The Biology" and Development of Cryptochaetum grandicorne (Diptera), an Internal
Parasite of Guerinia serratulae (Coccidae).
By
W. H. Thorpe, M.A., Ph.D.
( F r o m the Entomological D e p a r t m e n t , Zoological Laboratory,
Cambridge.)
W i t h 3 0 Text-figures.
C O N T E N T S .
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INTRODUCTION
BIOLOGY
AND
METHODS
o r T H E ADULT
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H o s t Relations a n d T i m e of A p p e a r a n c e
Mating and Oviposition
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STRUCTURE
O F T H EOVIPOSITOR
STRUCTURE
A N DBIOLOGY
THE
FUNCTION
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FILAMENTS
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STAGES
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o r T H E CAUDAL
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O F T H EE A R L Y
The Egg
First Instar Larva
Second Instar Larva
Third Instar Larva
Pupa
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C O R R E L A T I O N O FT H E L I F E - H I S T O R Y O FT H E P A R A S I T E W I T H T H A T O F
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HOST
COMPARISON
C H A E T U M
SUMMARY
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REFERENCES
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O FC R Y P T O C H A E T U M
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G R A N D I C O R N E
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I C E R Y A E
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A N D
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INTRODUCTION AND METHODS.
THE genus C r y p t o c h a e t u m includes some eight species
of small flies; five occur in the Tropics and sub-Tropics of the
Old World while three are native to Australia. One species
C r y p t o c h a e t u m g r a n d i c o r n e Rondani is found in
Europe, being confined to the Mediterranean region. As far as
is known all are parasitic in the larval stages within scaleinsects of the sub-family Monophlebiniae. The genus is so
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W. H. THORPE
isolated both structurally and biologically that, although
usually placed in the Agromyzidae it should really be
assigned to a separate sub-family or family. It clearly represents
a separate and restricted line of evolution of parasitic habits
among insects.
Bezzi (1919) places three of the eight species, f a s t i d i o s u m ,
aenescens, and g r a n d i c o r n e in a separate sub-genus.
On the basis of adult structure only this course hardly seems
justified, but when we consider the great differences in the
structure of larval mouth parts and spiracles revealed by the
present study, it appears thoroughly reasonable. Indeed, if
biology and larval characters are fully taken into account,
g r a n d i c o r n e and icery ae might well be placed in different
genera, or if the genus C r y p t o c h a e t u m be raised to family
rank, then perhaps in different sub-families. In fact this genus
provides a striking instance of the tendency to which Giard has
applied the term 'poecilogony'.
C r y p t o c h a e t u m i c e r y a e , an Australian species parasitic on I c e r y a p u r c h a s i , was introduced into California
in 1888 in the attempt to control the scale which was a serious
pest of Citrus. Some five years ago the present writer was
enabled to make a thorough study of this form (Thorpe, 1930).
Previous to this the larval structure and development of the
genus had been almost completely neglected, in spite of the
fact that the larval forms are among the most remarkable in
the Diptera.
The life-history of C r y p t o c h a e t u m iceryae may be
summarized briefly as follows: the egg which is laid in the
haemocoel of the host produces a minute ' embryo-larva' which
at first lacks external segmentation and shows no trace of
either heart, mouth parts, tracheal system, or sense organs. It
apparently absorbs food by diffusion from the blood of its host.
This embryo larva stage is followed by three other stages. The
first two are tracheate but apneustic. The segmentation is now
complete and the alimentary, sensory, and circulatory systems
are normal save that the mid-gut is closed posteriorly. The most
striking feature of these stages is the development of two taillike tubular lobes of the body-wall which contain blood and
BIOLOGY OF CRYPTOCHAETUM
275
tracheoles. These caudal filaments arise from the last segment
and ramify among the organs of the host, somewhat after the
manner of the ' roots' of the parasitic Crustacea S a c c u 1 i n a
and M o n s t r i l l a . A series of experiments with chemical and
biological indicators showed that respiration is carried on at
the general body-surface, but that the tails are particularly
important as tracheal gills, which increase the surface at which
gaseous exchange between the larva and the blood of the host
can take place. The final larval stage is omnivorous and the
hind-gut is now open. The tails are still present and respiration
is mainly cutaneous. Although both anterior and posterior
spiracles are now present they do not normally come into contact
with the air until towards the end of the larval life. Pupation
takes place within the dead body of the host.
Our knowledge of other species of this remarkable genus is as
yet very slight. Vayssiere (1926) in the course of a paper on
the Coccidae has described certain stages of C r y p t o c h a e t u m
g r a n d i c o r n e , and has recorded some interesting biological
observations, while de Meijere (1916) has figured the last instar
of C r y p t o c h a e t u m c h a l y b e u m . These observations
although fragmentary were sufficient to show that great differences in development and life-history must exist between the
different species.
The object of the present paper is to compare the structure
and biology of C r y p t o c h a e t u m g r a n d i c o r n e with that
of C r y p t o c h a e t u m i c e r y a e . Consequently it will not be
necessary to describe in detail those features in which the two
correspond; only the points of difference need be discussed.
In January 1983 Professor F. Silvestri kindly sent me some
living puparia of C r y p t o c h a e t u m g r a n d i c o r n e . These
were hatched in the laboratory at Cambridge and placed in
a heated insectary in cages containing Vicia faba infested
with first instar Guerinia s e r r a t u l a e . The flies, which
were fed upon moistened sugar and raisins, mated and laid
eggs readily. Prom these specimens good material of the first
and early second instars was obtained. Since there were certain difficulties in carrying the insects through their complete
life-cycle under laboratory conditions in this country, the study
276
W. H. THORPE
was completed during March and April in Professor Silvestri's
laboratory at the Istituto Superiore Agrario, Portici, Italy.
Throughout I have received most kindly assistance from
Professor Silvestri, and it is a pleasure to express my sincere
thanks to him for his help in obtaining material, and for the
generous hospitality of his department.
BIOLOGY OF THE ADULT.
Host E e l a t i o n s and t i m e of A p p e a r a n c e .
It seems probable that C r y p t o c h a e t u m g r a n d i c o r n e
Eondani is confined to a single host, Guerinia s e r r a t u l a e
Fabricius. Vayssiere attempted to induce development on other
species of Coccidae but without success. He did secure oviposition in I c e r y a p u r c h a s i , but in no case was the development completed and the majority of the scales died before their
first moult, apparently as a result of injuries inflicted during
oviposition. In addition C r y p t o c h a e t u m g r a n d i c o r n e
is recorded from I c e r y a seychellarum Westw. and
Warajicoccus (Drosicha) c o r p u l e n t u s Kuwana in
Japan, but as I have previously pointed out (1930, p. 935) there
is considerable doubt about these records and they certainly
cannot be accepted without confirmation.
As soon as the investigation was begun a marked difference
was observed between the behaviour of the two species in captivity. Whereas iceryae is intolerant of captivity and will
only behave normally in very large cages, g r a n d i c o r n e will
live, mate, and lay eggs when confined in the ordinary 'lamp
glass' cages. This made the investigation of the early stages
very much easier in the case of the latter species, and as a result
I was able to obtain material of the transient first instar suitable
for sectioning.
In 1933 the flies were emerging in the Portici district from
about January 10 till April 10, the latter half of February being
the period of maximum emergence. According to Vayssiere the
date of first appearance may vary from the middle of December
to the end of April. In the former case the adults are present
in the field over a period of three and a half months, whereas
in the latter they were scarcely a month on the wing. Duration
BIOLOGY OF CRYPTOCHAETUM
277
of life in captivity was about ten days. Males and females
emerged in approximately equal numbers.
Mating and O v i p o s i t i o n .
Mating was seldom attempted unless the cage was in sunlight,
when it took place readily. The method is similar in the two
species. If undisturbed copulation may last 15 or 20 minutes.
Oviposition is accomplished with such rapidity that it is very
difficult to observe exactly what happens. Often not more than
four or five seconds will elapse between the preliminary extension of the ovipositor to pierce the host and the time when the
female moves off in search of another scale. It appears that only
scales in the first instar are suitable for egg-laying, and these
only after they have settled down upon their host-plant. As
a rule female flies failed to show any interest in young scale
which were walking about the cages, although on one occasion
half-hearted attempts were made by two females to lay eggs in
a young scale situated on a piece of filter paper.
Although, in contrast to i c e r y a e , never more than one
parasite is produced from a scale, g r a n d i c o r n e females when
confined in cages will lay a large number of eggs in a single host.
One of a number of scale insects that had been exposed to the
attentions of several females over a period of two days yielded
on dissection no fewer than seventeen unhatched eggs and four
first instar larvae. The majority of this batch of scale, however,
were moribund, death probably being due to excessive oviposition by the parasite.
STRUCTURE OP THE OVIPOSITOR
One of the most striking characters of the adult female
C r y p t o c h a e t u m is the presence of a piercing ovipositor
quite different in appearance from the more fleshy rasping
ovipositor described in nearly related non-parasitic flies, e.g.
P h y t o m y z a (Miall and Taylor, 1907; von Schlechtendahl,
1901), and it is important to know how far this can be regarded
as an adaptation to a parasitic mode of life. In the T i p u l i d a e
(Snodgrass, 1903) there exists a sclerotized valvular ovipositor
which, however, cannot be homologized with structures found
278
W. H. THORPE
elsewhere in the Diptera. A retractile piercing ovipositor is
found in a number of widely separated groups and has evidently been produced by different methods. In general it is
formed by a sclerotization of the terminal abdominal segments,
but there are considerable differences in structure to be found
and these indicate that the organ has been evolved several
times within the order. It is most characteristic of parasitic
forms, e.g. Pipunculidae, Conopidae, Phoridae, and Tachinidae,
but is also known in the Trypetidae. It is only in the Tachinidae that the structure has been carefully studied and here
Pantel (1909) has shown that among those forms which lay
their eggs within the body of the host three different types of
ovipositor are found.
1. A piercing organ distinct from the ovipositor itself which
lies above it resting in a groove on its dorsal surface, e.g.
Compsilura c o n c i n n a t a , the only species which has been
studied in detail.
2. A piercing organ combined with the ovipositor to form
a single structure. Weberia (Cercomyia) c u r v i c a u d a
parasitic on H a r p a l u s (Coleoptera). Also probably
F r e r a e a , Besseria, and P h a n i a .
3. A complex apparatus formed by various strongly sclerotized pieces situated at the apex of the abdomen and presumably
employed as an ovipositor. Allophora (Phasia), H y a l o myia. Also probably L e u c o s t o m a , O c y p t e r a , Celat o r i a , N e o c e l a t o r i a (Chaetophleps) (Walton, 1914),
Blaesoxypha.1
If we extend this classification outside the Tachinidae the
parasitic Phoridae (Pseudacteon, Apocephalus) and
Conopidae should probably be placed in group 3. The ovipositor of the Pipunculidae does not appear to have been studied
in detail. Hendel (1916) has erected a super-family, the Tephritoidea, to contain the Trypetidae, Lonchaedaidae (Sapromyzidae), Tanypezidae, Pyrgotidae, and Agromyzidae. These
groups are characterized by the presence of an ovipositor which
he states is composed of the eighth and ninth abdominal seg1
There is no detailed account of any of these genera and further study
might well result in some of them being removed to groups 1 or 2.
BIOLOGY OF CRYPTOCHAETUM
279
ments, withdrawn, when not in use, Avithin the conical seventh
segment; the tergum and sternum of which are fused to form
a single piece. But it seems fairly clear from the account of
the Pyrgotid Campylocera given by de Meijere (1916) and
of the Trypetid Dacus by Miyake (1919) that very different
types of ovipositor occur within these groups and that Campylocera would fall in Pantel's group 1, and Dacus in his
group 2. As yet not enough is known of the Sapromyzidae and
Tanypezidae to enable a definite conclusion to be reached.
An examination of the female abdomen of C r y p t o c h a e t u m
shows that this too falls in the second of Pantel's groups and that
it is a very different structure from that found in the Agromyzidae and Phy tomyzidae, groups to which C r y p t o c h a e t ti m is supposed to be very closely related.
The abdomen of the female C r y p t o c h a e t u m is composed
of eight visible segments. Assuming that the true first segment
is atrophied, the first visible segment, which is much reduced,
is number two. Five clearly defined segments follow, the fifth
(no. 7) being conical but not with tergite and sternite fused.
The eighth abdominal segment is membranous with incompletely
defined tergum and sternum and is covered with minute backwardly projecting spines. Segment 9 bears at its apex dorsal
and ventral laminae between which is a spacious cavity. Immediately beneath the dorsal lamina is a papilla upon which
the anus opens and immediately beneath this again lies the
dagger-like ovipositor which is traversed by the genital duct.
At the very base of the ovipositor is a conspicuous rectangular
structure, a basal articulation which is moved by muscles
running to the outer and inner walls of segment 9 and which
apparently brings about the further extension of the piercing
structure. It is hoped that the accompanying figures (nos. 1-4)
are sufficiently clear to render farther detailed descriptions
unnecessary.
It is clear from this that the ovipositor of C r y p t o c h a e t u m
differs from both that of Dacus and of P h y t o m y z a . In
Dacus both the rectum and the oviduct are said to enter the
ovipositor although it seems from Miyake's figure that there is
some doubt about this and that the rectum may in reality open
NO.
306
u
280
W. H. THORPE
at the base of the piercing structure. In P h y t o m y z a the
genital duct is described and figured (Miall and Taylor) as
opening at the base of the terminal segment of the ovipositor
(i.e. true tenth segment, ninth segment in Miall's terminology),
whereas the rectum runs to the very tip. If this is a correct
v L
TEXT-FIG. 1.
Ovipositor of fly, not quite fully extended. Treated with potash and
cleared. Dorsal view, o, ovipositor proper; d I, dorsal lamina;
v I, ventral lamina; rp, rectangular piece.
interpretation, and there is no reason to doubt it,1 it follows that
the ovipositor of C r y p t o c h a e t u m i s structurally far removed
from the rasping ovipositor of the Agromyzidae and cannot
have been derived therefrom merely by a sclerotization of the
terminal segment. It is much nearer in structure to that found
in the Trypetidae. There seems little doubt, therefore, that it
may be regarded as a definite adaptation to a parasitic mode of
life.
1
Mr. G. C. Varley has kindly provided me with some living $ $ of
Agromyza sp. Examination of the abdomenshows that there is one duct opening at the base of the ovipositor, between the paired, valve-like blades, and
another at the very tip; and it appears probable that, as in P h y t o m y z a ,
the one at the tip is the rectum and the other the oviduct.
BIOLOGY
OF
281
CRYPTOCHAETUM
TEXT-FIG. 2.
Ovipositor of fly, lateral view, a, position of anus; other lettering
as Text-fig. 1.
v L
0-1 mm.
TEXT-FIG. 3.
Transverse section of ninth abdominal segment of fly with ovipositor
partially extended, r, rectum; other lettering as Text-figs. 1 and 2.
S T R U C T U R E AND B I O L O G Y OF T H E E A R L Y
STAGES.
The Egg.
The egg (Text-fig. 5) resembles that of C r y p t o c h a e t u m
i c e r y a e but is somewhat longer, more curved, and tapers
282
W. H. THORPE
more towards the posterior pole. There is also a definite difference in the structure of the micropyle. Measurement of ten eggs
gave the following: Largest 0-29x0'09 mm. Smallest 0-22
X0-05 mm.
There is not the very marked increase in size which was
ut
0-25mm.
Diagrammatic transverse section of segment 8 to show relationship
of alimentary canal and genital ducts, m, muscular mass surrounding terminal part of uterus; ut, uterus; v s, duct of central
seminal receptacle; r, rectum; ov, ovary (posterior region);
c, cavity formed by invagination of segment 9.
observed in the eggs of i c e r y a e during the course of development.
Dorsal closure of the embryo is complete on the second day,
but the gut is still incompletely developed. By the third day
the most characteristic feature of the larva, namely the caudal
filaments (Text-fig. 6), can be seen lying folded in the apex of
the egg. At glass-house temperature (50-90° ¥.) eggs were
ready to hatch towards the end of the fourth day after laying.
BIOLOGY OF CRYPTOCHAETUM
283
The egg opens by an irregular tear at the anterior end
(Text-fig. 7) and the larva escapes by means of very slight
bending movements of the body. This is the only movement
that the first stage larva seems capable of making and I have
never observed the slightest further movement after hatching
until the end of the instar when the animal is ready to moult.
TEXT-FIGS.
5—7.
n o . 5. Egg. nip, micropyle.
FIG. 6. Egg nearly ready to hatch.
FIG. 7. Hatching of larva from the eg]
First Instar Larva.
The first stage (Text-fig. 8) is an ' embryo-larva', which upon
hatching is very similar in appearance to that of i c e r y a e ,
but which is somewhat larger and more elongate. Externally
it shows no trace of segmentation, nor have I been able to detect
any sense organs, although there are some minute unpigmented
backwardly projecting spines ventral to the mouth region.
The most noticeable distinction between the first stages of
284
W. H. THORPE
i c e r y a e and g r a n d i c o r n e concerns the presence of a pair
of mandibles in the latter. I was quite unable to find mouth
TEXT-FIGS.
8—10.
n o . 8. Early first instar larva, m, mandibles.
FIG. 9. Late first instar larva.
n o . 10. First instar larva ready to moult. Characteristic spines of second
instar larva are now visible beneath the old cuticle.
parts in i c e r y a e . In g r a n d i c o r n e , however, although
unpigmented they are easily seen (Text-fig. 11). They lie sunk
within a cone-shaped depression behind which lies an equally
unpigmented and almost invisible pharyngeal sclerite. I have
BIOLOGY OF CRYPTOCHABTUM
285
never observed any movement of the mandibles, nor have I
been able to discern their muscles. The gigantic nuclei in the
tail hypodermis of g r a n d i c o r n e are a conspicuous feature,
even in living unstained specimens. Owing to the greater ease
with which material could be obtained it has been possible to
investigate the internal anatomy of the first stage larva by
means of sections. The oesophagus is open anteriorly and runs
back through the pharyngeal trough in the usual way, the lumen
•05mm
TEXT-FIG. 11.
Mouth structures of first instar larva, m, mandibles; ph s, pharyngea
sclerite; v s, ventral spines.
here being crescentic in section. Farther back, however, just in
front of the oesophageal valve, the lumen narrows and is lost
entirely, the gut for a short distance in this region appearing,
as in i c e r y a e , as a solid strand of cells. Although of course
one expects a narrowing of the lumen in the region of the
proventriculus of all dipterous larvae, these sections show a
definite closure and not a mere collapse of the gut wall. This
interruption of the lumen has been observed in three sets of
sections. Sections through the mid-gut again show a lumen.
A salivary duct is present but the salivary glands are nonfunctional and lack a lumen. The heart is still in porcess of
formation and is not yet functional. Rudimentary Malpighian
tubules are present. The ventral nerve chord is still in a
286
W. H. THORPE
partially embryonic condition. The structure of the tails is
identical with that of the first instar of i c e r y a e .
The larva increases in size somewhat during the first instar
and towards the end of this period segmentation becomes clearly
visible (Text-fig. 9). The tracheal system is also in process of
formation during this time and individuals nearly ready to
•Imm
TEXT-FIG. 12.
Newly emerged second instar larva.
moult may show a small section of the main tracheal trunks
containing gas, although normally they do not fill till the second
instar. Sections of early first instar larvae do not show any fully
formed tracheae although thick-walled embryonic tracheal
rudiments with imperfectly developed lumen can be discerned.
Sections of second instar larvae show fully formed thin-walled
tracheal trunks. Just before moulting the heart is complete
and the heart-beat can be seen. The digitate spines on the
288
W. H. THORPE
spines, giving the larva a very different appearance to that of
iceryae.
The structure of the mouth parts (Text-figs. 15 and 16) is
similar to that of the second and third stages of i c e r y a e .
The same median tongue-like structure is present. Such differences as there are, are shown clearly by the figures. The sense
organs including Keilin's organ are of the same type in the two
species.
The tracheal system (Text-fig. 13) is similar to that of the
third instar of i c e r y a e , but the subcuticular network in the
abdominal segments is very much less developed, and there are
as a rule only three or four tracheal branches in each caudal
filament, instead of the six usually present in that species. In
some young specimens all of these tracheoles may extend to the
very tip of thefilamentwhile in others, usually older specimens,
one or two may reach the tip while the rest end in hypodermal
cells half or two-thirds of the way down. The fact that the
tracheoles usually extend farther in the younger specimens is
probably accounted for by the increase in tail length referred
to above. Occasionally there may be a fifth or sixth tracheole
lying coiled loosely in the very base of the filament. Usually
these caudal tracheoles contain air for about two-thirds of their
total length, the terminal portion beingfilledwithfluid,although
in many specimens the column of air is visible nearly to the
tip of the structure. They are unbranched and under an oil
immersion lens are moniliform in appearance (Text-fig. 14).
Experiments in which larvae were punctured and placed in
hypertonic NaCl solution produced only an extremely slight,
barely appreciable, extension of the column of air.
Although during the first part of the instar the caudal
tracheoles terminate in the epidermal cells in the usual way,
it seems that, as in i c e r y a e , this connexion later becomes
broken, although the tracheoles may still contain air and be fulfilling their normal function. This was shown in a most striking
manner by some experiments in which uninjured larvae in late
second stage were placed in hypotonic (0-5 per cent.) NaCl
solution. The cuticle of the caudal filaments being very much
more permeable to water than the rest of the body surface,
BIOLOGY OF CRYPTOCHAETUM
289
water is absorbed rapidly into the lumen by osmosis. The fluid
then passes up from the filaments into the body-cavity of the
larva itself, and so rapid is the flow that the tracheoles are carried
16
TEXT-FIGS. 15—16.
FIG. 15. Mouth parts of second instar larva, lateral view. Ph,
pharyngeal solerite; Hyp P, hypopharyngeal plate; Md, mandible;
D, dentate sclerite; Ac, accessory sense organ; Ant, antenna; L P,
labial palp; K o, Keilin's organ.
FIG. 16. Mouth parts of second instar larva, ventral view. Lettering
as Text-fig. 15.
bodily up with it and in two or three minutes have been drawn
right into the last abdominal segment where they can be seen
lying as a tangled mass.
The heart is in action throughout the second stage and the
wave of contraction can be traced forward as far as the second
thoracic segment.
'290
W. H. THORPE
The second stage larva is capable of a certain amount of
movement, curling and uncurling its body, and keeping its
mouth parts in frequent motion. The larva now feeds upon the
blood of its host and the bright yellow contents of the mid-gut
are easily visible. Probably some of the diffuse fat-body of the
scale is also devoured during this stage. The mid-gut is still
closed posteriorly. The instar probably lasts for some three to
four months in all, though in certain seasons the period must be
shorter. It is a time of considerable growth as will be seen from
Text-fig. 13, a and b.
Thirdlnstar Larva.
The third stage larva (Text-fig. 18) is an ovoid-shaped maggot
tapering towards the anterior end.
The moult between second and third stage was observed on
several occasions. The skin splits along the back, beginning at
the hind end (Text-fig. 17). There is no trace of the evanescent
'second stage' without body ornamentation and lacking
tracheal system which was described by Vayssiere. This is
clearly an artefact and probably what Vayssiere saw was a larva
ready to moult. At this time the digitate spines on the abdomen
are extremely difficult to see and may easily have been overlooked.
In general structure and mode of life the third stage closely
resembles the mature larva of iceryae although there are
some very definite differences in detail.
The mouth parts show the same main pieces but the median
dorsal sclerite is represented by two spine-like structures on
either side, the bar uniting them having disappeared. Also I
have failed to distinguish discrete epipharyngeal and hypopharyngeal sclerites. The mandibles and the dentate sclerites
too are of very different form from those of i c e r y a e . The
same median tongue-like structure is jtresent in the two species.
This is perhaps homologous with the liguloid arch described by
Miller (1933) in Calliphora and with the ' labium 'of M i 11 ogramma Thompson (1928). It is worked by similar muscles,
the liguloid retractors. In C r y p t o c h a e t u m the liguloid
region is not sclerotized whereas in Calliphora a pigmented
292
W. H. THORPE
arch-like sclerite is present. During this stage the larva is
omnivorous, devouring all the internal organs of the host more
or less indiscriminately, and defaecation takes place in the
TEXT-FIGS.
21—2.
FIG. 21. Mouth parts of third instar larva. Lateral view. Lettering
as Text-fig. 15.
FIG. 22. Mouth parts of third inatar larva. Dorsal view, dor, vestige
of median dorsal sclerite. Dentate sclerite not shown in this figure.
normal way, the faeces being surrounded by the conspicuous
peritrophic membrane.
Anterior and posterior spiracles are present. As in iceryae
the anterior ones are kept withdrawn into their pockets until
towards the end of the larval life but they are totally different
in form, being pale and very slightly sclerotized and shaped
like a human hand with four very longfingersand a somewhat
shorter thumb. Those of iceryae, on the other hand, are
elongate, deeply pigmented structures shaped roughly like a
BIOLOGY OF CRYPTOCHAETUM
293
At
24
TEXT-FIGS. 23—i.
FIG. 23. Anterior spiracle of third instar larva. At, atrium,
n o . 24. Posterior spiracle of third instar larva.
10mm
TEXT-FIG.
25.
Pupa; lateral view.
barbed spear-head. The hind spiracles of the two species are
almost identical in appearance, but whereas in iceryae I was
unable to find any opening, in g r a n d i c o r n e an opening is
clearly visible on the inner side near the base of the spine. This
was confirmed by means of injection with benzene which had
previously been strongly stained with Sudan III.
294
W.
H.
THORPE
In the younger larvae (Text-fig. 18) the caudal filaments are as
conspicuous a feature as in the previous instar, but before long
the column of air in the caudal tracheoles begins to be withdrawn,
TEXT-FIG. 26.
Banding of P o l y t o m a culture with young second instar larva, a,
position of band at 10 niin.; 6, at 15 min.; c, at 20 min.; d, at 23
min.; e, at 25 min.; /, at 30 min.
the tracheoles appear to degenerate and the filaments themselves shrivel and are frequently broken off short (Text-fig. 20).
During the earlier part of the third stage respiration is still
entirely cutaneous since neither the anterior nor posterior
spiracles penetrate the cuticle of the host. Nor, in the course of
numerous dissections specially directed to this end, have I ever
found the posterior spiracles penetrating any of the tracheal
trunks of the scale. Later, however, they are thrust through
the host's skin in the dorsal posterior region of the abdomen
and connexion with the atmospheric air is thus established.
BIOLOGY OF CRYPTOCHAETUM
295
The third stage larva completes its feeding in about ten to
twelve days, by which time it has reduced its host almost to an
empty skin; the parasite at this time nearly fills the host in
TEXT-FIG. 27.
Banding of P o l y t o m a culture with a fully grown second instar
larva, a, 10 min.; 6, 15 min.; c, 20 min.; d, 25 min.; e, 30 min.;
/, 35 min.
which it lies longitudinally. Finally, the anterior spiracles are
thrust out of their pockets and through the cuticle of the host,
and the puparium is formed.
Pupa.
The pupal stage lasts for six months or more. The method
of emergence from the puparium is identical with that of
iceryae.
THE FUNCTION OF THE CAUDAL FILAMENTS.
In the case of C r y p t o c h a e t u m iceryae it was shown
that the caudal filaments, whatever other function they may
NO.
306
x
296
W. H. THORPE
have, served to increase the area of surface at which gaseous
exchange may take place. In other words, they act as a sort of
tracheal gill. Similar experiments in which the oxygen uptake
and the carbon-dioxide output was studied by means of biological
and chemical indicators respectively were undertaken in the
case of C r y p t o c h a e t u m g r a n d i c o r n e . The methods
employed were those described in detail in a previous paper
(Thorpe, 1932). For studying the oxygen uptake a pure culture
in glycocoll medium of the flagellate P o l y t o m a was employed.
TEXT-FIG. 28.
Diagram to illustrate area of colour change with a young third
instar larva immersed in cresol red solution, a, 12 min.; b, 15
min.; c, 25 min.
When a well-aerated suspension of this animal is run in under
a raised cover-slip, beneath which lies a clean freshly dissected
parasite larva, the organisms will shortly form aggregations at
those surfaces of the larva where oxygen is being absorbed from
the fluid. As the tension of oxygen falls below the optimum, the
flagellates will retreat from this region leaving a clear space and
moving outwards in crescentic formation as a retreating band
with the respiring surface as its centre, till they come to lie
as a narrow strip close to the edges of the cover-slip. For the
study of carbon-dioxide output, cresol red in normal salt solution adjusted with Na2CO3 to pH 7-6 was used. This solution
is used in the same way, the area of colour change being observed
under low power of the binocular microscope against an opaque
white background. In every case the results obtained for oxygen
consumption by means of theflagellatemethod were confirmed
for carbon-dioxide elimination by means of the chemical
method; the latter are therefore not described in full.
First stage larvae and very young second stage larvae are
BIOLOGY OF CRYPTOCHAETUM
297
too minute for these methods to be employed with success, but
the respiration of the later stages can easily be investigated by
their means.
When a half-grown or fully grown second stage larva is
exposed to a P o l y t o m a culture, clumping is noticed at the
general surface of the body, particularly the posterior region and
at the basal half or two-thirds of the caudal filaments, the
reaction to the anterior region of the body and the head being
negligible. The banding which follows (see Text-fig. 26) similarly
indicates that these regions are predominant in respiration.
TEXT-FIG. 29.
Diagram to illustrate area of colour change with a young third
instar larva immersed in cresol red solution, a, 10 min.; b, 12
min.; c, 15 min.
Text-fig. 27 is a characteristic example chosen from a series of
five experiments with P o l y t o m a . Although, of course, the
results of the experiments are not always identical, yet in no
case was the clumping and band formation more marked at
the head region; the bands were always of the type shown.
These observations were confirmed by a series of nineteen
experiments using cresol red.
Results obtained with third stage larvae are shown in Textfigs. 28 and 29, which are taken from a series of ten experiments.
A young third stage larva, in which the tracheal supply in the
caudal filaments is still undiminished, will give the same sort
298
\V. H. THORPE
of result as the second stage larva. A later third stage, in which
the caudal filaments have already begun to degenerate, shows
the predominance of the posterior region very much less
(Text-fig. 30). In one case the reaction at the surface of the
thoracic segments, where fat-body is rather less abundant than
elsewhere, was noticeably greater than at the abdomen.
From this it will be seen that in the second stage larva the
respiratory function of the caudal filaments is if anything somewhat greater in g r a n d i c o r n e than i c e r y a e . Although
the normal number of caudal tracheoles is four in the former
TEXT-FIG. 30.
Banding of P o l y t o m a culture with late third instar larva, a, 10
min.; b, 15 min.; c, 20 min.
species as against six in the latter, it must be remembered that
they extend farther in g r a n d i c o r n e . Moreover, the development of the tracheal supply of the caudal filaments relative
to that of the body surface is definitely greater in g r a n d i c o r n e than in i c e r y a e , since the subcutaneous tracheal
network in the body segments is so much more highly developed
in the latter species.
In the final larval stage the position is reversed, the caudal
filaments of g r a n d i c o r n e being of less respiratory importance than those of i c e r y a e . This is correlated with the fact
that whereas in i c e r y a e the filaments increase in length still
BIOLOGY OF CRYPTOCHAETUM
299
more in the final instar, these structures in g r a n d i c o r n e
not only do not increase in length but degenerate more rapidly.
This fact is perhaps in its turn related to the fact that in
g r a n d i c o r n e the spiracles are placed in contact with the
atmospheric air somewhat earlier.
CORRELATION OF THE LIFE-HISTORY OF THE PARASITE WITH
THAT OF ITS HOST.
The life-history of the scale is an unusual one and that of
the parasite is correlated with it in a remarkable way (Vayssiere).
This is in marked contrast to i c e r y a e. The larvae of G u e r i nia s e r r a t u l a e complete their development early in July.
They remain immobile for two or three weeks after their last
moult, and then leave the herbaceous plants on which they have
developed and wander in search of shelter in the crevices of the
bark of neighbouring trees. Here the females produce their
eggs, after which they die. The eggs hatch in about three or
four weeks but the young larvae remain immobile clustered
around the dead body of the parent for six or nine months,
during which time no food is taken. In December, January, or
February, the young scale, still in their first instar, commence
to wander in search of suitable herbaceous hosts. Shortly after
they have fixed themselves and have commenced to feed, the
first moult takes place. Feeding and development are completed
on the same host.
A great A'ariety of herbaceous plants have been recorded as
hosts of Guerinia s e r r a t u l a e . Leguminosae are particularly liable to infestation and among these Vicia faba is
one of the most favoured.
It will be seen that the life-history of C r y p t o c h a e t u m
g r a n d i c o r n e is adapted to that of its host in three important
ways. Firstly, there is a long pupal period which corresponds
to the dormant first instar of the scale. Secondly, egg-laying,
while confined to the first instar larvae of the host does not
normally take place until they have become fixed to their foodplant. Thirdly, the third stage larva of the parasite does not as
a rule commence to attack the vital organs of its host until the
300
W. H. THORPE
latter has returned to the shelter of the deeply cleft bark of the
olive or other convenient tree.
Although most of the work described in this paper was carried
out upon scales which had been parasitized in the laboratory, it
may be Avorth recording that dissection of a batch of fifty hosts
collected in the field near Sarno near Naples on April 3, 1933,
revealed a parasitism of approximately 12 per cent.
COMPARISON OF CRYPTOCHAETUM ICERYAE AND
CRYPTOCHAETUM
GRANDICORNE.
Although the life-history of the two insects is seen to be very
similar, the points of difference are more numerous and more
important than one expects to find between two such closely
allied species. It is worth while considering the exact significance
of the differences.
The first stage larvae, or ' embryo-larvae' of the two species
are identical in all essential points of structure save one. Whereas in iceryae I was unable to find any trace of mouth parts,
in g r a n d i c o r n e minute semi-transparent spine-like structures, presumably mandibles, are clearly visible projecting from
a cone-shaped mouth opening, and there is a very slightly
sclerotized pharyngeal trough. Whether vestiges of the mandibles are present internally in iceryae could not be determined without cutting sections. That there is a tendency for
suppression of the mouth apparatus in larvae of this genus is
also shown by the statement of de Meijere (1916) that in the
third stage larva of C r y p t o c h a e t u m chalybeum he was
unable to find any mouth parts at all."
The second important point of difference concerns the number
of larval stages. In g r a n d i c o r n e there are three. In
i c e r y a e , on the other hand, I found two larval forms apparently
intervening between the embryo-larva and the final omnivorous
larval stage. Although as stated in my previous paper (p. 967)
the moult between these was not observed, the differences in
size and structure of the mouth parts, in degree of development
of the tracheal system, and in the size of the abdominal filaments,
seemed almost conclusive evidence that they "were distinct
instars. Consequently it was assumed that there were four
BIOLOGY OF CRYPTOCHAETUM
301
larval stages in all. The present study of g r a n d i c o r n e ,
however, raises the question whether in spite of the apparently
important structural differences, these two forms of i c e r y a e
may not after all be different growth stages of the same instar.
If this were so then the differences in mouth-part structure and
abdominal ornamentation in the smaller larva must be put
down to incomplete sclerotization and expansion persisting for
a short while after the moult from the first larval stage. While
it seems improbable that the number of moults should differ
in iceryae and g r a n d i c o r n e , even though Bezzi (1919)
does place them in separate sub-genera, such a state of affairs
would not be entirely without parallel. Within the family
Oestridae, for instance, both Hypo derma bo vis and Hypoderma l i n e a t u m are now known to have five larval stages
(Laake, 1921) whereas Cephalomyia (Oestrus) ovis has
the usual three (Portchinsky, 1913). The only other alternative
would be to assume these smaller larvae of C r y p t o c h a e t u m
iceryae to be something comparable to the abortive asexual
larvae described by Silvestri (1906) in the Chalcid L i t o m a s t i x , but there is not at present any evidence in favour
of this view.
There remains a difficulty concerning the transient penultimate larval stage described by Vayssiere in g r a n d i c o r n e .
As mentioned above, all stages of the moult between the true
second and third instars were observed and no trace of Vayssiere's curious larval type was seen. It is clearly an artefact.
Another clear difference between the two species lies in the
structure and use of the spiracles in the last larval stage. In
i c e r y a e , as far as I have been able to ascertain, the hook-like
posterior spiracles are closed, and although they frequently
pierce the host-skin at the time of pupation, in many instances
when a number of parasites are contained in a single scale the
position assumed is such as to render this impossible. In
g r a n d i c o r n e , on the other hand, the posterior spiracles are
clearly open and as a rule puncture the host's skin some time
before pupation. Since there is never more than a single parasite
in each host this is always possible. In correlation with this
difference in the posterior spiracles it is found that the digitate
302
W. H. THORPE
anterior spiracles of g r a n d i c o r n e are less heavily sclerotized
and are not brought into play so early as are the dart-like
organs of i c e r y a e. The difference in the degree of development of the caudal filaments and its relation to the function
of the spiracles has already been dealt with above.
Finally, there may be mentioned the fact that with g r a n d i corne however many parasite eggs are placed in a single
host all but one invariably succumb in the early second stage,
whereas with iceryae as many as seventeen parasites are
capable of completing their development in a single scale.
SUMMARY.
1. In a previous paper was described the life-history of
C r y p t o c h a e t u m iceryae parasitic on Icerya purchasi, an Australian species introduced into California. It
was shown that the genus is highly specialized for life as a
parasite, and that it represents a separate and restricted line
of evolution of parasitic habits among insects. The present
study concerns C r y p t o c h a e t u m g r a n d i c o r n e which is
probably confined to the Mediterranean region, and which is
the only species of the genus known to occur in Europe. The
two species show notable differences in structure and life-history
although both are highly adapted to a parasitic mode of life.
2. The very minute eggs are laid in the haemocoel of the
host. The egg hatches to form a short-lived 'embryo-larva' at
first atracheate and showing no trace of external segmentation.
Mouth parts are present although the fore-gut is closed, food
materials presumably being absorbed by diffusion from the
blood of the host.
3. The second stage larva is tracheate but apneustic. Segmentation is complete. The fore-gut is now open but the hindgut remains closed. The food consists of the blood and fat-body
of the host.
4. The third stage larva is omnivorous devouring the
internal organs of the host indiscriminately and the hind-gut
is open. The tracheal system is amphipneustic. A few days
after the commencement of the instar the posterior spiracles
pierce the skin of the host and establish connexion with the
BIOLOGY OF CRYPTOCHAETUM
303
atmospheric air. The puparium is formed within the dead body
of the host.
5. As in C r y p t o c h a e t u m iceryae the larva is supplied
with a pair of long tubular caudal filaments, lobes of the bodywall containing blood and tracheae, which arise from the
posterior segment and ramify among the organs of the host.
Experiments indicate that they serve to increase the surface
area available for respiratory exchange between the larva and
the blood of the host. They are also readily permeable to water.
6. Although a large number of eggs may be placed within
a single host, only one reaches maturity.
7. The highly specialized ovipositor is described in detail
since it appears to be a striking adaptation to a parasitic mode
of life, and cannot be derived directly from the rasping ovipositor of the Agromyzidae.
8. In South Italy as in South France there is one generation
per year, the life-history of the parasite being closely correlated
with that of the host.
9. Special attention is paid to those features in which
g r a n d i c o r n e differs from iceryae and the significance of
these differences is discussed.
REFERENCES.
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2. Laake, E. W. (1921).—"Distinguishing Characters of the larval stages
of the ox warbles, Hypoderma bovis and Hypoderma lineatum",
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4. Miall, L. C, and Taylor, T. H. (1907).—"Structure and Life-history of
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8. Portschinsky, J. A. (1913).—"GEstrus ovis, sa biologie et son rapport a
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11. Snodgrass, R. F. (1903).—"The Terminal Abdominal Segments of
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12. Thompson, W. R. (1928).—"Dipterous parasites of the Earwig
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13. Thorpe, W. H. (1930).—"The Biology, Post-Embryonic Development
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(1932).—"Experiments on Respiration in the Larvae of certain
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