..
I
Reprinted from
TRXNSACTIOSS
OF THE ROYALSOCIETYOF TROPIUL ~UEDICIX~AXD HYGIEXE
Vol. 49. No. 6. pp. 505-527, 1953
ANOPHELES
GILES
GArMBIAE
COASTAL
AKD
AREA
ANOPHELES
OF LIBERIA,
MELAS
WEST
AFRICA
THEOBALD
IN
A
t
BY
HENRY
;_Tatiotlal Institutes
of Health,
hl. GELFAND,
Laboratory
of Tropical
3I.D.’
Diseases, Bethesda, Maryland.
Anopheles gambiae Giles has long been known to be one of the most common mosquito
species in West Africa, and the one mainly responsible for the transmission of malaria in
that part of the world. Only one year after its original description in 1902, a variety melas
was described for an extremely dark variation bred from brackish water in Gambia (THEOBALD,
1903). B_UBER and OLINGER (1931) extended knowledge of the variety with observations
and experiments in Ngeria.
Since that time, this designation has been applied to females
of A. ganzbiae which possessedmelanic coloration and an additional dark band in the terminal
white area of the palps, and which were associated with brackish water in coastal regions
(EVANS, 1931, 1938). Th e variety has been found in numerous localities along the coasts
of West and East Africa (MUIRHEAD-THOMSON, 1947, 1951a ; and HOLSTEIN, 1951). It
is worthy of note that var. ;meZas
was never found in Brazil during the invasion of that country
by A. gambiae (SOPER and WILSON, 1950).
During World War II, the greatly increased interest in the epidemiology and control
of malaria in West Africa resulted in a great extension of our knowledge of this variety.
As a result of work in Sierra Leone (RIBBANDS, 1944a ; MUIRHE.~D-THOISOX, 1945), it was
elevated to specific rank, and important differences in the bionomics of the two species were
discovered.
In Liberia, YOUNG and JOHNSON(1949) recorded the finding of Anopheles melas larvae
on Bushrod Island, near Monrovia.
I am unaware of other references to this species in
that country. The present report is based on work done in Marshall Territory, Liberia
from January, 1952 through July, 1953:.
Over 11,000 adult specimens of the A. meZasA. gambiae complex were collected, and over 3,400 egg batches identified.
t This work was done as part of the United States Technical Co-operation
Administration
Pro-.
gram in Liberia, and was a collaborative project of the National &IicrobioJogical
Institute of the U.S.
Laboratory
Public Health Service and the National Pubiic Health Service of the Republic of Liberia.
facilities were made available by the Liberian
Institute
of the American
Foundation
for Tropical:
Medicine.
and American,
without whose co-operation.
* I am greatly indebted to many persons, Liberian
and assistance this work would not have been possible.
I wish specifically to mention The Hon. R,
Francis Okai, Superintendent
of LIarshall Territory,
Justice Williams,
proprietor
of Cow Farm, and
Mr. Joseph K. Davis, faithful laboratory assistant.
1 Routine collections for June and July, 1953 were continued
after my departure,
under the
direction of Dr. Thomas A. Burch for whose assistance I am greatly indebted.
-‘ -._-.
509
Desuiption
Anopheles gambiae AP?D A.
melas IN LIBERIA
of the area.
Marshall Territory
is a small district on the Liberian Coast, lying between the counties of hlontserrado and Grand Bassa, about 30 miles east of Monrovia,
comprising the area on both banks of the
This is a tidal, brackish-water
stream lined for many miles by mangrove sxvamp.
lower Du River.
Marshall City, the principal town, is situated at the end of a long, narrow peninsula, betsveen the river
and the Atlantic Ocean.
Numerous
small villages are scattered along the river edge and throughout
There are few roads : travel is mainly by foot trails
the “ bush ” extending away from the river.
and canoe.
The one motorable road leads from the river opposite Marshall,
two and a half miles to
the Liberian Institute, thence to Roberts Field, Firestone Plantation, and hlonrovia.
The predominant
vegetation bordering the river is the red mangrove, Rlzizoplzo~a Taceznosa, &ich
is restricted to the margin subject to daily tides.
This band, of variable width, is a dense tangle of
mangrove aerial roots growing in bare, black muck.
Scattered specimens of the white mangrove,
FIGVRE1.
Lower
Du River,
Marshall
Territory,
Liberia.
Avicennirz nitida, are occasionally seen on higher ground where no mosquito breeding occurs.
In
almost all places investigated,
the slope of the land rises sharply, and the fringing mangrove swamp
In relatively few isolated areas, the slope of the river bank is more
gives way to dry, sandy grassland.
This second zone is above the
gradual, and a zone of salt-marsh grass, Paspalmz aaginatztm appears.
level of the daily tides, but is flooded twice monthly by the “ spring ” tides, which, at the lower Du
These zones and their influence on mosquito
River, average about one foot higher than “ neap ” tides.
breeding have been fully described by Mr;uEtE_4D-THOhlSoK
(1945) for Freetown,
Sierra Leone, and
The Marshail
area differs only in detail from both
by BRUCE-CHWATT (1949) for Lagos, Nigeria.
those places.
Figure 1 shows the general area of the lower Du River and the towns and villages where the studies
Marshail City, including Krutown,
Fantitown,
and Freetown,
to be reported below were conducted.
where both native thatched huts and two- or
is a semi-urban
community
of some 500 inhabitants,
The six villages-on the north bank of the
three-storied
wood and corrugated iron houses are found.
river, opposite Marshall, are small, primitive communities
of the predominant Bassa tribe with a popuLloydesville
and Wehn are somen-hat larger villages of about 100
lation of only 25-35 people each.
people each.
?eather.
However,
such data are
Ko meteorological
data are collected anywhere in alarshall Territory.
These may differ somewhat
available for Roberts Field, located about seven miles from the Du River.
Table I summarizes
from conditions in Marshall in day-to-day detail, but give a correct general picture.
H.
M.
GELFAXD
510
the available information
by months during the period of this study.”
in 1952 was about average and the monthly distribution
was typical.
were not marked.
The total of 138 inches of rain
Daily temperature
variations
The mosquitopopulation.
Table II lists all the adult mosquitoes collected during 17 months in Marshall Territory
by nighttime human bait-collectors,
in man-baited traps, and in native houses in the early morning.
It gives,
therefore,
some indication
of the relative numerical
importance
of the anthropophilic
night-feeding
species during the period of this study.
Day-feeding
mosquitoes were very infrequently
encountered
in this area and were never troublesome.
TABLE I.
Rainfall,
temperature
Rainfall
Month
(inches)
and relative
humidity
1 Average air temperature
i
i
Maximum
I
at Roberts
(“F)
Field,
Liberia,
1952-53.
1 Average relative humidity
Minimum
Maximum
Minimum
88.6
,!
73.0
89.2
j
;
73.7
>,
88.8
73.3
>,
60.0
89.3
73.9
>)
63.1
87.6
74.0
,,
68.4
72.9
,,
68.4
81.0
72.7
,,
75.2
32.48
79.4
71.8
,,
78.7
Sept.
34.54
81.7
72.6
,,
81.2
Oct.
18.13
84.3
72.4
I,
84.6
Nov.
7.44
85.7
73.0
9)
79.6
Dec.
2.46
86.8
72.2
,>
76.0
Jan.
0.17
88.9
73.0
f,
63.2
Feb.
1.66
89.0
73.2
>,
60.2
March
3.76
90.3
73.3
t)
58.4
April
9.48
89.8
73.1
,,
63.4
May
16.53
86.9
73.5
>,
69.6
June
29.80
July
60.23
Jan.
1.15
Feb.
3.67
March
7.13
April
2.05
May
12.30
June
31.93
85.7
JOY
33.41
Aug.
* Provided by the Manager,
is gratefully
acknowledged.
Ii
1
Pan American
j
1
i
I
I
1
World
Airways,
Saturated
Roberts
Field,
Liberia,
(%)
(
I
1
65.1
59.9
whose courtesy
Anopheles gambiae AKD A.
511
melas IN LIBERIA
Table II indicates the importance of the A. melas-A. gambiae complex.
These species were the
main pest mosquitoes, in addition to their importance
as disease vectors.
Also notable in this table
is the absenceof Anopheles funestus.
This species, an important
vector of malaria in most parts of
equatorial Africa, and present in large numbers only a few miles away from the Du River, was collected
only twice in Marshall Territory
during this time.
TABLE II.
Anthropophilic, night-feeding mosquitoes collected by all methods in Afarshall Territory,
3nc9
c7
17JL'JJ.
Anopheles melas-Ano-pheles
gambiae
CztLextJmllasizrs Theo.
compler:
11,331
1,113
Anopheles hancocki Ed\v.
521
Mamonia
103
(Man.)
ztn<fomis (Theo.)
Aedes spp.
Other
CzJex spp.
Other
Mansonia
180
78
spp.
52
Other Anopheles spp.
26
The identification of A. melas and A. gambiae : In a detailed study, RIBBANDS (1944a)
showed conclusively that the melanic “ 4-banded palp,” so long used for the identification
of ” var. melas,” is a genetically determined familial characteristic, and that the percentage
of proved A. melas demonstrating it varies from locality to locality. This was investigated
in a small series in Liberia.
Of 206 A. melus identified by eggs in Marshall Territory, 20,
or 10 per cent. had a recognizable extra band in the terminal white area of the palps. In
addition, 14 others showed a few black scales in this site (grade 1 of RIBBANDS). Of 149
A. gambiae none had a recognizable extra band, and only one had a few black scales.
RIBBANDS (1944a) also reported the finding of constant differences in the larval pectens
of fresh-water “ gambiae ” and brackish-water “ ntelas,” correlated with differences in
larval reaction to salinity which was made the basis of a physiological test to differentiate
between larvae of the two groups. In Liberia, both criteria were used in examinations of
hundreds of field collections of larvae, and of larvae reared from the eggs of wild-caught
females. Both characters were found to be absolutely reliable, and to correlate with each
other and with the morphology of the eggs from which the larvae hatched, if known. In
addition, in our laboratory, A. ganzbiae was maintained in a continuous colony for 9 months,
and A. nzelusfor 4 months, by use of fresh water only for rearing both ; and there was no
change in the morphological appearance of the pectens. Eggs from the A. melas colony
could be hatched and the larvae successfully reared to adulthood in sea-water ; eggs from
the A. gambiae colony hatched poorly, and larval and pupal mortality was high, in dilute
sea-water.
&IUIRHEAD-THOMSON (1945) reported the finding of constant differences in the eggs of
the two species. CHWATT (1945) reported that 8.6 per cent. of egg batches at Lagos, Nigeria,
In the series from Marshall Territory, of
were intermediate and could not be identified.
3,400 egg batches, only eight or 0.2 per cent. presented difficulty, and most of these contained
markedly abnormal eggs. In the great majority of cases, there was no question whatsoever,
H.
512
&I. GELFAND
and a single egg was sufficient for positive identification.
This difference in the eggs provides
the only reliable character now known for separating wild adult females of the two species,
and has been used in the work reported below. Eggs of the two species from Liberia have
been illustrated and described by me (GELFAXD, 1954) as follows :
A gambiae - platform
narrow, owing to the closer approximation
the float is separated from the frill by a distinct space.
A. melas - platform wide, and the float touches, or is very narrowly
distinct space.
of the frills
separated
on either
from,
the frill
side
;
by a
The morphology of the eggs was found to be a stable character in laboratory colonies maintained under identical conditions for the above-mentioned time.
Infectivity of A. melas and A. gambiae for malaria and bamroftian Jilariasis : Prior to
World War II, dissection series of “ Anopheles gambiae ” in coastal areas of WTest Africa
undoubtedly included numbers of A. melas as well. The comparative role of the two species
therefore required evaluation. This was done for malaria infections in Sierra Leone (TREDRE,
1946) and Nigeria (MUIRHEAD-THOMSON,
1945). In the latter report, based on 3,494
dissections, the over-all sporozoite rate for A. melas was 3.5 per cent. and for A. gambiae
10.0 per cent. Apparently no work had previously been done on the comparative capacity
of the two species to serve as vectors of filariasis.
In Liberia, there have been no reports comparing A. melas and A. ganzbiae as vectors
of malaria. The vectors of filariasis in Marshall Territory have recently been the subject
of a separate report (GELFAKD, 1955), and are fully discussed there. However, the results of
dissections of wild-caught females of these two anopheline species are included here in Table
III for comparative purposes. Since it is not possible to make a specific identification based
on filarial larvae from the mosquito host, their designation as Wirchmeria banmofti is only
an assumption. However, since A. n&s and A. gambiae were both readily infected in the
laboratory by W. banmofti from human volunteers, and since these mosquitoes are markedly
anthropophilic (see below), worms agreeing with the morphological description of this
parasite were so designated. Table III summarizes the dissections of both species, as well
as of unidentified members of the complex, performed throughout the year in Marshall
Territory.
These mosquitoes were collected in the same localities throughout the study but,
because of the seasonal variations in incidence (see below), not necessarily at the same time,
Because of the average delay of almost 3 days necessitated by identification by eggs, the
percentage positive for malaria may be slightly higher than would be found in mosquitoes
dissected immediately.
However, in the case of filariasis, the age of the worms was estimated
and, if it appeared that a mosquito was not infected prior to the date of capture, it was not
This was necessary because some of the mosquito trappers were
counted as “ positive.”
themselves infected with IV. bancrofti. All identifications and dissections were performed
by myself.
For malaria, these results are in general agreement with those from Lagos. The
sporozoite rate for A. gambiae (5.7 per cent.) is four times as high as that for A. melas (1.4
per cent.). Nevertheless, A. melas is a vector of considerable importance, and its significance
is increased by the prevalence of this species in the dry season when A. gambiae may be
almost absent. For filariasis, the infectivity rate in A. melas appears to be significantly
higher than that in A. gambiae considering both total infections (27.1 per cent. vs. 19.5 per
cent.) and mature proboscis infections (3.6 per cent. vs. 1.9 per cent.).
Anopheles gambiae AND A.
513
TABLE III.
Dissections
of A. melas and A. gambiue, Marshall Territory,
malaria or filariasis, or both.
Filariasis
NO.
NO.
dissected
positive
3G6
I
1
369
83
27.1
262
Salivary gland infections only.
Dissected on the day of capture
of the two species.
I
1
A. melas/gambiaeT
_L
NO.
I positive
21
1 5.7
1142
27
2.4
51
19.5
592
132
22.3
4
1.5
11
1.9
3.6
and therefore
for either
I I
NO.
positive
1.4
No. with mature
larvae
*
t
/O
1952-53,
No.
Y 0.
dissected’
I
01
Liberia,
A. gambiue
6
427
total
T
A. melas
~~
Malaria*
melas IN LIBERIA
not identified
$
‘6
specifically.
dissected,
j
This
group
%
is a mixture
In order to understand these discrepant results, which show A. gambiae to be a more
effective vector of malaria and a less effective vector of filariasis than A. meh, the infectability
of both species for the two diseasesmust be studied under experimental conditions. ROBERTSON (1945), in a very small series, found that A. melas seemed to be as susceptible to experimental infections with malaria as A. gambiae.
This work should be repeated on a larger
scale. In a limited experiment (GELFAND, 1955), I found no significant difference in the
laboratory susceptibility of both species to infections with IV. banmuftiwhen fed simultaneously
on the same human volunteer. If both these findings are verified, then the difference in
natural infectivity rates must be explained on a bionomical rather than on a physiological
basis. Several possibilities suggest themselves, but differences based on the degree of
association with man appear to be ruled out because the rates were based on dissections of
the same individuals in most cases. Hypothetical factors in the environment which differ
seasonally, or which are different because of responses in behaviour of the two species during
the resting periods between blood meals, may be considered. An attractive possibility,
worthy of investigation, is a difference in the feeding preference of the two mosquito species
for persons of different ages. MUIRHEAD-THOMSON (195 lb) found a marked preference
for feeding on older people in A. aZbimanus in Jamaica. If the same were true for A. meh,
and if by contrast, A. gamhiae exhibited a preference for infants, it would provide a simple
and biologically very interesting explanation. However, THOWYS (195 1) in Sierra Leone
has indicated that A. gambiae has a host-age feeding preference similar to that of A. albimanus,
and I\%IRHEAD-THOMSON (1954) in a later report assembled data on filariasis and its vectors
which suggest epidemiologically that anophelines in general may have a host-age preference
of this sort in contrast with a less discriminating tendency on the part of culicines. Obviously
a great deal more work is required on this entire subject.
Seasonal incidence of A. melas and -4. gambiae : Soon after the recognition of A. meh
species distinct from A. gambiae, it was realized that there were marked seasonal fluctuations in the populations of both species. For the 18 months, February 1952 to July, 1953,
routine adult mosquito collections were made in three Du River villages on the north bank
of the river, and for a shorter time in Marshall City. From these data and the results of
as a
H.
M.
GELFAND
514
concurrent egg identifications, it has been possible to describe a rhythm of seasonal population
rise and fall in one complete annual cycle and in part of a second.
Of the various collection techniques that were used, the one that seemed most reliable
was the use of supervised two-man teams of bait-collectors.
These consisted of two Africans,
equipped with kerosene lantern, flashlight, test tubes and small cages. The men were stationed
outdoors under the sheitering roof of an inhabited native hut, and worked for the 1Zhour
period 7 p.m. to 7 a.m. They had a regular schedule of stations, and always used the same
hut in each locality. Every few minutes throughout the night they searched each other for
the presence of feeding mosquitoes, which were captured by test tube and transferred alive
to a small cage. The results mere then expressed as “ females per night per two-man bait
The mosquitoes were carried to the laboratory in the morning, and placed in tubes
team.”
for oviposition. This technique for estimating two mosquito populations is valid only if
c
I
0
Marshall
25
20
I
0
2
City
= A. melos
= A. gombioe
I5
\
z
0
i=
u
IO
5
0
Du
River
Villages
FIGURE 2. Seasonal incidence of A. melns
in &la&all
Territory,
and A. gambiae
Liberia and monthly
rainfall at Roberts
field, Liberia, February, 1952 to July, 1953.
0
50
v)
W
I
v
z
-
Rainfall
c
- Roberts
Field
40
30
20
IO
F
fs! A
M.
J.
J.
1952
A
S.
0
N
D
J.
F
Fri. A.
M.
J.
J.
1953
both attack. man to the same extent under these conditions. That both A. nzelas and A.
gambiae do readily feed on man outdoors will be shown in a later section.
One bait-collector team operated on the north bank of the river. It had a varying
schedule which always included, however, one night each week at each of the three Du River
Table IV gives the combined average
villages, Taryeweh, Dyochewehn, and Cow Farm.
collections per night of A. ~neZu.s
and A. gamtiae in these villages, expressed on a monthly
basis. A second team operated on the south bank of the river, spending one night each week
in Krutown, Fantitown, Marshall City proper, Freetown, and two tiny villages between
Table V gives the combined average collections per night for these
the city and Freetown.
six localities on a monthly basis. This material, plus monthly rainfall, is graphically presented in Figure 2.
It is evident that there is a marked seasonal alternation in species dominance in Marshall
Anopheles gambfae
515
A.
AND
melas IN LIBARIE
Territory, and that this is related to rainfall.
The direct and indirect effects of variation
in rainfall on larval breeding of the t\vo species are discussed in a later section. The relative
importance of these factors may vary from locality to locality. In Marshall Territory,
A. meLasappears to be almost completely absent during the rainy season, to reappear during
the dry season, to be most abundant during the period of intermittent showers at the beginning
of the rainy season, and to decline in numbers with the onset of heavy continuous rainfall.
TABLE IV.
Average
/
/
Month
1952
Feb.
nightly
collections
Average
nightly
collection
of A. melas and A. gambiae by bait-catcher
Identified
I
1
A. melas
27.4
54
by eggs.
per night
I
j ya A. melas i A. n&as
c
A. gambiae
’
Average
teams, Du River
1
98.2
A. gambiae
//I
26.9
0.5
1
39.0
0.0
/
16.4
March
1
39.0
70
0
100.0
April
/
18.1
49
5
90.7
12.1
19
12
61.3
34.9
2s
42
17.2
13
61
17.6
3.0
14.2
62
6.1
0.7
11.5
0.0
May
June
i
July
Aug.
/
12-2
Sept.
j
8.0
0
102
7.0
0
92
0
72
0
27
Oct.
Kov.
I
I
I
]
Dec.
8.0
1
1
I
2.3
4
)
1
I
I
I
1
40.0
/
7.4
I
;
14.0
1.7
4.7
)
20.9
0.0
1
8.0
0.0
I
7.0
0.0
1
8.0
0.0
0.0
j
2.3
1.5
0.0
0.0
I
1953
Jan.
1
!
2.3
/
20
10
66.6
Feb.
I
1
30.
I
34
2
94.4
1
4.6
i
120
4
I
96.8
4.5
0.1
3
I
i
]
97.4
10.5
0.3
89.5
18.8
2.2
March
April
May
I
[
j
10.8
21.0
June
16.3
July
10.8
I
112
I
153
1
44
1
4
I8
104
1
128
I
I
/
I
/
2.8
0.8
,
0.2
29.7
j
4.8
11.3
0.3
j
3.2
10.5
This is quite different from data presented by RIBB.GDS (1944b) from Aberdeen, Sierra
Leone, which show the A. melas population at its maximum during the rainy season and from
those of MUIRHEAD-THOMSOX
(1948)
f rom Lagos, Nigeria which showed considerable
variations bet\veen different collecting stations, the peak months being different and not
necessarily related to rainfall.
Furthermore, in Liberia A. melas was abundantly found in
H. AI. GELFAND
516
iMonrovia in October and November, 1951, by me, and in November, 1952, by Mr. C. E.
KOHLER (personal communication) at a time when it was essentially absent from the Du
River. It is apparent, therefore, that the varying combinations of environmental conditions
which will occur even in the limited area of the Liberian coast necessitate a study of the local
conditions in each place where work is contemplated.
rA.EtLE \'.
i\lonth
Average
I
--
1952
Sept.
Oct.
nightly
Average
nightly
collection
coIlections of A. melas and A. gambiae by bait-catcher
Identified
A. melas
-I,
I
I
NOV.
I
Dec.
1953
Jan.
Feb.
March
April
May
June
July
0
0
0
2
1
,
1
I
I
!
I
I
I
!
j
A. gambiae
i 3; A. melas
-_
1.5
0.5
1;‘:
2615
29.5
22.2
14
8
10
58
69
33
5
/
/
I
per night.
-j A. gambiae
_/_
0.0
0.0
0.0
32
20
11
2
0.0
0.0
0.0
50.0
0.2
5.2
4.0
1.7
0.2
2
1
12
34
38
276
324
87.5
88.9
45.5
63.0
64.5
10.7
1.5
1.3
0.4
1.2
11 .o
17.1
3.2
0.3
0.2
0.1
1.4
6.4
9.4
26.3
21.9
i
-
A. melas
I
I
5.2
4.0
1.7
0.3
Average
_.
_/_
I
I
by eggs.
teams, hIarshal1 City.
j
,
I
-
-
It is also worth while to note the variations that may occur in the same locality from year
to year. In 1952, March was the month of peak abundance of A. melas but in 1953 the peak
occurred 2 months later and was much lower. This indicates that even a prolonged study
of this sort cannot serve as a guide to forecasting, and that great caution must be observed
in interpreting reductions in mosquito incidence following the institution of control measures.
In contrast to A. melas, A. gambiae is most abundant during the rainy season and declines
to low population levels during the dry season. This is the usual observation in many parts
of Africa (DE MEILLON, 194’7).
Distribution of A. melas : In order to determine the limits of the area infested by A.
melas and its dissemination away from its brackish-water breeding grounds, collections and
ovipositions were obtained from villages at increasing distances from the Du River during
months xvhen A. melas T;siaspresent in large numbers. Table VI presents these data and
indicates that A, llzelns becomes progressively less abundant in comparison with A. ganzbiae
TABLE VI.
Collections
Village
Du River villages
Gai-bli
Samuel
Zopablo
Liberian Institute
of A. txeZas and A. gambiae at increasing distances from the Du River,
July, 1952 and January-May,
1953.
Distance from
Du River (miles)
0.0
0.8
1.0
2.0
2.4+
A. gambtie
A. melus
493
16
40
2
0
i
I
146
24
224
79
40
April-
o/o A. melas
I
I
’
77.1
40.0
15.2
0”:;
517
AnopheZes gambiae AXD A.
melas IK LIBERIA
one moves inland, and will probably not be found beyond two miles from brackish water.
This is in agreement with observations in other countries.
as
To determine the range of A. rrze2a.s
up a brackish river, collections and identifications
by eggs were made at various localities on the Du River during April and May, 1953, when
this species was abundant (Table VII).
All these villages are within the limits of mangrove
swamp and brackish water, analyses for XaCl at Wehn showing at least 25 per cent. sea-water
at high tide during these months. This Table indicates that the importance of A. melas
rapidly declines away from the mouth of the Du River. The extreme range had not been
reached, however.
Feeding habits of A. melas and A. gambiae : a. Tims of biting To obtain information
about relative feeding activity at various hours during the night, the teams of bait-collectors
described above were used. Mosquitoes were collected without interruption throughout
the night, but were placed in separate small cages marked in 2-hourly intervals : 7 - 9 p.m.,
9 - 11 p.m., 11 p.m. - 1 a.m., 1 - 3 a.m., 3 - 5 a.m., 5 - 7 a.m. In this latitude, sunset and
sunrise are nearly uniform throughout the year. The men collected mosquitoes which
, actually landed on their bodies and attempted to feed. These two-man groups were stationed
outside inhabited native dwellings, on the sheltered “ piazza.”
In this site, the factor of
time of entry into houses was eliminated.
Because the number of specimens suitable for positive identification by egg examination
was too small for an hourly analysis, for each species those months of the year were chosen
when it was preponderant in Du River villages, the number of specimens of the minority
species being so small that the error thus introduced was insignificant.
TABLE VII.
Collections
Village
Wehn
Lloydesville
Poortown
Freetown
Cow Farm
Fantitown
of A. meZa.sand A. gambiae at increasing
May, 1953.
Distance
COW Farm
from
(miIes)i
A. melas
6.5
5.5
4
1
1.5
3.0
-1.0
3;
261
43
-
distances up the Du
A. gambiae
4-4
45
67
24
17
10
River,
o/o
A.
April-
melds
9.1
2.2
9.5
57.9
93.9
81.1
Table VIII shows the total number of mosquitoes of each species collected during each
time period. When only a few anophelines were collected per night, they were randomly
distributed through the night. As the total number increased, there appeared to be a tendency
in each species for an increasing percentage to be collected later during the night, but this
tendency was never marked, and the activity of these anophelines throughout the night is
(1943) found the
notable. Working with these two species in Nigeria, MUIRHEAD-THOMSON
period of greatest feeding activity inside huts to be between 4 a.m. and dawn. HADDOW et al.
(1947) working with A. gambiae only, in uninhabited forest, also found a peak in the few hours
before dawn.
b. Outdoor L
‘S
’ _indoor feeding - The preference of A. melas and A. gambiae for feeding
outdoors and indoors was investigated by stationing the same bait-collecting team, on alternate nights, outside on the “ piazza ” of an inhabited house, and inside one of the sleeping
H.
hf.
GELFAND
518
rooms (vacated by other persons for this occasion) of the same house.
Table IX shows
the results from Cow Farm village and for comparison, collections of A. hancocki in Samuel
Village are also included. These data indicate no marked preference : the three species
fed readily outdoors if a suitable host was available, and all moved indoors without hesitation
in search of a blood meal. Observations in other villages, and during periods when both
species were present, lead to the same conclusion.
TABLE VIII.
Percentage of A. nteZasand A. gambiae collected by human bait-collectors at 2-hour
intervals throughout the night, Du River villages, 1952-53.
L
I
I Average i
)
x0. of
Species 1collections
I
Time
per.
j
collection
7-9
j 9-11
j M
/
1-3
j
3-5
ii-7
-‘
A. melas
A.gambiue
1
24
16
4
Total
9.2
19.6
64.5
38
7.8
36.6
17.2
21.6
14.7
18.2
20.6
9 .3
16.3
To&
TABLE IX.
1 17.2
17.8
20.1
18.4
i
15.4
19.1
34.8
23.2
I,
I
19.2
12.0
16.5
15.4
16.9
12.0
14.9
10.1
18.6
13.3
Outdoor and indoor night collections by bait-collector team, COW Farm village, 1952-53.
i
I
Outdoors
Species
A. melas
A. gambiae
Indoors
No. of
collections
Average per
collection
No. per
collections
Average per
collection
7
35.3
7
39.3
13
8.4
I2
8.8
10.1
I4
10.1
I
A. hancocki
14
1
A brief investigation was made of the vertical distribution of these anophelines in
dwellings of more than one floor. HADDOW et al. (1947) sh owed that A. gambiae was found
at all levels up to 82 feet in the uninhabited Semliki forest in Uganda. In Marshall City
in May, 1953, six collections were made in the same two-floored house. First-floor and
second-floor collections were made on alternate nights by the same team of two men. A
total of 41 A. melas and A. gambiae was collected on the first floor and 28 on the second.
Of five identifications of egg batches from the second floor catches, two were A. ganzbiae
and three were A. m&s.
Although too limited for statistical significance, the results do
suggest that these species will readily ascend to upper levels in a house.
c. Host prefuence - A limited experiment was designed to compare the relative attractiveness to A. melas of man and the other animal species commonly found in villages in this
area when all were available simultaneously in separate traps. For this purpose, small stable
traps were constructed of wood. They were 5 feet long x 24 ft. wide, without flooring,
with a removable, slanting, tarpaper-covered roof, and with a single door at one end protected
Anopheles gambiae AND A. melas IX LIBERIA
519
with a heavy cloth curtain on the inside. Mosquito entry lx-as possible through a plasticscreening louvre running the entire length of both sides, which followed the “ Egyptiantype ” design of BATES (1949).
The experiment was run for 18 nights in March, 1953, in Cow Farm village. Six
different species of animal bait were used -man,
monkey, goat, hog, dog, and chicken.
No cattle nor horses are normally present in the area. The traps were placed in a radiating
pattern, like the spokes of a wheel, at one edge of the village. The anthropophilic tendency
Because of the difficulty in procuring and keeping
of the anophelines was striking (Table X).
animals here, the volume of bait in each species was not comparable. However, man attracted
207 anophelines compared to only 12 for all other bait species. These results apply only
to A. TFzelas
since of 65 identifications by eggs, 64 were A. metas and only one was A. gambiae.
was not possible to study entrance of anophelines
Activity iu houses : a. &try -It
into habitations by direct trapping, but a few points of interest were established by other
methods. Both A. melas and A. gambiae freely enter African huts - rather flimsy affairs
in Marshall Territory with woven “ mat ” or thin mud walls and a thatch roof which is not
Openings in the eaves are often a foot or more in width, and, despite
tightly tied to the v-a&.
doors tightly closed and windows shuttered at night, almost unrestricted entrance is provided
for mosquitoes. Entry can be readily inhibited, however, by reducing the apertures available to hungry mosquitoes. For other studies (see below), huts were constructed into which
mosquito entry was permitted only through numerous small holes, about one half inch in
diameter,
punched through the mud at the eaves. Although numerous elsewhere in the
village, never more than one or two A. ntelas were found in these baited huts in the morning.
The openings were then enlarged to chinks about two inches in diameter, and mosquitoes
became numerous inside the huts. To determine to what extent A. melas vi-as still being
excluded, on four consecutive nights in March, 1952, the same t\\-o men bait-collected inside
the experimental hut at Cow Farm village. On two nights the door was kept closed and a
window exit trap was left in place all night (the eave openings thus providing the only means
of entrance). On two alternate nights, the door and window were left open until just before
dawn, and then the door was closed and the trap put on the window.
Table XI shows the
results of these four collections, and, incidentally, provides some information about the
percentage of the mosquitoes attracted to them which are actually captured by the baitcollectors. This table shows that, even with openings of this size, mosquito entry was
reduced by two-thirds.
TABLE X.
Mosquitoes
attracted
to different
baits offered simultaneously
March, 1953.
!
Species
Man
Goat
Ho&
Chicken
Monkey
Dog
No. of
individuals
in stable traps,
A. melas/A. ganzbiae
No. of
nights
TotaI
16
18
10
13
11
5
207
9
2
1
:
I Average per
1
I
night
12.8
0.5
0.2
0.1
0.0
0.0
Du
River,
I
Culicines
total
6
16
0
0
2
0
H.
M.
520
GELFAX’D
Time of entry was indirectly studied by all-night collections by the bait teams stationed
inside native sleeping rooms. The results showed no significant difference from the hourly
distribution of outdoor collections. No detailed records were kept of moonlight, but no
obvious effect was noted (see RIBBAXDS, 1946).
b. EL&t - Because large numbers of freshly-fed A, gambiae can often be found resting
in the morning in dark native huts, it has been generally assumed that this is a predominantly
house-haunting species, using outdoor resting places to only a slight extent. More recent
work, however, has demonstrated that this is not true, and that A. gambiae females, and A.
melas females to an even greater extent, leave the house at some time after feeding to spend
a part of the gonotrophic cycle outdoors (see review by MUIRHEAD-THOMSON,
1951~).
TABLE XI.
Collections
made in an experimental hut with restricted
mosquitoes, Cow Farm, Du River.
I
Door
Open
and window
(1)
(2)
Average
Closed
(1)
(2)
Average
/
,
(
1
I
Number
collected
Number
trapped
58
and unrestricted
entry for
Total
89
67
78
60
59
147
127
137
35
7
21
13
25
19
48
32
40
To study mosquito exit under controlled conditions, five similar experimental huts
were constructed, one each in TaryeTveh, Cow Farm, Fantitown, Marshall City, and Freetown. Modified from the general design of MUIRHEAD-THOMSON
(1948),
these were built
to simulate African huts as closely as possible, but with mosquito exit limited to one window,
to the outside of which a trap was tightly fitted. These huts were just sufficient in size to
accommodate txvo beds, with a low, tightly-thatched roof tapering to a central point. There
was a door in the western end, made light-tight by a double layer of heavy cloth on the inside,
and a one foot x one foot lvindow in the eastern end. The walls were made of woven reed
“ mat ” ; two were lined with mud, and two with old newspaper and cassava starch in the
local manner. In the Taryeweh and Cow Farm mosquito houses, the space between wall
and roof was filled in with mud, into which chinks about two inches in diameter were made
at intervals. In the Marshall huts, this space was left open and instead, a mat louvre was
made on the inside, leaving a one inch entry space all around the walls close to the roof.
A half-ceiling of mat was built into the eastern end of the hut just above the window, producing
a dark, undisturbed recess, seemingly ideal as a mosquito resting place. In the dim light
of the moon, at dusk and at dawn, the only light visible was that coming in through the
window opening. Even during the day, the recess above the window was still dark.
The removable exit traps were one foot cubes, screened except for the surface opening
into the window. At this face, a wire screen cone was attached, tapering into the trap and
slightly upward, to a point about two inches from the far surface, with an opening about one
cm. in diameter. Mosquitoes attracted to light readily entered this trap, and were unable to
leave until removed through a cloth sleeve in one wall.
A series of observations, by using empty mosquito houses, showed that only rarely did
mosquitoes enter these huts for shelter alone. By employing two volunteer “ sleepers,”
521
Anopheles
gambiae
AND
A. melas
IN
LIBERIA
it was possible to determine daily entry, exit at any time-interval desired, and resting preferences.
To estimate the time elapsed following the last blood meal in female anophelines,
the
scheme was used. Laboratory observations had sho1v-n that the gonotrophic
following
cycle lasts 2 days in the great majority of A. meZasand A. gambiae, never less, It was therefore possible, by examining the female abdomen, to estimate the stage of the mosquito in
the gonotrophic cycle. JT7henexamined in the morning, those which were both unengorged
and non-gravid were “ stage 1.” Freshly engorged females had fed during the night before,
Gravid or part-gravid females had fed no more recently than
and were called “ stage 2.”
2 nights before, and were “ stage 3.”
To determine how long A. melas and A. gambiae remain inside an undisturbed hut, the
Cow Farm hut was occupied by “ sleepers ” every night for periods of time, with a window
exit trap attached continuously except when changed once daily. The contents of the trap
were examined each morning, and the anophelines were “ staged ” according to the scheme
outlined above. It was assumed that all freshly-fed females (stage 2) had departed during
the night of feeding or at dawn, and that all gravid and part-gravid (stage 3) had been in the
hut for at least one full day. Stage 1 was ignored in the calculations. At times, mosquitoes
were collected by hand from the walls in the morning, after the trap was changed. When
this was done, those females were included in the stage 3 class for the day following.
Almost 90 per cent. of the female A. melas which fed indoors left an undisturbed hut
on the night of feeding to seek outdoor shelter (Table XII).
There were very small numbers
of A. gambiae, but these indicated that only 40 per cent. left the hut on the night of feeding.
To determine the time of departure of the freshly-fed and gravid A. melas, during 12
nights in April, 1953, the window trap was changed at intervals during the night. A trap
was left in place from 7 a.m. to 7 p.m. Since there appeared to be no tendency for mosquitoes
to leave during the daytime after 7 a.m. this trap collected those females leaving at dusk.
The trap was then removed and replaced at 9 p.m., 11 p.m., 1 a.m., 3 a.m., 5 a.m. and 7 a.m.
Table XIII summarizes the results by Z-hour periods. Here it can be seen that the majority
of all A. melas females left at dawn. Of those which had fed on the night in question, 72.7
per cent. left at dawn ; of those that remained in the hut during the day, 91.8 per cent. left
at dusk that evening.
Of the engorged females that did not leave the hut during the night of feeding or at dawn,
i.e. 10 per cent. of A. nteh and 60 per cent. of A. gambiae, the percentage that left at dusk
on the following night can be estimated in another way, by analyzing the morning collections
in native huts. The mosquitoes were “ staged ” in the same manner. Table XIV shows
TABLEXII.
Window
exit trap collections,
Cow Farm
experimental
hut, 1952-53.
I
Stage
(1)
(2)
(3)
Total
Unengorged,
non-gravid
Freshly engorged
Gravid and part-gravid
(2 + 3)
76 Remaining
I
A. melas
A. ganhiae
76
1
16
27
745
43
10.2
1
H.
TALE
XIII.
Time
of departure
M.
GELFAND
of A. mela-s from experimental
Stage
1
Hours
hut, Cow Farm,
Total
2
3
April,
1953.
Per cent.
(2 + 3)
2
(2 + 3)
I_
7
7
9
11
1
3
5
a.m.
p.m.
p.m.
p.m.
a.m.
a.m.
a.m.
-
7 p.m.
9 p.m.
11 p.m.
1 a.m.
3 a.m.
5 a.m.
7 a.m.
0
0
0
2
2
0
4
1:
7
:
Total
22
1
0
0
0
1
4
lo9
20
/i
0
1
151
i
I
24
22
I
10
7
1;:
I
12.6
0.6
2.9
5.7
4.0
62.8
11.4
72.7
175
100.0
the results of collections in Du River villages. Assuming that, on the average, equal numbers
of mosquitoes enter every night, the ratio gravid : fed would be 1 : 1 if all remained for the
entire Z-day gonotrophic cycle. However, for A. m&s, the ratio found was 1 : 18. Therefore, 17 out of every 18 which remained in the hut in the morning subsequently left at dusk
later that day. , Thus only 0.57 per cent. of A. melas females spent the entire gonotrophic
cycle indoors in this study. Similarly, 8.6 per cent. of A. gambiae remained in the hut for
the entire cycle. Therefore, both species appear to be exophilic in resting habits in this
area.
TABLE XIV.
Morning
collections
in huts, Du River
Stage
(1)
(2)
(3)
A. melas
Unengorged, non-gravid
Freshly engorged
&avid
or part-gravid
Ratio gravid
: fed
villages,
A. gambiue
4
304
17
1
: 17.9
1952-53.
28
204
28
1
: 7.3
c. Resting places in houses- Two walls of the experimental huts were surfaced with
mud, and two were covered with old newspaper applied with cassava starch. The thatch
roof was left bare. A half ceiling of “ soft mat ” covered half the length of the beds. The
beds were of rough wood, the eaves (in Marshall) were of “ hard mat,” and the door was
protected by a cloth screen. Therefore a great variety of surfaces was availables for mosquitoes
that remained in the huts in the morning.
Of 181 females of mixed A. melus and A. gambiae
collected in all five huts, the surface distribution was as follows : paper-covered walls - 70,
soft, mat ceiling - 59, mud walls- 30, mat eaves - 11, thatch - 6, under beds - 5, cloth
door cover - 1. Statistically these results prove little, for the surfaces were not equal in
area and the ease of collecting (or missing) on the different surfaces was not taken into account.
Nevertheless, the contrast between thatch roof and “ mat ” ceiling, and between mud and
paper-covered wall, was marked. In African huts, the preference of these anophelines for
resting on “ soft mat ” and paper-covered walls when available was often noticed.
Outdoor resting places of A. melas : It is apparent that both A. melas and A. ganzbiae
use outdoor resting places to a considerable extent. Some effort to locate those resting
places in Marshall Territory was made, but was almost completely unsuccessful. Only
523
Anophles
gambiae
A-ND A.
melas
IN
LIBERIA
a few unfed females were found under rotting logs near Cow Farm.
However, in this
area, there are few specific sites where a concentration of resting adults might be expected.
In the enormous area of surrounding mangrove swamp, many thousands of mosquitoes could
be dispersed with little likelihood of discovery. That such dispersion does occur is suggested
by night bait collections on the uninhabited island just offshore from Cow Farm. This
island consists almost entirely of red mangrove swamp, and no larval breeding has been
found there. Nevertheless, large numbers of A. ~zelnswere attracted to human bait stationed
on the island, probably to remain there after feeding.
In Monrovia, outdoor resting A. melas including both freshly engorged and gravid
females, were readily found in large numbers at the buttressed bases of kapok trees and in
old termite hills within or near native villages. A few were even collected from the walls
of crab holes near the mangrove swamp.
I@Lence of rainfall and tides on the adult population of A. melas : RIBBANDS
(1944b)
made a detailed statistical analysis
of
this
relationship
at
Freetown,
Sierra
Leone.
He
.
concluded that the incidence of A. n&s is controlled seasonally by rainfall, and this has been
shown to be true also in Marshall Territory.
On a short-term basis, however, tidal fluctuations
were the controlling factor. For Marshall Territory, this is graphically demonstrated in
Figure 3, and the time relationships between peaks of mosquito abundance (A. nzekasonly)
and the occurrence of the spring tides are shown in Table XV.
The mosquito population
was measured at Cow Farm by the use of an experimental hut baited with two human sleepers.
To minimize the irregularities resulting from adventitious daily fluctuations, each day’s
collection was expressed as a 3-day average of the catch the day preceding, the day in question,
and the day following.
The tide was recorded by daily observation of an arbitrary scale at
5
8
9
6
%
II
7
4
2
0
5
IO
15 20 25
JANW
FIGURE 3.
Rainfall
I
*
3',
i
4
I
3
'L&_lf-LI
14 i9 24
FEEFiUARf
tides,
r.
!_ /_ I _'L
I ni,
I
6 I! I6 21 26
MARCH
and the adult
1 Al
31 5
P'r) I j n f! IL
IO I5 20 25 30: 5
fiPR!L
female population
Liberia, 1953.
~,n,&
0
15 20 25 Y)
MAY
4
9
14 19 24
JUNE
of Anopheles melas, Cow
29
4
Farm
9
bt 24
JULY
Village,
The daily high tides were plotted to give the alteraa dock on the Du River near Taryeweh.
From
this
figure and table, it is very apparent that in the
tions of spring and neap tides.
absence of rain, the A. ?neZa.s
population was totally dependent upon the bi-monthly spring
tides, reaching a peak about 11 days after each high tide. With the advent of occasional
showers, the effect of the tides was reinforced by the prolonged submersion of the larval
H.
M.
524
GELFAIW
breeding areas, and the adult population peaks were higher and longer. After the last distinct
spring tide on May 10, a period of continuous heavy rainfall set in, and no further tidal
fluctuations were discernable, the level of the river being dependent wholly upon rainfall.
The larval breeding areas T-t’ere then continuously submerged, breeding almost ceased, and
the population of A. m&s rapidly declined, never to rise again during the period of this study.
TABLE XV.
Relation between matimum
tides, rainfall, and peaks in the population of A. melas,
Cow Farm, 1953.
1 Date
Date of matimum tide 1
l/14
l/27
2j13
2125
3/14
3131
4/10
4528
5110
I
I
average of
of population
3-Day
I
1 population at peak.
peak following.
1128
214
2120
316
3/26
4/15 (?)
4/24
517
5126
5.7
10.0
4.0
16.7
16.7
24.7
46.3
27.7
55.5
Interval between maxi.
mum tide and population
peak
(days)
14
8
7
9
12
15
14
1:
Lamal breeding of A. melas and A. gambiae : The importance of the topography of
the land, rainfall, and tidal fluctuations on the larval breeding of A. melas have been suggested
above, but can be summarized for this area as follows : No breeding occurred in the swamps
of red mangrove where tidal influence was felt twice daily throughout the month.
Repeated
collecting in water-holding
depressions around mangrove roots and in crab holes was always
unsuccessful. The second tidal zone, that of salt-marsh grass, was above water level during
neap tide, but was submerged by the spring tides which occurred twice a month at the new
and full moons. A. melas larvae promptly appeared after flooding, and development was
rapid. During the dry season, brackish water remained in pools and low areas for about a
week after the tide receded, and only one mosquito generation was possibIe. Larvae in
collections at this time were all of approximately the same age. Early in the rainy season,
the occasional showers caused the flooded areas to remain submerged for a longer period of
time. Several generations of larvae were possible, and the total adult population was much
greater. As the rainy season advanced, rain occurred almost every day and the volume of
fresh water coming down the river became of greater significance than the pressure of the
ocean tides. Fluctuations at a greater level still occurred, but depended upon the amount
of rain rather than upon the phases of the moon. The river overflowed the grassland areas
completely, and larval breeding was prevented. The adult population then rapidly declined,
This situation is unlike that encountered in Freetown ( RIBBANDS, 1944b) and Lagos ( MUIRHEADIn both those places, complete flooding of the breeding area did not take
THOMSON, 1943).
place, and usually breeding was maximal in the rainy season. Elsewhere in Liberia, limited
collections in the Monrovia
area indicated two distinct A. meZas cycles.
On Bushrod Island,
where breeding occurred in extensive Paspalum grass marshes reminiscent of Lagos lagoon,
it was maximal in the rainy season. In the Sinkor district, where breeding occurred in large,
permanent, brackish pools which have lost their connection with the sea, it was maximal
in the dry season.
Anophdes
525
gambiae
AND
A.
meIas IN LIBERIA
Extensive collecting in Marshall Territory failed to locate the breeding of A. nzel~.~
in other than brackish water. In Monrovia, however, larvae of this species were found
occasionally in salt-free rainwater puddles on the shores of the ocean and the Mesurado
River.
The larval breeding of A. gambiae was almost restricted to the rainy season in Marshall
Territory.
Larvae of this species were found only in the “ typical ” breeding places, i.e.
small, open, sunlit, freshwater pools. These included : rainwater puddles, hillside seepages,
semi-permanent pools in a sand quarry, hog wallows, and discarded calabashes. A few
larvae were found in water tanks inside a brick pumphouse. These types of water collections
are almost absent during the dry season, and A. ganzbiae adults were then rare. No larvae
of this species were ever found in brackish water in bIarshal1 Territory.
FIGURE
4.
Comparison
- - - - - .. ... . .
l
of adult mosquito
collecting
techniques,
Du River Villages.
Night bait - collector team (Females per night per two-man team).
Human-baited
hut traps (Females per night per two-man team).
Morning hut collections (Females per 10 sleeping rooms).
The selective breeding of A. melas in brackish water is interesting, since larvae develop
equally well in fresh and saline water in the laboratory. It is apparently not due to conscious
selection by the gravid female of saline waters. This was experimentally tested in our
laboratory colony. Two similar vessels for oviposition were placed in the cage on several
nights, one containing distilled water and the other 37 per cent. sea water. Of 4,031 eggs,
48.7 per cent. were laid in fresh water. In a similar test in the A. gambiae cage, 75.0 per cent.
of 5,538 eggs were laid in fresh water. The selection of breeding places must be made by
the ovipositing female, but on some basis other than that of salt content.
Contpartion of adult collecting techniques: During the course of this study, three different
techniques were used for the routine collection of adult anophelines, i.e. two-man teams of
bait-collectors, human-baited hut traps, and hand collecting in native huts by means of glass
tubing aspirators. When these three techniques were used in the same locality at the same
time, an excellent opportunity was provided for comparing them as population sampling
devices. Figure 4 shoxs the collections of the A. melas - A. gambiae complex obtained in
Du River villages. The mosquitoes caught by bait-catchers and baited huts were expressed
as the average number of females (A. meh - A. gambiae complex only) per night per two-man
H.
M.
GELFAND
526
team, and those collected in huts in the mornings as the number of females per 10 sleeping
rooms. The striking similarity in the general character of the three curves is most interesting.
Different individuals were involved in each collecting method, who were unaware of each
other’s results. Since the three methods seem to support each other, any one n-ould appear
to be a valid sampling device for this group of mosquito species. Each will have its own
special field of usefulness.
%JMMARY
(1) The bionomics of Anopheles llzelas and Anopheks gambiae were studied in Marshall
Territory, a rural, coastal district of Liberia, West Africa, from January, 1952, through July,
1953,
(2) The A. mehs - A. gam-biae complex constitutes the most abundant group of mosquitoes in this area, as well as being the main vectors of malaria and bancroftian filariasis.
(3) A. melas and A. gambiae are distinct though closely related species, separable by
well marked and constant characters in the egg and larval stages. Adults cannot be identified
directly.
(4) The over-all infection rate for malaria (salivary gland dissections) was 5.7 per cent,
in A. gambiae and 1.4 per cent. in A. hzelas ; for filariasis it was 19.5 per cent. in A. gambiae
and 27.1 per cent. in A. melas.
(5) There was a marked seasonal periodicity in both species in Marshall Territory,
A. melas being most abundant in the late dry season and early rains (January - June), and
A. gambiae being most abundant during the rains (June - November).
The seasonal incidence is a local characteristic and will probably vary from place to place in Liberia.
(6) A. melas was restricted to coastal districts. It probably does not occur inland
beyond two miles from brackish water, and, in Marshall Territory, was not common beyond
three-four miles from the mouth of the Du River, although it occurred in small numbers
at least 10 miles upriver.
(7) Both species fed readily outdoors but entered human habitations without hesitation.
Feeding occurred at all hours of the night, from dusk to dawn. A. melas was shown to be
markedly anthropophilic.
(8) Window exit trap studies showed that 90 per cent. of the A. melas females feeding
in a hut during the night left for outdoor resting places by dawn, the majority at dawn.
Of those remaining, 90-95 per cent. left at dusk later the same day. For A. gambiae, the
figures were 40 per cent. and 85 per cent. Few females of either species spent the entire
gonotrophic cycle indoors.
(9) Outdoor resting places could not be located in Marshall Territory.
In Monrovia,
freshly-fed and gravid A. melas females were found abundantly on the buttressed trunks
of kapok trees and in old termite hills.
I
I
I
I
I
II
(10) Routine adult collections of A. melas showed a marked correlation of peaks in
the adult population with tidal fluctuations, population peaks occurring about 11 days after
each “ spring ” tide in the dry season. With the onset of the rains, the effect of the tides was
reinforced. During the height of the rains, the river rose, tidal influence vanished, and the
population of _&I.szrplasdeclined.