.
Ascending pharyngeals.
Lateral pharyngeals.
Muscles
Anterior
Posterior
Lateral
of the valve.
dorsal dilators
dorsal
dilators
of antlia.
dilators
of antlia.
of antlia.
Probablyonly the posteriorportionof the musclecomplexthat
[ have called the lateral dilator of the esophagus,
givesriseto the
lateral dilator musclesof the imago'santlia, the anterior portion
being histolyzed. The hypopharyngeal
musclesof the adult are
derivedfromthe fibersbeneaththe hypopharyngeal
sclerite. Those
musclesdorsad of the alimentarycanal which have forerunners
amongthe larval muscles
appearfrom the twenty-fifthhouronward
and the clypeusbeginsto differentiate
at thistime (pl. 17, fig. 58).
The esophageal
valveof the new-formedpupais partlydrawnout
of the cardia,sothat the annulusof regenerative
cells(a•) liesin
line with the walls of the cardia and the upperbend of the valve
has beenshiftedto a point low down on the reflectedface. The
regenerative
annulusalsohaswidened,
probably
throughmodification of adjacentcellsof the epithelium
of the valve. In a pupafive
hoursold (pl. 16, fig. 38) the valve is completely
withdrawnfrom
the cardia. The regenerative
annulusis very prominentand the
annularmuscleof the valve(an• m) has begunto degenerate
in
advanceof the histolysis
of the muscle-coats
generally. An hour
later,theregenerative
annulus
is represented
bya lowcircularridge ß
of columnarcellswhichprojectsinto the lumenof the esophagus
justabovethe cardia. The valveliesimmediately
cephalad
as a
low ri,g-fold of the wall of the gut, girdledexternallyby the
THOMPSON: ANATOMY OF MOSQUITO.
187
remainsof the annularmuscles.By the tenthhour(pl. 16, fig. 43)
the partsare hardlyrecognizable;
for the valve (v) is flattened
andalmostobliteratedwhilethe cellsof the annulus(ann) have
largelylost their characteristic
histologicalstructureand resemble
cells of the adjacent esophageal
epithelium. The eleventh-hour
pupahasalmostthe imaginalrelations(pl. 16,fig. 40). The regenerativering and larval valve havedisappeared,
the epithelium
is
everywhereof esophageal
type and near the cardia the walls are
foldinginwardto buildthe imaginalesophageal
valve. The rapidly
diminishingbelt of d4briswhich remainsfrom the degenerated
annular muscleslies cephaladof the new valve. The infolded
regionclearlyis epithelium
that at onetime formedpart of the
regenerative annulus.
At aboutthe tenth or eleventhhourthe esophageal
diverticula
pushoutjustcaudad
of the histolyzed
annularmuscles.Theyprobablydo notinvolvecellsthat haveformedpart of the regenerative
annulus,but arise from esophageal
epitheliumwhich has not been
altered(pl. 16,figs.40, 43,f •'es). Theout-pushings
quicklyenlarge
to smallpouches,
the ventralone openingwidelyinto the lumenof
the gut, the dorso-lateraloneswith narrow stalks. Growth is then
slow ,mtil the thirtieth hour of pupal existence,after which the
pouches
rapidlyattainthe respectiveimaginalproportions.The
epitheliumremainslike that of the esophagus,
columnar,till the
veryend of pupallife, whenthe characteristic
imaginalepithelium
is developed,
seemingly
by a flatteningof the cells,accompanied
by
a foldingand wrinklingof the walls. I am not certainwhether
thesepouches
secretea pupalas well asan imaginalintima. When
the mosquitoleavesthe pupacasethe sacsare empty,but air
bubbles
appearshortlyafteremergence.
It is not possible
to escape
the feelingthat a relationexistsbetweenthe formationin Culex of the annulusof regenerativecells
closeto the cardiaand the developmentof the "anteriorimaginal
ring" at the samepoint during the •netamorphosis
of otherflies.
Such a ring of regenerativecells has been describedfor Chironomus, Anthyomyia,Strafiomya,Tanypus,Gastrophilus,and Musca
(Vaney, :02). Its fate has been determinedfor Gastrophilusand
Musca. In the formerthe wholestomodaeum
is destroyedand is
replaced"entirely by proliferation of cells derived from the buccal
discsand the interiorimaginalring" (Vaney,:02). With Musca
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the stomodaeal
epithelium
is partiallydestroyed
at leastandisrebuilt
from scatteredregenerativecells,as well as from the imaginalring
(van Rees, '88). In Culex the esophagealepitheliumseemsto
sufferno lossof cellsand the regenerativeannulusgivesriseto the
imaginalesophageal
valve by simpleremodeling.
The salivaryglandsfrequentlybeginto degeneratebeforepupation occursand the destructiveprocesses
advancerapidly during
the first hours of pupal life. The cytoplasmof the epithelialcells
become's
more and more spongy,large vacuolesappear here and
there, and the nuclei decreasein size. Meanwhile the small re-
generativecells at the neckof the glandare steadilyincreasing
in
number. By the twentiethhour the glandshave shiftedto a more
ventral positionand the regenerativeepitheliumstainsdeeplywith
haematoxylin,a characteristic
of regenerativeepitheliumgenerally.
The nuclei of the glandcells are now still morediminishedin size
and the vacuolizatiof•is extensive,especiallynear the belt of regenerative cells. Frequently cells are so distendedby vacuolesthat
they obliteratethe lumen of the gland. About this time the
imaginalsalivaryglandsappearas threesmall cylindersdeveloped
front the regenerativeepithelium. As soon as these form, the
larval glands histolyze completely,becomea massof granular
detritusfilled with fragmentsof chromatinand are absorbed
without phagocytosis
in five or six hours. 'In their placeson either
side of the thorax lie the three slendertubes of the imaginal
glands. Just beforethe mosquitoleaves
the pupa casett•e glands
enlarge,shift to the adult positionbeneaththe alimentarycanal,
and the central
and lateral
acini become unlike.
Whether
the
final increase in size is due to increase in the number of compo-
nent cells or only to growthof cellsalreadypresent,could not be
determined. After the emergenceof the fly, the nucleiof the cells
enlargeslightly,andthe cellsbecomedistended
with the storedsalivary secretions
untilthreetimestheir formerdimensions.
The epitheliumof the caraliaand stomachis totally histolyzed
and a new epitheliumis formedfrom regenerative
nucleiwhich
appearamongthe older cells. This courseof development
has
beenobservedfor the midgut of all flieswhosemetamorphosis
has
been studied (Korscheldtand Heider, '99). The larva of Culex
reachespupationwith degenerative
andregenerative
processes
well
established.The cytoplasmof the epithelial cells is vacuolated,
THOMPSON:ANATOMY OF MOSQUITO.
189
producinga spongyappearance,
the striatedborderis dissolving,
and occasionally
cells protrude into the lumen of the canal. Between the basesof the older cellsthe regenerativenuclei occurin
increasingnumbers. With the larvae of Tenebriomolitor (van
Rees,'88) the regenerativenucleiare foundevenin the first instar.
I do not know how early they appearwith the mosquitowriggler.
In the last larval instar and the newly formed pupa they are very
numerous. As these nuclei increasein number,they diminish in
size, which suggeststhat they are derived by repeated divisions
from thosepresentin the larval stage.
The changesadvanceswiftlyafter pupation. In a pupaonehour
old (pl. 14, fig. 28) the epithelial cells are separatingfrom oneanother, are elongateand protrudeinto the lumen,the striatedborder
hasbeen everywherelost, and the cytoplasmis dissolving. By the
third hour of pupal life, the regenerativenuclei, which are now
about the sizeof the epithelialcellsof the gut of the imago--less
than onehalf aslarge asthe regenerativenucleiof thelarva--have
forced the older cells from the basement membrane
and form a deft-
nite pavementepitheliumwith recognizableeytoplasm. The older
cells lie in fusedmassesin the lumen of the gut, their nuclei reduced
to dark granulate spheres. Ilere they are rapidly absorbed(pl. 16,
figs. 40, 43, ca). This metamorphosis
doesnot proceedwith equal
eelerityin all regionsof the midgut. The caeca,the eardia,and the
iliae end of the stomachlag behind the major part of the stomach.
The caecadecrease
in sizeastheir epitheliumhistolyzes,
andby the
fifteenth hour they have vanished. The epitheliumof the eardia
degenerates
first at the posteriorend of the chamber. The new
epithelium,unlike that of the caecaand the stomach,consistsof
columnarcells from the beginningand appem-sto be formed as an
advanceforward beneaththe oldercells(pl. 16, fig. 38, ca), instead
of simultaneously
over all parts of the wall.
At the beginningof pupallife the hind gut regions,ileum, colon,
andrectumarethe sameas in the maturelarva,exceptthat they
are shortenedand that a ring of small dark cells lies between the
feetran and the anus (pl. 17, fig. 54, r an). This ring develops
during pupationand the first hour of pupal life, but whether its
cellsare derived from the cells of the posteriorend of the rectum
or from eelIsof the hypodermisin the region of the anuscould not
be determined. The rectumand ileumare involvedin degenerative
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and regenerativeprocesseswhich run the samecoursein all parts
of both regions,but could only be studiedin detail for the ileum.
The metamorphosis
of the colonis very peculiar.
Accordingto Hurst ('96) the epitheliumof the intestine"divides
into a thin outer and a thick inner layer. The latter becomesloose,ed, breaksup and appearsto be digested." The processfor the
speciesof Culexthat I studiedis not as simpleas this wouldimply.
The flattened epithelium first becomescolumnar. Then the cells
degenerate,separatingmore or ]essfrom one another. Large vacuoles make their appearance,
the cytoplasmbecomesspongy,and a
smallamountof d4bris,in whichgranulesof chromaticmaterialare
noticeable,is given off into the lumen of the canal. In all regions
considerablemitosiswas observedtoward the end of the changes.
The processes
begin at the upper end of the ileum, run their course
here in aboutsix hoursand then involve simultaneously
the rest of
the ileum and the rectum,lastingeightor ten hours. They result
in the formationof an epitheliumof small,dark-staining
cells,somewhat like the cellsof the new anal ring. These graduallyalter to
the adult charactersfor each region. Enlargementand probably
proliferation of cells in certain parts of the rectal walls form the
rectal papillae (pl. 17, fig. 53, p). The muscle-coats
are lost about
the seventeenth
hour and are later replacedby new muscles.
It is evidentthat in the metamorphosis
of the ileum and rectum
cel•tainof the epithelialcells are eliminated and new cells,derived
from epitheliumwhichescapes
destruction,
replacethem. Probably
the degenerative
process
involvesall the older'cellsto a greateror
lessdegree. The nucleiof the reconstructed
epitheliumare barely
one half the size of the nuclei of the older cells.
The nature of the
preliminaryalteration by which the epitheliumbecomescolumnar
is obscure. It doesnot seem,however,to dependon anycontraction of the walls of the gut.
The alterations
recorded above are not without
relation to the
postembryonic
developmentof the hind gut in suchfliesasMusca
or Gastrophilus.In the lattergenus,the hindgut is destroyedand
is reconstructed
from au anteriorimaginalring of regenerative
cells
near the Malpighian tubulesand a posterior'ring near the anus
(Vaney,: 02). The hind gut of 5[uscaevidentlyundergoes
a similar metamorphosis,
but observersdo not agree as to the amountof
epitheliumdestroyed. Kowalevsky ('87) assertsthat all the older
.THOMPSON: ANATOMY OF MOSQUITO.
191
cells are swept away while van Rees ('88) believesthat a considerablepart of the older wall remains. In Culex the appearanceof the
regenerativechangesin the neighborhood
of the Malpighiantubules
before the remainder of the gut is involved, and the formation of
an anal ring of new cells--whatever their ultimate source--furnishinterestingparallelsto the conditionsfoundin Muscaand Gastrophilus. The resemblancesof course mean no more than the
presentationin varied degree of certain tendencieswhich exist
amongthe Diptera. A closergeneticconnection
is not implied.
As an illustration of the improbability of such a connectionwe
may take the changeswhich occur in the colon of Culex. The
developmentof this region recalls the reconstruction
of the hind
gut in the morespeci,•lized
flies from anlagenat either end of the
canal. But the processesare not really comparablewith those
found in Musca or Gastrophilusand when we considerthe simple
metamorphosisof the remaining hind gut there is marked and
peculiar specialization. The colon of Culex is destroyed and
replacedby a tube formed from a backwardgrowth of the cells
of the ileum and a forward growth o• the cells of the rectum.
The proliferationbeginsas soonas the iliac and rectal epiCbella
are reconstructed, i.e., about the seventeenth hour.
At this time
the cells of the shrunken colon are spongy,and have separated
from one another.
Their
nuclei
have diminished
in size but are
intact (pl. 17, fig. 52, co). The invadingcellsenterthe lumen (pl.
17, fig. 59), and active mitosisis observableat and near the head of
either advance. The two cell armiesseemto progresswith equal
speed. As soonas the advanceis well under way, the nucleiof the
colon cells lose their reticular structure
chromatic material within the nuclear
and reduce to masses of
membranes.
Nuclei and
cells,however,remain distinctuntil individuallydisplacedby the
invading cells, and then quickly histolyze. In a nineteen-hour
pupa, cells already displacedlay outsideof the •ew epitheliumas
a massof f•'agments,while cells still in positio• remained •listinct
units. By the twentiethhour of 1)upalli•'e (1)l. 17, fig. 53) the
new canal•ilc) is completedand the old colon(co) is represented
by d•b•'is. This is q•ickly absorbedwithout the interventionof
phagocytes. The musclesof the larval colon histolvze •', sit•e.
The musclesof the new ileo-colonare developeelfrom myoblasts,
probablyfrom the ileum and rectum.
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The Malpighiantubulespassfrom the larva to the imagowithout
visible change. This absenceof metamorphosisappearsto be the
rule for thesetubulesin the NematocerousDiptera, if observations
on Simulium,Chironomus,and Psychoda(Vaney, :0•-) added to
thosefor Culex, give a sufficientbasisfor generalization.
•Y-otes
o• the metamorphosis.- The organsof Culex outsideof
the alimentarycanalundergoa metamorphosis
assimplein gradeas
that of the mouthparts or alimentary canal. A few pointswith
respectto these organsmay be noted. The dermisof the head
and the body is passeddirectly over to the imago. The musclesof
the thorax and abdomenalso are seemingly
not involvedin any
alterations. The wing and leg musclesare new with the pupa,the
rudimentsof the former beingl)resentbeneaththe hypodermis
of
the dorsumof the thoraxduringthe last larval instar. My observations on the fat body accordwith thoseof Vaney. "In the
lower Diptera (Culex, Simulia,Chironomus)the fat elementsare
maintainedin their integrity from the larva to the adult." In the
fat bodydorsadof the nerve cord in the abdomen,there can be
noted during the pupal stagescattered cellswhich resembleleuco-
cytesmore than other cellsof the body. These do not, however,
appearto functionin any manner.
The eyes of the larva formpart of the compoundeyesof the
perfectinsect. Hurst ('90) hasgiven a good accountof the evolution of the imaginal eye, which "consistsin the additionof new
elementsat the edge[of the larval eyeI whichariseby directmodificationof the .... epidermis
aroundthe marginof the eye,epidermis whoselast functionwas to secretethe pupal cuticle." He notes
that "cornealfacetsare never formedin the pupalcuticle"and that
the ocellusof the larva is inconspicuous
in the adult. I find that this
ocellusdegenerates
duringthe pupastage. The cells becomevacuolated, the pigment agglomerates,and the whole structuresinks
beneaththe dermis. In the perfectinsecteachocellusis hardly
more than a small massof black pigmentat the rear of the compoundeye.
Both CulexandAnopheleshavea pair of rudimentaryocelliwhich
do not seem to have been described. They are situated on the
vertex of the head, caudad and roesad of the basesof the antennae.
Eachconsists
of a plateof enlarged
derrealcells,lyingimmediately
over a smallganglionicmassof spindle-shaped
cells. A fine nerve
THOMPSON: ANATOMY OF MOSQUITO.
193
connectsthis ganglionxvith the supra-esophageal
ganglion. The
overlying cuticle is not modified (pl. 12, fig. 2; pl. 13, fig. 8, oc).
These ocelli are not presentin the larva, but developduring the
pupationmoult. Probably they representin a vestigial form the
lateral members of the three ocelli found on the head of many
Nematocerous flies. If this is the case, no trace of the median
member
can be found.
During the pupal stagethe nervous system undergoesgradual
changeswhich are growth rather than a metamorphosis,
increasing
greatly in size and probably also in complexity. The brain of the
imagois relatively enormousand there are two huge opticganglia
united above to the supra-esophageal
ganglion (pl. 13, fig. 9-11,
o gang). A smallbuccalganglionlies abovethe pharynx,between
the last pair of elevatorsof the palate and the valvular muscles.
The larva,onthe other hand,hasa small,transverse
supra-esophageal
ganglionand an insignificantinfra-esophageal
ganglion(pl. 15, figs.
29, 30). The swollen lateral apicesof the supra-esophageal
mass
correspondto the optic ganglia of the imago (o ga,g). The buccal
ganglionis very small(pl. 15, fig. 32, b ga•,g). The thoracicganglia
of the imago are united to form a single elongatemass. In the
abdomen six ganglia are distinct and occupythe joints from the
secondto the seventhinclusive. In the larvae,however,(pl. 14,
fig. 26) there arethreeseparate
thoracicgangliaand the firsteight
abdominaljointsare furnishedwith a ganglionapiece. During the
later larval instars,the gangliaof the first andeighthjointsof the
abdomenshift to the anteriorbordersof their respectivesegments,
and beforepupationoccurs,the ganglionof the first joint entersthe
thorax(pl. 17, fig. 54). La.ter, it fuseswith the thoracicganglia
to formthe thoracicganglionmass. Somehoursafter pupation,the
ganglion of the eighth abdominalsegmentpassesinto the seventh
joint (pl. 17, fig.55) andultimatelyfuseswith the ganglionalready
presentin the segmentto form the sixthabdominalganglionof the
imago.
The imaginalbudssharein the simplicityof the metamorphosis.
With the exceptionof those for the antennae,noneare deeply
invaginated,and as far as the mandibles,maxillae,and the tail fans
of the pupa are concerned, the formative dermis does not alter
beyondthe thickeningwhichis the invariablepreliminaryto the
formationof an imaginalbud. The labium,maxillarypalpi, tho-
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racic limbs,respiratorytrumpets,wings,halteres,andgonapophyses
are developedfrom shallowinvaginatebuds. The labrumis formed
from an evaginatebud. The thoracicand antennalbudsare evagihated immediatelyprior to the pupationmoult (pl. 16,fig. 48), the
labial and maxillary palp buds after the larval cuticleis partly
removed. The buds for the gonapophyses
are situatedon the ventral face of the ninth joint of the abdomen. The patchesof altered
hypodermiswhich representin a rudimentaryform budsfor the tail
fans of the pupaare formed low.down on the sidesof the eighth
segmentof the abdomen. In the speciesof mosquitostudiedby
Hurst these buds are describedas "plate-like bodieslying immediately beneaththe cuticle of the larval siphon." They lie high
up on the sidesof the segmentiu the larva of Anopheles,and in
Corethraare ahnostdorsalin position. Thesedifferencesare prob-
ably correlatedwith the variedsizeand shapeof the eighthabdonfihal segmentin the threegenera: wideandunincumbered
in Corethra,
narrower in Anopheleswith a small dorsalrespiratorysiphon,and
very narrowin Culexwith a hugedorsalsiplion. The tail fan buds
are the last to appearin the Culex larva. The orderfor the developmentof the imaginalbudsis: antennalandthoracic;labialand
maxillary palp; labral; gonapophysial;
mandibulara•d maxillary;
tail fan.
The antennal buds are of especialb!terest. They are deeply
invaginatedinto the head cavity,and when completedthe baseof
the new antennaat the bottomof the invaginationpocketliesbehind
the eye spot (pl. 15, fig. 29, ant bd). Miall and Itammond ('92)
comparethe type of antennaland eye developmentpresentedby
Culex,wherethe eyesare maintainedindependently
of the antennal
invaginations,
with that found in Chironomusor Sinmliumwhere
invaginations
give riseto both antennaandeye,andfinallywith the
extensive invaginationsof Musca from which eye, antenna,and
the whole head are formed: the so called "brain appendages."
On a basisof this comparisonhe placesCulex at the bottom of a
series,Simuliunfand Chironomushigher, and Musca at the summit.
This arrangementis perhapsjustified as a statementof progressive
elaborationof a tendencyto form eye and head from an invagination
that
is connected with
the antennal lind.
But it does not
sufficientlyacce•tuatethe mechanical
aspectsof the phenomenon.
Kellogg (:o2) appearsto me to expressthe casebetter whenhe
THOMPSON:
ANATOMY
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195
says: "Whether an organ, as wing, leg, antenna,or mouthpart,
shall begin as an invagination or an evaginationof the derm is
chiefly a matter of mechanicalnecessityor easeand of the radicalnessof the metamorphosis."The "brain appendages"of Musca,
the deep cephalicpouchesof Chironomus,
the shallowcephalic
pouchesof Simulium,and the independenteye and antennalformation of Culex do not constitutea closelylocked series. The
characterand positionof the invaginationdiffer in eachinstance.
The generacan be comparedonly with respectto the radicalness
of the metamorphosis
in eachcase:by whichvariedmodifications
of tendenciesthat existthroughoutthe Diptera are presented. That
mechanicalnecessities
are more potent than relationshipin determining the characterof a budis well shown•
by a comparison
of the
antennal'budsof Culex, Anopheles,and Corethru. The larva of the
first germshas a broad,capacious
head, but the head of the larva
of Anophelesis laterally compressed,
and that of the Corethrularva
is markedlyflattenedfrom sideto side. The antennalbudsin Culex
are thrust back into the cavityof the head. In the other genera
the antennal buds are thrust back in deeplongitudinalfurrows on
either sideof the head,and the invaginationpocketis narrowly
openfor its entirelength.
SUMMARY.
The alimentarycanalof the mosquitohasa midgutand hind gut
which are not subdividedinto numerousor well definedregions,but
the fore gut exhibitsthe maximumamountof differentiation
that has
beenfound amongthe flies. It may be dividedinto proboscis
canal,
pharynx,antlia,andesophagus.The pharynxandantliaarepumping organs. Their suctorialactiondependson the modification
of
the chitinous
intimato formdefiniteareasor "plates,"which spring
inwardand occludethe lumen,to be withdrawnagain1)ymuscles.
This duplicationof the pumpingapparatusplacesthe mosquitoes
in
the diantlial classof flies,a group which probablyincludesall the
Nematocera. The Muscidaeand others of the Cyclorrhaphaare
monantlial,having only the anterioror pharyngealpump. There is
no secondary
union of the pharynxwith the wallsof the head,sothat
it is not convertedinto a fulcrum. In this region,as is the case
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with all otherflies,the wallsandfloorare rigid and the dorsalroof
constitutes
the plungerof the pump. The pyriformantlia on the
other hand has walls that are modified to form three racquetshaped plates, one dorsal, two ve•tro-lateral. In the narrower
anteriorendof tile pump, forwardof the circumesophageal
nervering, the walls becomeunyielding,so that the dorsalplate alone
retains
a limited
amount
of motion.
In correlation with the i•ac-
tivity of this part of the pumpthe anteriordorsaldilator muscles
are vestigial. The Anophelesmosquitoes,
which otherwiseclosely
resemblethe membersof the genusCulex in the structureof their
fore gut, have a more cylindrical antlia, the anterior end of which
is muchlessrigid, and the dilator musclesare well developed. A
valve separatesthe pharynx from the antlia and a sphinctermuscle surroundsthe rear of the latter pump as it givesplace to the
esophagus. The esophagusis a thin-walled tube with flattened
epithelium and poorly developedmuscle-coats.The esophageal
valve is small and immediatelyin front of it the three esophagcal
diverticula arise. The tracheal supply to the fore gut is scanty,
but tile
number
of branches
which
run
to
the
muscles
of the
pharynx at•d antliaindicategreatactivityfor both pumps.
Each of the two salivaryglands is composedof three acini, the
middle member being unlike the lateral in the character of its
secretions. The ducts from these glands unite in the rear of the
head and the resultingmedian duct empties into a cup-shaped
chitinouspump at the baseof the hypopharynx. This pumpforces
the saliva along the salivarygutter of the hypopharynxinto the
wound.
The two regionsit•to which the midgut may be divided,the
cardia and the stomach,are not sharplydistinguishedfrom each
other. Their epithelium is low a•d columnar,the muscle-coats
are represented
by slenderlongitudinaland circularmuscles,
and
caecalpouchesare not developed. This last is a point of difference between Culcx and Anophcles, as the latter genusseemsto
possess
rudiments
of suchdiverticula. The trachealsupplyto both
caraliaand stomach is extensiveand the abdominal portion of the
stomachis capableof great dilatation,servingas a storageplacefor
the meal of bloodwhile it is being digested.
A prominentvalve is developedat the posteriorend of the
st9mach,opposedto a similarvalve at the upperend of the first
THOMPSON: ANATOMY OF MOSQUITO.
197
region of the hind gut, the ileo-colon. Between the two sets of
valvesthe orificesof the five Malpighian tubulesare situated. The
two regions of the hind gut, the ileo-colon and the rectum, are
distinguishedmainly by the greater diameter of the latter and the
six rectal papillaeon its walls. The epitheliumis uniformlyflattened, the musclesare much like those of the stomachor cardia, and
the trachealsupplyis extensive. A thin chitinousintima lines the
hind gut, and is differentiatedto form a hirsutebelt immediately
behindthe valve at the upper end of the ileo-colon.
The regionsto the alimentarycanal of the imago and larva are
noticeablyunlike in severalpoints, and the high degreeof adapt-
ability of the I)ipterous larva generally,has producedwithin the
group markedvariationsin the morphologyof the alimentary tracts
in the young of closelyallied genera. Hence in a sense,the morphologyof the larval and imaginal gut are separateproblems. In
the larva of Culexor Anophelesthe followingregionscanbe distinguished:buccalcavity, pharynx,esophagus,
cardia,stomach,ileum,
colon,andrectum. Of these,the pharynx is not well markedoff
from the esophagusbelow and at the sides,the cardia is more
distinctfrom the stomachthanin the imago,but clearlya differentiation of its anteriorend, and the colon and rectum pass
gradually into one another.
The buccalcavity has columnarepitheliumand an epipharynx
is differentiated
on its dorsalwall. The epithelium
of the pharynx
is flattened. This regionhasthe heaviestintimaof anypart of the
larval gut and is an organof peculiarshapewith anelaboratemusculature. The buccalcavityof the larva corresponds
to the posterior
endof the proboscis
canalandtheanteriortwothirdsof thepharynx
of the imago. The pharynxof the larva forms the posteriorthird
of the pharynxof thefly,whilethe anteriorend of the esophagus
becomes
the antlia. The esophagus
hasflattenedepithelium,
strong
circular,and well developedlongitudinalmuscles. Esophageal
diverticulaare lacking,but the esophageal
valveis morehighly
specialized
than the corresponding
valveof the imago. It is less
modified,however,than the esophageal
valveof the larvaof Anopheles,the alimentarycanalof whichotherwisecloselyresembles
that of Culex. The salivaryglandsof the mosquito
wrigglerare
cylindricaland the salivaryductis undifferentiated
throughout
its
length. It opensfreely on a small area of heavy chitin in the
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floor of the buccal cavity, the hypopharyngealsclerite. This
structureis not holnologouswith the hypopharynxof the imago.
The epitheliumof the cardia and stomachis very unlike that of
the sameregionswith the perfectinsect,being composedof large
polygonal cells which have a striated border and large nuclei.
Eight cylindricalcaecaprotrudein a circle just behindthe cardia
and there is no valve at the posteriorend of the stomachnor at the
cephalicend of the ilemn. The muscle-coats
are weak.
Of the threeregionsof the hind gut,the ileum hasan extremely
thin epitheliumandwell developedcircularismuscles,the colonan
epitheliumof huge polygonalcells and slenderbelts of circular
musclesat intervals,while the rectumhas a very thin epithelium
indistinguishable
from that of the colonand similarbelts of circular muscles. The hind gut is lined by a strong chitinousintima.
The metamorphosis
of the mosquitoes
as shownby Culex is of
low grade comparedto that of other flies. No phagoc•'tosis
has
beenobserved. The fat body,the Malpighiantubules,the ventral
thoracicmuscles,
and the musclesof the abdomenseemingly
are
passedon intactto the imago. The eyesof the larva are the eyes
of the pupaandareelaborated
to formpart of the compound
eyesof
the adult, the remainderof these organs being developedfrom
the surroundinghypodermis. The larval ocelli degenerate.The
nervoussystemincreases
greatly in bulk and complexity,two of
the abdominalseriesof ganglia changetheir position,a Johnston's
organ is formed within either antenna,and two vestigial ocelli
appearon the vertex of the head. These changesare rather of the
nature of growth than metamorphosis.The dermis of the head
and bodymouldsitself to the l)upal and imaginal contoursand
secretesthe successive
pupaland imaginalcuticularstructureswithout visiblemitosis or histolysis. The epitheliumof the fore gut
altersgreatly in al)pearance
, the pharyngealand antlial puml,s are
differentiated,the larval esophageal
valve fades away and a new
valve is formedby the out-foldingof the gut walls,andtheesophageal diverticulaare pushedout. Apparentlythe epipharynxalone
undergoeshistolysisduringthesechanges.
No true imaginalbudsare developedfor the mandibles,the maxillae, or the tail fansof the pupa. The hypodermismerelythickens
and stains more deeply. The lat•rumdevelopsas an evaginate
pouch of altered tiypodermison top of the head. The labium,
THOMPSON: ANATOMY OF MOSQUITO.
199
maxillary palpi, thoracic limbs, wings, halteres, and gonapopl•yses
are developedas invaginatebuds. But the invaginations
are very
shallowin most casesand in the deepestof them--those for the
antennae--the
connection
with
the surface
of the head
is a wide
channel. During the pupal stagethe hypodermisof the mid-dorsal
face of the lablure thickensand formsa low ridge, beneathwhich
cellsbuild the salivarygutter. In male pupaethe ridge disappears
as soonas the salivarygutter is formed. In female pupae it separatesfrom the lablure and becomesthe hypopharynx.
The longitudinaland circularmusclesof the walls of the alimentary canaland most of the head musclesare completelyhistolyzed.
It seemslikely, however,that the head musclesand the musclesof
the walls of the gut in the perfectinsectare derivedfrom myoblasts
which
are descendants
of nuclei of the
older
muscles.
A
few
of
the musclesin the head undergoa partial degenerationonly and
in a sensegive rise directlyto imaginal bands. The musclesof the
wings and legs of the imago are new with the pupa stage. The
rudimentsof the former seriesappear during the last part of the
larval period.
The epitheliumof the midgut, i.e., of cardia,caeca,and stomach,
is sloughedand is replacedby a new epitheliumderived from
scatteredregenerativecells. These appearduring the larval stage,
multiply rapidly,and duringthe early part of the pupal stageform
an epitheliumwhich displacesthe older cells. The latter fall into
thelumenandareabsorbed.The larvalcolonis completely
histolyzed and a canal which grows into its lumen by proliferationof
cellsof both ileum and rectumreplacesit. Prior to this growth,the
el,itheliumof the ileum undergoesa partial degeneration. Hence
the posteriorend of the imaginal ileo-colonis a new structure.
The anterior end consistsmainly of remodeledcells of the older
epithelium. Iu the rectmn the anteriorportion has a metamorphosissimilarto that of the ileum. The posterior end is rebuilt from
an anal ring of regenerativecells the originof which is uncertain.
The metamorphosis
of the alimentarycanal of Culex presentspar-
all•lstothepostembryonic
development
ofthemostspecialized
flies.
Thus, a belt of differentiated cells forms near the cardiac end of the
foregut, recallingin its positionthe anteriorimaginalring of Musca,
from which much of the foregut of the imagois derived. In Culex
the ring is remodeledto form the esophageal
valve. The anal ring
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already noted,from which part of the rectum comes,and the fact
that the degenerative
and regenerativeprocesses
in the epitheliumof
the ileum commenceand are completednear the Malpighiantubules
beforethey involvethe restof the hind gut, recall the positions
of
the anterioran(][posteriorimaginalrings in the postgutof many
flies. Even the peculiarmetamorphosis
of the colonmay be comparedwith the formationof the hind gut in the specialized
Cyclorrhapha by a forward growth from the anal and a backward
growth from the Malpighianregenerativerings, whether the older
epitheliumis destroyedcompletely
as in Gastrophilus,
or partly as
in Mnsca. The alterationsof the midgutalsoarepreciselythe same
as thosewhich are foundin all flies,even the mostspecialized.
Moreover,comparisons
may be madefor otherorgansthan those
of the alimentarycanal. For example,on the basisof the antennal buds,a seriesof genera may be formed, beginningwith Culex
where the larval --and pupal--eye is independentof the antennal
bud, through flies where the eyes developfrom the sameinvaginationsas the antennal buds,up to Musca, a speciesin which the
whole head is formed from a pair of large invaginations. But all
sueIt comparisons
must be taken to atean no more than this, that
Culex and the other fliespresentin varying degreeand undervary-
ing conditions,tendenciescommonto the insectgroup of which
they are representatives.
TItOMPSON: ANATOMY OF MOSQUITO.
201
LITERATURE.
Annett, H. E., Dutton, J. E., and Elliot, J. H.
:01. Report of the malaria expedition to Nigeria. Part 2. Liverpool
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Christophers, S. R.
:01. The anatomy and histology of the adult female mosquito. Reports
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Dimmock, G.
'81. The anatomy of the mouth-parts and of the sucking apparatus of
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van Gehuchten, A.
'90. Rechercheshistologiquessur l'appareil digestif de la larve de la
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: 02. A handbook of the gnats or mosquitoes. Edition 2.
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:00. Les moustiques,anatomie et biologi9. Contribution • l'•tude des
Culicid•es et principalement des genres Culex et Anopheles de leur rble
dans la propagation de la malaria et de la filariose et des moyens de
s' en preserver. Mauritius: 8vo, iv -]- 59 pp., 5 pls.
Hurst, C. H.
'90. The pupal stage of Culex. Studies from biol. lab. Owens college,
vol. 2, p. 47-71, pl. 5.
'96. The post-embryonicdevelopmentof a gnat (Culex). Proc.and trans.
Liverpool biol. soc.,vol. 4, p. 170-191,pl. 5.
Kellogg, V. L.
:02. The developmentand homologuesof the mouth parts of insects.
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Korscheld•, E., and Heider, K.
'99. Textbook of the embryologyof invertebrates. English edition;
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Kowalevsky, A.
'8?. Beitr•igezur kenntnis der nachembryonalenentwicklungder Musciden. Zeit. wissensch.zool., vol. 45, p. 542-594, pl. 26-:]0.
Kr/ipelin, K.
'82. Ueberdie mundwerkzeuge
der s•ugenden
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vol, 5, p. 574-576, 3 figs.
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'88. The poison-apparatusof the mosquito. Amer. naturalist, vol. 22, p.
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Meinert, F.
'81.
Fluernes munddele. Trophi Dipterormn.
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6 pls.
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L. C.
'93. Dicranota; a carnivorous Tipulid larva. Trans. ent. soc. London,
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Miall, L. C., and Hammond, A.R.
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Oxford: viii q-196 pp., illus.
Miall, L. C., and Shelforal, R.
'97. The structure andlife-history of Phalacrocera replicata. Trans. ent.
soc. London, 1897, p. 343-361, pl. 8-11.
MiM1, L. C., •nd Walker, N.
'95. The life-history of Pericoma canescerts(Psychodid•e). Trans. ent.
soc. London, 1895, p. 14'1-153, pl. 3-4.
Nuttall, G. F. H., •nd Shipley, A. E.
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'87. Die l•rve yon Culex nemorosus. Arch. fiir n•turgesch., vol. 53, pt.
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van Rees, J.
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'96. Ueber die veritnderungen des darmepithels bei Tenebrio molitor
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:02. Contributions '• l'Stude des larves et des m•tamorphoses des DiptSres. Ann. de l'univ. Lyon, new set., fasc. 9, 178 pp., 3 pls.
Printed October, 1905.
EXPLANATION
OF
PLATES.
All figures, unless otherwise specified, are drawn from camera outlines and
represent Culex.
ABBREVIATIONS.
ann, ring of regenerative cells below
esophagealvalve.
an•t m, annular muscle of esophageal
valve.
ant, antenna.
aai bd, antennal bud.
ani dfl, anterior dorsal dilator muscle
of antlia.
aug hd p, anterior hard palate.
aag m, antennal muscle.
apo, apodemeof maxilla (imago). .
apo, apodeme supporting pharynx
wall (larva).
ascph, ascendingpharyngeal muscle.
bc, bnccal cavity.
b gang, bnccal ganglion.
blks,black spot area of larval head.
bord l, bordering line of larval head.
ca, cardia.
cae, caeca of stomach.
cia, cingulnm muscle.
CO,colon.
coav mad, converger mascle of
mandible.
cres, crescentic sclerite of pharynx
roof.
el pal m, elevator musclesof the palate.
epi, epipharynx.
'
epi m, epipharyngealmuscles.
epi tr, epipharyngeal traverse.
exi ira, external trachea.
ß
fi, fibers passing from esophagusto
shoulder
of cardia.
fi, fiabella.
fold, "fold" in front of labial bud.
f res, esophagealdiverticula.
ga•g, brain.
h tr. traverse to hypopharynx and
pharynx.
hyp,hypopharynx (imago).
hyp, hypopharyngealsclerite (larva).
hyp m, hypopharyngeal muscles.
il, ileum.
ilc, ileo-colon.
i• ant, inner muscle of antenna.
iai ira, internal trachea.
ir fl, insertion inner retractor fiabellae.
la, labrum.
la bd, labral bud.
la m, labral muscle.
lag dil, lateral dilator
muscles of
antlia.
cu, "cushion" at entrance to pharynx.
dep l•, depressor of lablure.
lag dil oes.lateral dilator of esophagus.
lat m, lateral muscles of pharynx
dep maz, depressorof the maxilla.
diag, diagonal mascle.
div mud, divaricator of the mandible.
lat ph, lateral pharyngeal muscles.
of larva.
do reir, double retractor muscle.
less lat, lesser lateral muscles.
li, lablure.
dor dil, posterior dorsal dilator mascle
l• •, labial nerve.
of antlia.
dot ph, dorsal muscles of pharynx.
el d p, elevator musclesof dorsal pla•e.
li tra, labial trachea.
m, circularis muscle of colon (larva).
real g, Malpighian tubules.
max, maxilla.
max-li, maxillo-labial muscle.
max•, maxillary palpus.
maxp bd, bud of maxillary palpus.
mad, mandible.
rand m, mandibular
muscle.
o .qa•g, optic ganglion.
oc, ocellus.
oes, esophagus.
or f/, insertion
outer
retractor
flabellae.
ou ant, outer muscle o• antenna.
pgl, palatum.
pc, probosciscanal.
per, peritrophic membrane.
ph, pharynx.
prot infix, protractor of the maxilla.
p•mp, antlia.
r a•n, ring of new cells near anus.
rec.
rectum.
retrfi, retractor muscles of fiabellae.
retr max, retractor muscle of the
maxilla.
retr ph, retractors of pharynx.
s pump, salivary pmnp.
sca,scythe-shapedthickening on head.
sd, salivary duct.
sg, saliYary glands.
s-o m, subocular muscle.
sph m, sphincter muscle of antlia.
st, stomach or midintestine.
tent, tentorium.
teat m, tentorial
muscle.
tr, mental sclerite ("black triangle ").
tr I. "transverse
line ".
•b, "upper bend" of esophagealvalve.
v, esophagealvalve.
val m, valvular muscle.
vent r ph, ventral
pharynx.
retractors of
THOSIPSON. -- Anatomy of Mosquito.
PLATE
12.
Fig. 1. Pharynx and salivary pump of female mosquito; seenpartly in section and partly ill surface view.
Fig. 2. Semi~diagrammatic
section of head o,f femalemosquito,showing
pharynx, antlia, and muscles. The section is supposedto passnear the
midline above and anteriorly. It cuts acrossthe base of the maxillary
palp (mxp).the "head" of the tentoriumof the left side,and then sweeps
outward to the origin of the lateral dilator muscles (lat dil). The
labium is sectionedalong the midline. Only the proximal ellds Of tile
mouthparts are shown. On the "head" of the tentorimn tile origin of
tile inner (in ant) and outer musclesof the antenna can be noted. A
portion of the retractor of the maxilla (retr max) is removednear tile
origin of the muscle to display the insertions of the protractor of the
maxilla (prot max) and the maxillo-labial muscles (max-li) on the
free end of the maxillary apodeme (apo). The anterior end of the
retractor musclesand apodeme are removed to show the insertion of the
double retractor muscle (do retr) on the articular spin' of the maxilla
and the attachment of the hypopharyngeal muscle (hyp m) to the salivary
pump. The origin end of the hypopharyngeal musclecan be seenon the
pharynx xvall above the double retractor. The brain is sectioned, and
the buccal ganglion (b gang), labial nerve, and one of the rudimentary
ocelli (oc) are indicated. The left member of each pair of the elevators
of the palate on the roof of the pharynx and of the pair of valvular
muscles dorsad to the buccal ganglion at the elltrance to the antlia has
been cut off, leaving the musclesof the right side in place. The labral
musclesill the elypens alsohave been partly removed (lam). Behind the
ascendingpharyngeal muscleswhich bind the posterior endsof the pharynx to the vertex of the head (ascph) part of the vestigial anterior
sal dila[or muscle of the antlia is visible. Below, the lateral pharyngeal
muscle can be seen, passing from tile outer face of tile articular spur
of the pharynx to the upper end of the tentorium. All the •nandibular
muscle, which runs from the "head"
of the tentorium to the inaudible, is
removed except the insertion end (rand m) and below this can be seen
the small maxillary musclewhich extends from the w,•ll of the pharynx
to tile maxilla. The slender te•torial muscle parallels the nerve cord
and
runs
figured.
back
into
the
thorax.
The
minute
subocular
mnscle
is not
TtlOMPSON.•ANATOMY
PLATE
OF MOS0,.UITO.
/
PROC.
BOSTON
SOC.
NAT.
HIST.
HEL|OTYPE CO., nOSTO•.
VOL.
32.
12.
Anatomy of Mosquito.
PLATE
13.
Figures 3-12 are sectionsof the head of a female Culex, eachrepresenting
an actual section schematized; 3-6 and 9-11 are from one specimen, 7-8 from
a second•and 12 is from a third. Figures 13-15 are similar schematizedsections of the head of a female Anopheles. The figures are drawn to vf•rious
scales.
The median
trachiole
of the internal
trachea
is starred.
Fig. 3. Through basesof mandible and maxilla.
Fig. 4. Insertion of mandibular muscles; secondsection behind fig. 1.
Fig. 5. Salivary pump and insertion of •naxillo-labial muscles; 9th section
beyondfig. 1. Sections10 • in thickness.
Fig. 6. "Heads" of the tentoria and origins of mandibular muscles; 13th
section beyond fig. 1.
Fig. 7. Through posterior spurs of pharynx and the pharyngeo-esoph•geal
valve; specimen B.
Fig. 8. Anterior border of optic ganglion; specimenB.
Fig. 9. Midbrain section of specimen A.
Fig. 10. Origins of retractor maxillae muscles; 39th section beyond fig. 1.
Fig. 11. Through posterior ends of tentoria and insertions of protractors of
the maxillae; 41st section beyond fig. 1. Specimen A.
Fig. 12. Region of bulbousportion of antlia; specimenC.
Fig. 13. Section of head of Anopheles; comparable in position to fig. 5.
Fig. 14. Section of head of Anopheles; comparable in position to fig. 11.
Fig. 15. Section of head of Anopheles; comparable in position to fig. 7.
TIIOMPSON.--ANAT()M¾OF MOSQUITO.
PLATE
13.
14 't•n•m '
]am
•/d
ral•
o
o
•ax.lt ]3
ma•,
tt•"
max
15
0.•anq
Pe•rmaX
1o
S.O1it
max.It 6
1)llOC.BOSTONSoc. NaT. IhsT.
VOL. 32.
TItOMPSON.- Anatomy of Mosquito.
PLATE
14.
Figures16-19 representvarioustypesof pumping apparatusin the Diptera.
Fig. 16. Sectionof head of Anthyomyia; monantlial type.
Fig. 17. Sectionof headof female mosquito; diantlial and post-neuraltype.
Fig. 18. Sectionof head of Tipulid fly; diantlial and amphineural type.
Fig.'19. Headof Tabanusatrat•s; diantlial andpreneuraltype. (Free hand.)
Fig. 20. Alimentary canal of female mosquitowith outline of body. (Free
hand.)
Fi•g.21. Longitudinal section through stomachand ileo-colonof female
mosquito.
Fig. 22. Rectum of female mosquito.
Fig'. 23. Salivarygland of a newly emergedimagoof Culex$timula•8. cc••--•
central
acinus.
ß
Fig. 24. Mouthpartsof a female pupa in which the hypopharynx(hyp) and
probosciscanal (pc) are forming; transversesection.
Fig. 25. Hypopharynxand labium soonafter the formeris separatedfrom
the latter.
Female pupa, transverse section.
Fi•. 26. Alimentary canal of mosquitolarva with outline of body. (Free
hand.)
oe$-- esophagealvalve.
Fig. 27. Transversesectionof wail of colonof a mosquitolarva.
Fig. 28. Degeneratingmidintestinalepithelium of a pupa one hour old.
re9 -- reg'enerativenuclei.
TiiO•tI'SON.--ANATO•IY
14.
oF MosQI•lTO.
Pl/oc.
BOSTON •qOC. NAT.
11mT.
Vo•,..•2.
--Anatomy
ot Mosquito.
PLATE
15.
Fig. 29. Semidiagrammatic view from the ventral aspect of the head of a
mosquito larva; picro-carmine preparation.
Fig. 30. Similar head from dorsal aspect; younger specimen.
Fig. 31. Exuviated head cuticle to show relations of traverses, apodemes
of the fiabellae, pharynx, epipharynx, etc.
Fig. 32. Median longitudinal sectionof head of full grown larva; compounded
several sections. It is distorted so that the epipharynx lies too far back
relatively to the labium and adjacent organs; (comparewith fig. 36);
labial bud cut in midline
between eminences.
Fig. 33. Dorsal view of pharynx of mosquitolarva.
Fig. 34. Sectionof lower anterior region of head of mature larva to show
relations of labial bud; bud cut to one side of midline through one of
the two eminences.
Fig. 35. Lateral aspectof larval pharynx, dorsalplate removed.
Fig. 36. Section of part of head of pupating larva to illustrate fate of ventral
fold. epi f-- cuticular sheath of epipharynx. Cronparewith figs. 32
and 34.
TIIOMPSON.--ANATOMY
OF MOSQUITO.
Proc. BosTON Soc. NAT. IllsT. VOL. 32.
PLATE
15.
THOSIPSON.-- Anatomy of Mosquito.
PLATE
16.
Fig. 37. Esophagealvalve and cardia of a full grown larva, striated border
on cardia cells not distinct.
Fig. 38. Esophageal valve in a pupa fore' hours old; the left hand wall is
ventral.
Fig. 39. Separation of the degenerated dermis of the epipharyngealpit (ep,3
from the buccal cavity; longitudinal section of pupa ten hours old.
Imaginal muscles (x) are beginning to form in the old massesof the
retractors of the fiabellae and epipharyngeal muscles and the hypopharyngeal mu•les (hyp m) are forming at the base of the salivary duct.
Fig. 40. Esophageal valve of a pupa eleven hours old. The left hand wall is
ventral. f res-- ventral esophagealdiverticulum.
Fig. 41. Esophagealvalve of adult female mosquito; the right hand wall is
ventral.
Fig. 42. Surface of ventral food reservoir or diverticulum of adult female
showing the "striae" of the intima, A muscle fiber, and the nuclei of
the epithelium.
Bismark brown.
Fig. 43. Esophageal valve in a pupa ten hours old; the left hand wall is
ventral.
v -- valve, f res --- ventral diverticulum.
Fig. 44. Colon and rectum of a larva of the mosquito, longitudinal section.
Fig. 45. Portion of the wall of the stomach and ileum of a mosquito larva.
showing ring of naked epithelimn (x) at the junction of the two regions
where the Malpigbian tubules enter.
Fig. 46. Transversesectionof mouthpartsof male mosquito.
Fig. 47. Transversesectionof mouthparts of female mosquito; leve• of tip of
palps. The groove on the dorsal face of the labium in which the o•her
stylers are received,has been obliterated by swelling during dehydration.
Fig. 48. Ventral aspect of head of Culex pupa at the moment that the larval
head cuticle is ruptured. Dissected out and viewed as an opaque
object. fold----fold in front of labial bud (/i), see pl. 15, figs. 34 and 36.
Fig. 49. Side view of head of Culex pupa at moment when the respiratory
trmnpets free themselves by rupture of the thoracic cuticle; dissected
out and viewed as an opaque object.
Fig. 50. Side view of the head of Culex pupa at a time just prior to the complete release of the mouthparts and legs from the larval cuticle. Their
apices were still enclosed in the old sheaths. Dissected out and viewed
as an opaque object. Drawn on a larger scale than the other figures.
THOMI'SON.--2•NATOMY
OF•/•OSQUITO.
16.
•etr
38
39
THOMPSON.- Anatomy of Mosquito.
PLATE
17.
Fig. 51, Ileum of pupa twelve hours old, showing the columnar character of
the degenerating cells and some reconstructed epithelium (reg). cir
m -- circularis
muscles.
Fig. 52. Colon and rectum in a pupa five hours old; temporary degeneration of the epithelium of the rectum.
Owing to the collapsed walls
many cells are cut tangentially and apparently lie in the lumen.
Fig. 53. Sameregion in a pupa twenty-one hoursold, showingthe beginnings
of a rectal papilla (p) and the new-formed ileo-colon (ilc). Outside of
this lie the remnants of the larval colon (co). A few of its muscles(m)
are still recognizable.
Fig. 54. Diagrammatic longitudinal section of a pupa five hoursold.
$'ig. 55. Diagrammatic section of a pupa twenty-two hours old. The antlia
is not yet formed. f res -- ventral esophagealpouch.
Fig. 56. Outline of section of head of pupa four hours old. ß -- pharyngeobuccal boundary.
Fig. 57. Outline of section of head of pupa seventeenhours old, epipharynx
degenerated, musclesaltering' '-- pharyngeo-buccalboundary.
Fig. 58. Outline of section of head of pupa twenty-five hours old, the imag-
inal musclesa'ndpump perfected. ß--phary•geo-buccalboundary,
val m-- el d p, ascph-- retr ph of figures 56, 57. epi-- epipharyngeal
muscles.
•'ig. 59. Invasion of larval colo• by cells of the ileu:n; longitudinal sectio• of
pupa seventeenhours old.
THOMPSON.--ANATOMY
OF MOSQUITO.
PROC.BOSTON
SOC.NAT. HIST. VOL. 32.
PLATE
17.