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Circadian flight activity in the males and females ofAnopheles gambiaeGiles species A and B, A. melas (Theo.) and A. merus Dön. has been investigated using the acoustic actograph technique. In each species the pattern and timing of activity in males and females is almost identical. Cyclical activity continues in constant dark; the period of the cycle is approximately half-an-hour shorter inA. gambiaespecies A and half-an-hour longer inA. merusthan in the other two species. In alternating 12 h light: 12 h dark, both sexes are active during the dark period, with peaks of activity following light-off and light-on. After light-off, the species become active in the following order:A. gambiaespecies A,A. melas, A. gambiaespecies B,A. merus. A. merusis approximately 20 min later than species A.

1. The behaviour of A. gambiae and A. melas in houses is described in relation to possible use of pyrethrum and DDT.Pyrethrum.2. Mass spraying of isolated villages with pyrethrum in kerosene four times a week reduces the day-time catch of Anopheles to one-third. The sporozoite rate of mosquitos in sprayed villages remains much the same as before treatment.3. Observations in experimental huts with window traps attached show that pyrethrum exerts a repellent effect on many hungry Anopheles for a day or two after spraying, provided blood meals can be obtained easily in adjacent untreated houses.4. Where all available huts occupied by man are sprayed, there is no great repellence to hungry mosquitos, which continue to feed in large numbers even in huts sprayed every day.5. In untreated huts about 20 per cent. of the blood-fed females leave the house at dawn after feeding. In huts sprayed daily the proportion increases to about 80 per cent.6. The fall in the house catch in villages sprayed 4–6 times a week is not due to any great reduction in the mosquito population, but to a shift from indoor to outside resting places.7. As intense house spraying with pyrethrum fails to bring about any substantial reduction either in the mosquito population or in their infectivity, it is not likely to be of great anti-malaria value in West Africa.DDT.8. Mass spraying of village houses with 5 per cent. DDT in kerosene produces a dramatic fall in the day catch of resting Anopheles in houses. The catch remains very low for 4 weeks after treatment and it is still fairly low after 2 months.9. Observations in experimental huts (fitted with window traps) treated with DDT in kerosene show that mosquitos may enter and feed within a few days of treatment. By the second week after treatment large numbers of Anopheles may feed every night in treated huts.10. There is no evidence of mosquitos succumbing to the effects of DDT in kerosene inside the hut. Nearly all the Anopheles feeding in such huts leave after feeding and show no appreciable mortality in the following 48 hours.11. The DDT in kerosene has a marked residual irritant effect on Anopheles, driving them out of the house after they have fed, and preventing mosquitos resting long enough on treated surfaces to absorb a lethal dose of DDT.12. The few days complete protection from biting mosquitos which follows spraying inside the house with DDT in kerosene, is shown to be due mainly, if not entirely, to the repellent effect of the heavy dose of kerosene which accompanies the DDT.13. Treatment of all rooms in an isolated village reduced the day catch to nil during 5 weeks after treatment. In outside resting places beside the village, bloodfed and gravid Anopheles, of which 2 per cent. had sporozoites in the salivary glands, Were taken regularly during this period.14. The apparent elimination of mosquitos from houses following treatment of rooms with DDT in kerosene is due to a complete shift from inside to outside resting places, on account of the residual irritant, but not lethal, effect of DDT-in-kerosene treated surfaces.15. The sharp fall in the house catch of Anopheles following treatment Of West African village houses with DDT in kerosene can not, therefore, be accepted as evidence of mosquito reduction or effective control.

Polovodova's technique for determining the physiological age of mosquitos was used in a study in 1962–64 of the age-composition of populations of Anopheles gambiae Giles and A. funestus Giles resting in houses in two areas of Tanzania. One area was around Muheza, 25 miles from the coast, where the climate is humid and equable, and the other was around Gonja, 80 miles inland, where hardly any rain falls for five months of the year.It was found that the age-composition was almost identical in populations of A. gambiae and A. funestus at Muheza, about 20 and 23 per cent., respectively, being 3-parous and older and 1 per cent. 7-parous and older in both species. At Gonja, the population of A. gambiae was much younger, 14 per cent, being 3-parous and older and only 0·3 per cent. 7-parous and older. The oldest mosquitos found at Muheza included one 12-parous female of A. gambiae and one female of A. funestus believed to have laid eggs 14 times. No examples of A. gambiae older than 8-parous were found at Gonja.Dissections to determine the condition of the ovariolar sacs in A. gambiae at Gonja showed that in 87 per cent, of freshly fed parous females an interval of at least 24 hours had occurred since oviposition. At Muheza, 72 per cent, of individuals of this species in the cool season and 52 per cent, in the hot season were in a similar condition.Marking and recapturing experiments were carried out with females of A. gambiae in order to be able to correlate calendar age with physiological age. The oldest recaptured was 34 days old and was found to have laid eggs 10 times. From data on 60 recaptures, it was concluded that, although there was some irregularity, the first gonotrophic cycle lasted 3–4 days and later cycles 3 days.Age-specific sporozoite rates in A. gambiae rose from 4·1 per cent, for 3-parous to 32 per cent, for 7-parous and older females, and in A. funestus from 3·2 per cent, for 3-parous to 30 per cent, for 7-parous and older females. Most of the infected 3-parous females were gravid, indicating that few were infective at the beginning of the fourth cycle. On this account it was concluded that some 80 per cent, of malaria infections were transmitted in the fifth, sixth and seventh cycles.Analysis of the distribution of age-groups indicated that both A. gambiae and A. funestus showed a deficiency of nulliparous females, presumably because greater numbers of this group rested outside houses. From the second to seventh cycles the proportions of successive age-groups in both species at Muheza declined regularly at a rate corresponding to a mortality of 37·8 per cent, per cycle for A. gambiae and 38·6 per cent, for A. funestus, or 14·6 and 15·0 per cent, per day, respectively. Beyond this age the mortality was considerably higher. At Gonja, the population of A. gambiae declined at a rate corresponding to a mortality of 51·5 per cent, per cycle for the second to sixth cycles, or 20·9 per cent, per day. Above this age, the mortality was estimated to be higher still.From the regression of infectivity on age it was estimated that 6·8 and 6·1 per cent, of A. gambiae and A. funestus, respectively, became infected at each blood-meal.These findings are discussed in the light of current epidemiological theory.

SUMMARY. 1 A description is given of the topography and vegetation of Bwamba County, Uganda, an area where human yellow fever has recently occurred. The county is mainly low-lying and is extensively forested. It lies in the extreme west of Uganda, and includes part of the edge of the Ituri Forest. 2 The zoogeography of the mosquito fauna is discussed and it is shown that Bwamba belongs to the faunistic district known as the Lower Guinea Forest. 3 It is pointed out that the behaviour of well-known species of mosquitoes in forest areas may differ from that usually considered normal. Aedes aegypti and Anopheles gambiae are taken as examples, and evidence is given to show that in Bwamba these species, normally considered to be domestic and homophilic, appear to be largely sylvan and zoophilic.

Summary From June 1, 1955 to July 24, 1956 Anopheles gambiae were collected in villages in Marshall territory, coastal Liberia, using three collecting techniques: hand-catching mosquitoes on human bait in the open, trapping with human bait, and hand-catching mosquitoes found resting in the houses. All three methods yield valid population samples for estimating population size, and for studying both malaria and filariasis infectivity rates, demonstrating that gambiae feeds readily in the open and does not need to enter houses. The gambiae population is greatest during the rainy season from May to October, reaching peaks at the beginning and at the end of the rains, which are also the two periods of highest transmission. The gambiae population thus varies inversely with the temperature, which is lowest during the rains. The oocyst, sporozoite and total infection rates were found to yield the same epidemiologic pattern, the last being the most accurate and the easiest to interpret. The relationship between the sporozoite rate and the oocyst rate can be used to estimate the average age of wild caught mosquitoes when the timing of the developmental cycle of Plasmodium in the mosquito is also known. The average age thus derived for the mosquitoes studied was 27 days. Species identification of oocysts was possible in a third of the positive midguts; of these falciparum made up 82.4 per cent, and malariae and ovale each 8.8 per cent. Vivax was not found in wild gambiae. The disposition of the oocysts in the midgut differed from that described in A. atroparvus and A. stephensi; the density was greatest in the middle third (45.5%), somewhat less in the anterior third (36.2%), and least in the posterior third (18.3%). The overall rate of infection in A. gambiae with third stage bancroftian filariae (3.6%) was much higher than rates found by previous workers here, but is probably nearer the truth since it is based on many more dissections. Previous reports that A. melas is a more efficient vector of filariae than gambiae were based on small samples and are not convincing. Seasonal variation both in rate and in density of infection was noted, both increasing with the increase in the gambiae population at the beginning of the rainy season, both being lowest during the dry season. Geographically, rates of infection were found to be highest near the tidal rivers. becoming progressively lower inland, away from these rivers, and on higher ground.

A quantitative study was made of naturally acquired malaria infections among over 37,000 Anopheles gambiae and Anopheles funestus , collected from huts in a hyperendemic area in north-east Tanzania. The numbers of sporozoites estimated to be present in the salivary glands commonly lay between 2,000 and 4,000. The medians and means were higher, with those in A. gambiae exceeding those in A. funestus . The difference is attributed mainly to the circumstance that almost all the heaviest infections encountered, in both guts and glands, occurred in A. gambiae . Over 98% of the oocysts were identified as those of P. falciparum ; the remaining 4 infections, considered to be due to P. malariae , were all in A. funestus .

Wood-and-fibre-board huts fitted with one-way entry and exit traps were sprayed with DDT and BHC wettable powders at dosages 3·3 and 1·5 times the dosages reported by Muirhead Thomson in Tanganyika. Mud huts similar to his design were sprayed at 1·4 times his dosage of DDT and 1·1 times his dosage of BHC. Catches of Anopheline mosquitos continued for 16 weeks.In the first-named huts kills of A. gambiae were 97 per cent. for both treatments. Kills of A. funestus were 100 per cent. and 97 per cent. for DDT and BHC respectively.In the mud huts kills of A. gambiae were 62 per cent. and 98 per cent. for DDT and BHC respectively. For A. funestus kills were 78 per cent. and 93 per cent. in the same order.The advantages claimed for the wood-and-fibre-board huts include :—(a) freedom from ants, which may remove dead mosquitos.(b) ease of catching live adult mosquitos.(c) rigid construction of exit trap.(d) lack of adventitious exits other than exit trap.

The louvre-trap hut was designed to be cheaper and simpler to use than the verandah-trap hut. The eaves are closed, and mosquitoes enter by way of a row of five louvre frames and leave by a window fitted with a window trap. In three trials at Magugu, Tanzania, window traps fitted over one or all louvre frames showed that the efficiency of the louvres in preventing egress was 66–79% for Anopheles gambiae Giles, 51–73% for Mansonia uniformis (Theo.) and 74–78% for Culex pipiens fatigans Wied. Egress through the louvres of A. gambiae is only slightly more, and of M. uniformis much less, than through the eaves of a verandah-trap hut.

Malaria is most prevalent in the Gezira between October and January, following a well-defined seasonal periodicity. All three types of infection occur, malignant tertian being most prevalent. The incidence of the disease follows the rainfall curve at an interval of approximately two months. The only infecting mosquito worth considering is Anopheles gambiae which is most prolific from September till February and possibly aestivates during the summer months. Naturally infected mosquitoes never exceed 2 per cent. The human carrier of the disease is chiefly the child who is most heavily infected with gametocytes between October and January. Experiments have been carried out to test the efficacy of plasmoquine in causal and gametocyte prophylaxis. Small daily doses of the drug for causal prophylaxis are not recommended as they tend merely to conceal infection. The results of experiments in gametocyte prophylaxis tend to show that a biweekly dose of 0·02 gramme plasmoquine simplex to children might be of considerable value in antimalarial work. Conclusions drawn from the short period under review are suggested with diffidence.

Laboratory strains of Anopheles gambiae A and Culex pipiens fatigans from Tanzania, East Africa were experimentally infected with Wuchereria bancrofti to compare vector potential in the 2 species. Uptake of microfilariae, survival of the vector after an infective blood meal, number of filarial larvae reaching the infective stage and percentage of mosquitoes harboring infective larvae after 14 days were compared in the laboratory. C. p. fatigans ingested more than 3 times as many microfilariae when the 2 species were fed simultaneously on the same carrier. Fifty-four percent of the C. p. fatigans contained infective larvae after 14days compared to 9.0% of the A. gambiae. Feeding on a carrier with lower microfilaremia produced comparably lower values for each species. The mean number of infective larvae per positive mosquito was nearly twice as high in C. p. fatigans. A. gambiae showed a lower mortality rate than C. p. fatigans after an infective blood meal, but data indicated that the extremely low infection rates in A. gambiae nearly equated infected groups with controls. The extreme differences in Vector potential between the 2 species, in conjunction with the apparent role of A. gambiae as a vector in nature, suggest that strain rather than natural susceptibility may have been a major factor in the results of this study.

An intensive study was made of the ecology of species A and B of the Anopheles gambiae Giles complex at Kaduna, Nigeria, in October 1971. In collections comprising 383 larvae and 678 adult females from six villages, species A represented 82% of the larvae and 86, 89 and 91% respectively of the adult females caught by spray catches indoors, by biting catches outdoors and by exit traps fitted to windows. Larvae of species A and B occurred together in 42% of 57 breeding sites and showed no differential distribution among the various categories of site. Minor variations among the proportions of A and B females captured by different methods in the six villages showed no consistent contrasts. The only significant variation of the A/B ratio occurred at a temporary Fulani camp of cattle herdsmen, near Anguwun Maaza village, where an enhanced number of species B females were present indoors. At this Fulani camp a considerable number of both A and B females were feeding on cows outdoors and resting indoors during the day. Human blood indices of indoor-resting females were 88% for species A and 39% for species B at this camp, compared with 100% for both species at villages without cattle. Larval breeding places harboured an expectable range of insect predators. Predators of adult A. gambiae were not investigated although a predacious Emesine bug, Tinna elongata Villiers, was recorded in some huts. Numerous pathogen infections of larvae were observed, namely, Thelohania in 19%, Coelomomyces in 16%, unidentified gregarines in 7% and black-body virus in 3% of gambiae-positive breeding sites. Thelohania, Coelomomyces and gregarine infections were also encountered in adult females. Malaria sporozoite rates were 9·3% in species A and 5·5% in species B females. Some infections of stage III larvae of Wuchereria bancrofti were also seen in species A females.

The effects of tetramethrin on the behaviour and mortality of Anopheles gambiae Giles entering verandah-trap huts have been studied with the assistance of gaschromatographic techniques.Fifty-three to 39% of A. gambiae were deterred from entering a corrugated-iron-roof hut treated indoors with a nominal dosage of 2 gm/m2 tetramethrin active ingredient.Overall mortalities were 53–71% in the first month of application, but fell two to three months after application to 22–46%.The tetramethrin deposit had marked effects on the behaviour of A. gambiae entering treated huts. Feeding was greatly inhibited, 69–90% of the mosquitoes being unfed. Also, as the residual toxicity of the deposit declined, increasingly higher proportions (40–70%) of recently blood-fed A. gambiae left the huts due to the irritant action of the insecticide. The chemical basis of the irritant effect was indicated to the extent that survivors leaving the treated hut showed 9·5–14·0 ng tetramethrin/individual. The minimum lethal dose was indicated by 17·5–21·7 ng/individual picked up by mosquitoes showing delayed mortalities after leaving the treated huts. Mosquitoes that died indoors picked up amounts ranging from 27·0 to 42·3 ng/individual.The effects of tetramethrin on A. gambiae were in general similar to those of DDT and pyrethrum, and differed only in degree.

1. (1) The bionomics of Anopheles melas and Anopheles gambiae were studied in Marshall Territory, a rural, coastal district of Liberia, West Africa, from January, 1952, through July, 1953. 2. (2) The A. melas - A. gambiae complex constitutes the most abundant group of mosquitoes in this area, as well as being the main vectors of malaria and bancroftian filariasis. 3. (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. (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. melas; for filariasis it was 19.5 per cent. in A. gambiae and 27.1 per cent. in A. melas. 5. (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. (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. (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. (8) Window exit trap studies showed that 90 per cent. of the A. melas females feeding in a but 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. (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. 10. (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 A. melas declined. 11. (11) The larvae of A. melas breed almost exclusively in brackish water, conditions being optimal in the zone of salt-marsh grass which is flooded only by the bi-monthly spring tides, The larvae of A. gambiae breed exclusively in fresh-water — small, open, sunlit pools forming the most suitable places. 12. (12) A comparison of adult collecting techniques showed that night-bait collections, human-baited hut traps, and morning collections in huts are all valid sampling methods for following the seasonal variations in incidence of the A. melas - A. gambiae complex.

The effects of residual sprays of methoxy-DDT wettable powder at 2 g/m2 on the mortality and behaviour of Anopheles gambiae Giles, Mansonia uniformis (Theo.) and Culex pipiens fatigans Wied. females were investigated in four verandah-trap huts, one of which was used as control. The overall mortality of no species ever exceeded 29%, and no consistent deterrent effect on entry was demonstrated. The proportions of mosquitoes in the exit traps were, for A. gambiae 80–90% in treated huts and 29–70% in the control, for M. uniformis 90–100% in treated and control huts, and for C. p. fatigans 26–84% in treated huts and 38–66% in the control. There were more unfed mosquitoes of all three species in the treated huts than in the control, for A. gambiae 35–64% and 10–53%, respectively, for M. uniformis 38–84% and 35–64%, respectively, and for C. p. fatigans 22–58% and 21–43%, respectively. Bioassays with wild-caught A. gambiae exposed to the treated surfaces for one hour, showed that the deposits remained highly toxic for 16 weeks on grass, six weeks on iron and two weeks, or less, on mud.

(1958). A Modified Technique for the Age-Grading of Populations of Anopheles Gambiae. Annals of Tropical Medicine & Parasitology: Vol. 52, No. 3, pp. 261-273.

Arising out of a routine mosquito survey, carried out in a coastal district of the Gambia in 1946, it was discovered that specimens of typical A. gambiae showed a variation in maxlllary indices, suggesting the possible differentiation of the species into two races distinct from the known variety “ melas ”.Material from catching stations, considered beyond flight range from possible breeding places of “ melas ”, was subjected to statistical analysis as were also collections of adults emerging from different categories of fresh-water breeding places.The analysis showed that the maxillary indices of the samples obtained from control house catches were distributed in an abnormal manner and were therefore unlikely to have been drawn from the same population. At the same time, the samples collected by window-traps and spray-catches were found to differ significantly in their means, and this supported the view that a mixed population was being sampled. The samples obtained by trapping emerging adults from different categories of breeding places, where any adulteration by “ melas “ was a physical impossibility, showed significantly different mean maxillary indices. It was therefore very unlikely that they could have been drawn, by random sampling, from a pure population. When the samples, obtained by trapping over-breeding places, were divided according to their mean indices, it was found that those falling into the group with a mean index of 13·107 were taken from collections of water which could be classified as casual and those falling into the group with a mean index of 15·33 were collected from permanent pools.Preliminary research on the micro-flora of fresh-water pools has shown that certain distinct phases, marked by dominant plants, can be distinguished. The distinction between casual water and permanent water is capable of more accurate definition by reference to the dominant species. It has been observed that, within the limits of statistical differentiation, the race having the larger mean maxillary index is never found in water where photosynthesis, and hence chlorophyll, is absent whereas the race with the smaller mean maxillary index is never found in water where photosynthesis has been established amongst the micro-flora.The apparent difference in behaviour between the two races, as shown by the predominance of the 13·107 index group in the window-traps, and the predominance of the 15·33 index group in the spray-catches, cannot it is thought, be accepted as evidence unless the existence of a fatigue factor, accounting for house-resting habits in members of the latter group which have travelled far and exhausted their energy reserves, can be excluded.It is considered that the results of the work described in this paper may be of value in permitting control house catches to be analysed. From such analysis it should be possible to assess the proportion of adults which had originated in casual water and the proportion coming from more permanent breeding places. If, of course, it should be proved that the apparent differences in adult behaviour are, in reality, linked with differences in mean maxillary indices, it will be necessary to take this into consideration in assessing the value of residual insecticides and mass spraying. It is further argued that, even if the differences in behaviour can be explained on the fatigue factor hypothesis, it will still be necessary to take them into account in attempting to assess the value of mass spraying technique.It is considered that the method of statistical analysis could be more widely utilised in exploring many problems arising in the field, and that other variations, apart from those of maxillary dentition, might prove even more fruitful as a basis for analysis, although they would call for more time and labour than can usually be spared.

The brackish water form of A. gambiae on the East African coast—and probably in Mauritius—is not the same as A. melas of West Africa.In salt-water gambiae a variable proportion of the females have an additional dark band on the palps, resembling 4-banded melas, but the remainder are indistinguishable from typical gambiae.Eggs and larvae of salt-water gambiae show no morphological differences from those of fresh-water gambiae, thereby differing from A. melas of West Africa.Larvae of the two forms show a clear-cut difference in reaction to sudden changes in salinity, and a simple test has been worked out whereby wild-caught females can be accurately identified by the reactions of their progeny.This physiological test has formed the basis of all work in comparing the incidence, habits, and infectivity of salt and fresh-water gambiae in Dar-es-Salaam.Exposed to equal chances of infection in the same village during 1947 and 1948, fresh-water gambiae had a sporozoite rate of 9·4 per cent. while that of salt-water gambiae was 0·8 per cent.About 4 per cent, of both forms were infected with filaria larvae, but monthly figures showed that infection rates in salt-water gambiae may rise to 22 per cent.Fresh-water gambiae show little tendency to leave African houses at dawn after feeding, whereas in salt-water gambiae over one-third of freshly blood-fed females leave the house at dawn.In fresh-water gambiae many half-gravid females leave the shelter of the house at dusk on the night after the blood feed. There is no marked difference in infectivity between those which leave the hut and those which remain indoors at this stage.Blood-fed and gravid females of fresh-water gambiae, funestus, and salt-water gambiae have been found in outdoor resting places, gravid females predominating in the case of the first two.Although larvae of salt-water gambiae can complete their development in pure sea water, in nature increasing salinity becomes a limiting factor before it reaches that of sea water, continuous breeding being no longer possible at salinities over 83 per cent. sea water.Salinity as a limiting factor explains the rather restricted breeding of salt-water gambiae on the coast, and suggests that certain coastal fresh-water swamps at Dar-es-Salaam could be cleared of all Anopheline breeding by salinifying with sea water.

Collections of Anopheles gambiae Giles complex, A. funestus Giles group and other mosquitoes were made by spray-catch from twelve catching stations indoors and by hand-catch from pit shelters at two catching stations at Segera, Tanzania, between January 1970 and June 1971. Females of A. gambiae were identified cytotaxonomically as sibling species A or B of the complex. In houses during 1970, A. gambiae species B was more numerous at first than A, but A became predominant during the long rains of March-May. In the cool dry weather of June-November both A and B densities declined and the A:B ratio surpassed 50:1. The short rains in December produced a population explosion of species B and less multiplication of species A, the B:A ratio reaching >11:1. In 1971, hot dry weather during January-March caused declines of species A and B with a maximum B: A ratio of 13:1. Long rains, coming in late March, provoked a resurgence of A and a concurrent decline of B, so that the A:B ratio again reached 20:1 in June. Similar cycles of species A and B were observed outdoors, although the relative numbers outdoors/indoors averaged 2·3 times more for species B than for species A. In A. funestus, A. gambiae species A and A. gambiae species B Human Blood Indices were 97·5%, 91·2% and 60·9% indoors and 24%, 2% and 7% outdoors, respectively. Respective malaria sporozoite rates were 1·62%, 4·23% and 0·32% and minimum rates of stage-Ill filarial infection were 0·33%, 0·44% and 0·57%. Sporozoite-positive and sporozoite-negative mosquitoes exhibited similar HBF's in species A and discrepant HBI's in species B. The HBI's were higher in filariapositive A and B females than in filaria-negative females. Gregarines occurred in 1·36% of species A and 0·38% of species B. Trematode cysts were seen in two specimens of species A.Of A and B females 28% and 4%, respectively, had four-banded palps. It is shown mathematically that the discrepant malaria sporozoite rates in species A and B may be explained by extrapolating from the man-biting rate and probable daily survival rate for each species. This implies that no unrecognised factors play a major role in causing the contrasting efficiency of these two sibling species as malaria vectors.

1. 1. The environment surrounding Kisumu in Kenya is described, emphasizing the importance of the Nyando valley as a highly malarious area as compared with the Nandi and Kisii hills where only epidemic malaria occurs. 2. 2. Available data on malaria transmission are summarized in detail. The varying seasonal importance of Anopheles gambiae as a vector in the different topographical areas is discussed. 3. 3. Data resulting from a brief survey of the A. gambiae complex (November 10th–December 3rd, 1970) conducted during very hot and dry weather are presented to show that Species A and Species B resting in houses are differentially distributed in this part of Nyanza. Species B predominated in the lowlands and was prolific in the artificially irrigated areas. Species A predominated in the highlands. Both sibling species were well represented in sympatric association among foothills. 4. 4. Only 3 females of A. gambiae (1A and 2B) were obtained from the experimental area of 23 square miles employed by the WHO for hut testing of fenitrothion for anopheline control. 5. 5. Circumstances indicated that both Species A and Species B would have formerly occured more abundantly in this zone. 6. 6. It is suggested that although Species A was found to be virtually restricted to the highlands and foothills at this season, populations of A would probably encroach into the lowlands during rainy periods. 7. 7. Finally, WHO data on parous rates of A. gambiae at 2 localities near Kisumu are presented to show that Species B may continue reproducing at a greater rate than Species A as the dry season progresses.

The present paper is the first of two dealing with the seasonal distribution, hourly variation in biting activity, and vertical distribution of certain Southern Nigerian mosquitos. The technique employed in the studies under discussion resembled that used by Haddow (1947) in Uganda. Human bait was used in both cases and the agreement between the results obtained in the two localities is felt to be such as fully to justify the employment of such methods. Over fifty species of mosquitos were taken in all and fourteen of these proved to be sufficiently abundant to yield significant figures. Four of these are discussed and the remainder will be dealt with in the second paper.The most remarkable feature of the seasonal distribution was the very restricted period during which Anopheles gambiae was present in any abundance. This species occurred in large numbers only during the heavy rains of June and July and, to a less extent, in association with the small rains of September–October. During the dry season it almost disappeared. Aëedes africanus showed a similar distribution to that of gambiae but Taeniorhynchus africanus and Anopheles hargreavesi were most numerous during the dry season. It is suggested that in the area under consideration the latter species may well be a more important malaria vector than gambiae owing to its greater abundance and more uniform seasonal distribution.

Sporozoite rates found in Anopheles gambiae and A. funestus in an area of hyperendemic malaria in coastal Tanzania, by various investigators, have shown a marked and significant decline from 1934 to 1965. The environmental conditions of the mosquitoes have not changed in that period, and residual insecticides have never been used. The habits of the people of the area, however, have changed in respect of treatment of fever. Whereas formerly they relied on traditional remedies, more recently they have turned to chloroquine or mepacrine, either in the homes or at dispensaries. These drugs tend to reduce the infectivity of patients for mosquitoes and probably account for the declining sporozoite rates. The change of habit has a social background.

The mosquitos of two villages on the Kenya coast were studied over a period of two and a half years, which included years of very light, average and very heavy rainfall. A survey was made of the species occurring in the bush and in the houses, and in addition 24-hour biting catches were done and window-trap catches were examined to obtain information on the movement of species in and out of houses.Sixty five species were taken in net catches in the bush but only 19 were taken regularly; five (Taeniorhynchus uniformis (Theo.) and T. africanus (Theo.), Aëdes woodi Edw., Culex invidiosus Theo. and C. guiarti Blanch.) were abundant. In the houses, two species, C. pipiens fatigans Wied. and Aë. aegypti occurred regularly, the former being abundant and the latter not uncommon. Anopheles gambiae Giles and A. funestus Giles were seasonal; A. funestus appeared only after exceptionally heavy rain. The in-going window trap attracted only an occasional mosquito seeking shelter during the day and only six species were taken. Thirty species were found in the out-going trap but only seven occurred regularly. A. gambiae, A. funestus, T. uniformis, T. africanus and C. p. fatigans were most active during the hours of darkness, with the main activity in the four hours before dawn. Aë. aegypti had two waves of activity, in the four-hour periods before dawn and before sunset, respectively; the sunset peak was higher and the main activity probably occurred between 18 and 19 hr., as previously recorded for this species in Mombasa.Thirty five species were taken biting in the 24-hour catches in situations designated bush, house and compound, respectively. The biting cycles of A. gambiae, Aë. aegypti and C. p. fatigans have been compared in the three situations. The biting rhythm of each species was much the same in the different environments but the time and magnitude of the main peaks varied. In the case of Aë. aegypti, which probably includes two different forms, there is a marked difference in the magnitude of the evening peak in the bush and house. The biting cycles in the bush are given for A. squamosus Theo., T. uniformis, T. africanus, Aë. fryeri, Aë. metallicus (Edw.), Aë. albicosta (Edw.) and C. antennatus (Becker) and their occurrence in the other two situations is noted. The cycles of T. uniformis and Aë. fryeri agree with previous findings, but that of T. africanus differs from other cycles recorded for this species by having the main biting after midnight. The remaining species were rare, but one catch of 274 specimens of A. funestus was recorded.Resting mosquitos were more numerous in the second half of the year from August to December, a period of light rainfall and high temperatures. Biting mosquitos were more commonly taken, especially in the bush, in the second quarter of the year, during the main rainy season. The biting activity of some species increased in the houses in the second half of the year, when there are high winds, and biting in the bush was minimal.

A portable wooden hut with two windows, on opposite sides, fitted with traps to catch mosquitos as they entered the hut was tried, with human bait, as a sampling device for mixed populations ofAnopheles gambiaeGiles and A.gambiaevar.melasTheo. in and near Keneba village in the West Kiang district of the Gambia.In eighteen night catches, between 24th August and 22nd September 1954, 5,901 females of this species and its variety were taken in the traps, five high catches (642 to 1,240 females) being made before 3rd September and thirteen low catches (27 to 306 females) between 6th and 22nd September. Possible causes for the different size of catches in the two periods are discussed with the conclusion that the differences in catch are not simply attributable to erratic functioning of the hut as a trap.Four night catches of 186, 741, 797 and 1,240 females ofA. gambiaeand its variety are reported in detail. They reveal that mosquitos were taken virtually exclusively in the leeward trap if the wind persisted from one direction. But if the wind was variable, or conditions were calm, they were taken in both traps, entering the traps alternately as the wind shifted in the first instance and, perhaps simultaneously in calm. The results show that the use of only one trap could give misleading information about the numbers of mosquitos available outside the hut.There is no doubt that, with wind, the mosquitos approached the host in the hut from down-wind. Since much of the catching occurred in darkness it would seem that the mosquitos were attracted by the emanations of the host and that their approach to the hut was not directed by a visual mechanism controlling up-wind flight.Heavy rain for about seven hours did not prevent flight.From hourly collections, made from dusk to dawn, a curve of activity forA. gambiaewith its varietymelaswas obtained resembling the biting cycle reported elsewhere by other authors, the greatest activity occurring between midnight and 0600 hr. All the females (except two instances of incomplete fresh blood-meals) were unfed and, in a small sample, ovarian development was stage II or earlier. First entries occurred just before complete darkness in the evening and the last in good light about 30 minutes after sunrise.No males ofA. gambiaeor var.melaswere taken in the traps.The numbers, all females, of other species taken were: 1 ofA. funestusGiles, 1 ofA. rufipesvar.ingramiEdw., 59 ofTaeniorhynchus (Mansonioides)spp., 30 ofCulex nebulosusTheo., 19 of C.thalassiusTheo., and 15 of unidentified species. SomeCulicoidesoccurred in the traps and in the hut itself.The hut, if modified to include a window trap in each of the four walls and a cowl over each window to keep out rain, appears to be a simple and convenient sampling device for certain types of investigations on the biology and behaviour ofA. gambiaeand its varietymelas.

In the course of an experiment in malaria control in an inland region of Kenya and Tanganyika, by the use of house spraying with dieldrin, routine catches were maintained of mosquitos resting in artificial outdoor shelters. During the 18 months of the pre-spraying period, catches in the South Pare district of Tanganyika mainly consisted of the principal vectors, Anopheles gambiae Giles and A. funestus Giles, together with small numbers of A. rivulorum Leeson.During the three years following the spraying, A. funestus disappeared almost completely from the catches, while A. rivulorum showed an increase of about seven times above its former level.

Experimental huts similar in construction to the dwellings commonly used in East Africa, but with exit window traps, were sprayed with various formulations of the three residual insecticides, DDT, BHC, and dieldrin, and the effect on the A. gambiae and A. funestus entering them was observed.The almost complete absence of kill recorded by Muirhead Thomson (1950) in experiments in similar huts in Tanganyika treated with DDT Ditreen was not confirmed by these experiments.A significant proportion of the A. gambiae and A. funestus entering huts treated with DDT did, however, escape unharmed, even immediately after treatment, whereas with the other insecticides, BHC and dieldrin, none of these mosquitos escaped the effect at least in the first month after treatment.In preliminary experiments in which observations were carried on for nine months after treatments, BHC P.530 still showed some effect after seven months. This was almost certainly due to the fumigant effect of the small amount of insecticide still remaining below the wall surface. The irritant properties of the two DDT formulations, Ditreen and the oil-bound suspension “Supona” D, still existed after nine months.In a second group of experiments, dosages of less than 80 mg. DDT and less than 60 mg. BHC (8 mg. of the gamma isomer) per sq. ft. gave over 50 per cent. kills of A. gambiae and A. funestus for only one month.In a third group of experiments, using two formulations of BHC, five of DDT, one of a mixture of DDT and BHC and one of dieldrin:—(a) Dieldrin was by far the most efficient insecticide and gave very high kills for over seven months.(b) The DDT formulations, Murphy paste, Murphy wettable powder, suspensions of DDT crystals (c) The BHC formulations, P.520 and the oil-bound suspension “Supona” B, gave high kills for three to four months only.(d) The mixture of BHC and DDT in oil-bound suspension “Supona” DB gave the high initial kill of BHC and the long-lasting moderately high kill of DDT.(e) Against C. fatigans all the DDT formulations used in the third group of experiments gave very low kills, the BHC formulations high initial kills and dieldrin high long-lasting kills.BHC has marked fumigant and particulate properties lasting for three to four months. Dieldrin has a remarkable particulate action, which produces for the whole six-month period of the experiment, very high kills among mosquitos suspended without actual contact with the insecticidal surfaces; DDT only shows this particulate effect to a slight extent.It is probable that the differences in the toxicities to mosquitos of the insecticides used in these experiments is due partly to differences in the irritant properties of the insecticides. In the case of DDT many of the mosquitos having contact with this insecticide are irritated and escape from the treated surface before acquiring a lethal dose.

Mosquito larvae derived from eggs laid in earthenware pots in various habitats on the site of University College, Ibadan, in Nigeria were collected over a period of 26 months.Anopheles gambiae Giles, Aëdes aegypti (L.) and Culex (Lutzia) tigripes Grp. were the species most frequently collected.The relation of these results to the fluctuation in the populations of the species concerned is discussed. It seems unlikely that pot collections give results which can be directly related to the numbers of mosquitos in the field, but they can elucidate questions of mosquito biology and behaviour.

Mosquitos continue to enter occupied, untreated native huts throughout the night, with peak periods of entry at dusk and dawn. Early morning mosquito catches do not give a true picture of the numbers entering and leaving huts during the night.In a series of catches 63 per cent. of 5,576 mosquitos and 79 per cent. of 506 Anopheles gambiae were caught resting on the underside of the thatch roof.By using five traps inserted in apertures one foot below the top of the wall, the numbers of mosquitos attempting to leave a hut were determined. Of 1,014 mosquitos entering huts before 10 p.m., 63 per cent. remained inside until 6.30 a.m., that is for 8½ hours. Catches to estimate numbers entering and leaving at different times during the night were also made.Treatment of huts with DDT wettable powder and DDT-kerosene solution did not interfere with the normal behaviour of mosquitos as far as entry was concerned. Biting occurred in the treated huts.The DDT wettable powder appeared to be more effective than the DDT-kerosene solution.Some mosquitos entered the treated huts, fed and then left before acquiring a lethal dose. After making contact with treated surfaces mosquitos became restless but, under the conditions existing in the huts during the experiments, activation did not result in more leaving the treated huts than the untreated one. Unfortunately there were few A. gambiae and the predominant species entering the huts was Taeniorhynchus fuscopennatus.Some of the female A. gambiae released into unoccupied DDT-treated huts escaped into the traps before acquiring a lethal dose. Although there was a tendency for more to enter the traps of a DDT-treated hut than those of an untreated hut, the data are insufficient to show a significant difference.The majority of mosquitos entering the traps did so within one hour of their release.No mosquitos were still alive 12 hours after their release in huts treated 17 weeks previously with DDT wettable powder or DDT-kerosene solution, or in the hut treated 12 weeks previously with "“Gammexane” wettable powder.

Observations on the behaviour of Anopheles gambiae Giles and A. funestus Giles in experimental huts and ordinary African houses were carried out in a low-lying area of Tanganyika. Special attention was paid to the exodus of females at different stages of the gonotrophic cycle and to the composition in terms of ovarian development of the house resting population.About 5 per cent. of the biting population of gambiae and the same number of funestus were shown by the use of window traps to be leaving experimental huts after feeding.Forty three per cent. of half-gravid gambiae left an experimental hut at dusk, 12–24 hours after entering for a blood meal, whereas only 3 per cent. of funestus left under the same conditions.Daytime spray catches of gravid gambiae and funestus made in ordinary African houses during the hot season were about 50 per cent. and 30 per cent. lower, respectively, than the corresponding numbers of recently fed specimens. A. funestus therefore makes rather less use of outside shelters than gambiae during the hot months.In the cool season about 50 per cent. of half-gravid and gravid funestus were absent from house catches.No mortality could be detected amongst funestus females resting under natural conditions for 24 hours in an experimental hut.Re-entry of gravid funestus into unoccupied and open types of inhabited houses occurs at dawn.

In sugar-fed A. gambiae females, light may affect flight activity directly or by changing the phase of the circadian rhythm; both responses depend on the phase of the rhythm. The phase-response curve (1 h, 70 lux, signals given in the first cycle in DD following LD 12:12) shows a sharp swing, at about 3 h after normal light-off, from a maximum phase-delay to a maximum phase-advance, each of about 2 h. When signals are given at this time, phase re-setting is very variable; cyclical activity continues but the individuals are out of phase. Phase shifting appears to be a function of the energy of the signal. A 5 min, 70 lux signal has no apparent effect. The effect of a 1 h signal increases with intensity, up to at least 500 lux, but does not appear to be significant below 10 lux. Light normally inhibits flight activity, but there is a burst of activity at light-on (light-on response) if it occurs during the active half of the cycle following the initial activity peak. A vigorous light-on response occurs even at the lowest intensity used (0·3 lux).

1. 1) Two insectary strains of Anopheles gambiae, a typical “freshwater” strain and a “saltwater” variant, from East Africa, were found to be susceptible to infection with local strains of Plasmodium falciparum. 2. 2) The data indicated that the “freshwater,” or “Kisumu,” strain was slightly more susceptible to infection than the “saltwater” strain when fed on subjects with moderate to high gametocyte densities.

(1944). Differences Between Anopheles Melas and Anopheles Gambiae. Annals of Tropical Medicine & Parasitology: Vol. 38, No. 2, pp. 87-99.

Summary From January, 1952, through April, 1953, a series of dissections of wild-caught anthropophilic mosquitoes for larval worms resembling Wuchereria bancrofti was performed in Marshall Territory, Liberia. Advanced-stage infections were found in Anopheles gambiae, A. melas, and A. hancocki. In addition, natural infections have been found in A. funestus in a nearby locality in Liberia. No advanced-stage infections were found naturally in any culicine mosquito. Five species of laboratory-reared indigenous mosquitoes were fed on a human donor with microfilariae of W. bancrofti. A. gambiae, A. melas, and Culex thallasius were readily infected and carried the infection to maturity within thirteen to fourteen days. Culex fatigans and Aedes aegypti became infected in a much lower percentage, and development of the worms tended to be slower. In all three culicine species, many young nondeveloping larvae were noted in the thoracic muscles. In the small series reported here, Anopheles melas was more heavily infected with the intermediate stages of W. bancrofti in nature than Anopheles gambiae.

Assessment of the effects of dichlorvos (DDVP), released from a Ciba XI dispenser, on females of Anopheles gambiae Giles and Mansonia uniformis (Theo.) entering a verandah-trap hut in the Umbugwe area of Tanzania was made over a period of two months in 1964. Of the numbers of A. gambiae that entered one treated and one untreated hut, 27 per cent, of those entering the hut treated with dichlorvos and 48 per cent, of those entering the untreated hut left again. Of the numbers leaving each hut, 38 per cent, left through the eaves of the treated hut as compared with 9 per cent, in the untreated one. In the case of M. uniformis, 88 per cent, of those entering the treated hut and 94 per cent, of those entering the untreated hut left again. Of the numbers leaving each hut, 59 per cent, left through the eaves of the treated hut as compared with 61 per cent, in the untreated one.Over-all mortalities were 56 per cent, for A. gambiae and 34 per cent, for M. uniformis when the eave-egress fraction from the treated hut was taken into account, compared with 62 per cent, for A. gambiae and 43 per cent, for M. uniformis when the eave-egress fraction was ignored.The results of bioassays and of chemical analyses showed that the problem of mortality from fumigation in situ was considerably less in verandah traps than indoors or in window traps fitted with funnels of cotton netting.

The validity of the MLD (Minimum Lethal Dose) as a practical concept for measuring toxicity, despite its obvious lack of precision, is discussed. Its use has justification.The 48-hour MLD of acetone-water suspensions and oil films of DDT and dieldrin for early fourth-stage larvae of Anopheles gambiae Giles have been measured.The results show A. gambiae to be relatively resistant to DDT.In acetone-water suspension dieldrin appears to be about six times as toxic as DDT. In oil solutions this difference is more marked, the ratio being in the neighbourhood of 20:1.

Within the possibilities offered by an emergency laboratory in Aracati, Ceará State, we studied the resistance of the eggs of Anopheles gambiae, maintained out of water under different conditions of temperature and humidity. The resistance observed was insufficient to justify special mosquito-control measures. The same results were obtained with larvae and pupae at low temperature. The analysis of the data concerning the egg resistance showed that hatching depends on stimuli.

The eggs of A. gambiae var. melas are distinctly different from those of typical gambiae, and it is now regarded as a distinct species, A. melas. Other workers have found that the larvae also differ, physiologically and structurally. All adults with an extra dark band on the palps—4-banded forms— are known to be melas, but those adults with normal 3-banded palps can so far only be distinguished from typical gambiae by egg characters.A. melas is now known to be an important vector of malaria in coastal districts in West Africa. In some estuarine and mangrove swamp areas it may be even more important than typical gambiae. In melas caught in Freetown estuary (mostly from Wellington village), 42 out of 1,000 glands dissected were positive, giving a sporozoite rate of 4·2 per cent. for all months of the year. The oocyst rate was 4·7 per cent., and the total infection rate was 7·8 per cent.In Freetown estuary melas is rare in Freetown itself, and in the adjoining village of Kissy, but in all other parts of the estuary is at least as important as typical gambiae. In many places it is the dominant vector. In Wellington to the east of Kissy, and on the Bullom shore which forms the north shore of the estuary, melas forms about 90 per cent. of Anophelines caught in houses.

Towards the end of 1931 a study of the eggs of Anopheles mosquitos was commenced, primarily with a view to establishing any characters in the egg which may prove useful in differentiating closely allied species, and in particular those suspected of being concerned in the transmission of malaria.The eggs of ten species of Anopheles are described and figured below : mauritianus var. ziemanni, Grünb., implexus, Theo., pharoensis, Theo., theileri var. hancocki, Edw., christyi, N. & C., gambiae, Giles (costalis, Theo.), transvaalensis, Carter, funestus, Giles, marshalli, Theo., and moucheti, Evans. Of these the writer has had the opportunity of examining specimens of Anopheles gambiae and funestus from four different regions in Uganda, namely, Kampala (3,900 ft.), Jinja (3,800 ft.), Mbale (4,000 ft.) and Fort Portal (5,100 ft.), and no local variations were observed. The eggs of two species dealt with in this paper have been previously figured by workers in other parts of the world. Edwards (1921) reproduced a drawing by Patton of the eggs of Anopheles gambiae (costalis) from Arabia, and Theodor (1924) published a drawing of those of A. pharoensis from Palestine. Both illustrations show eggs which in general structure are very similar to those of the same species in Uganda ; the sculpture of the membrane is not figured. Individual variation within the eggs of a single adult was found to be constant in the case of A. marshalli, the three forms figured being represented in each of the three batches examined.

The effects of smoke from mosquito coils containing pyrethrins or DDT on Anopheles gambiae Giles and Mansonia uniformis (Theo.) were investigated by burning coils for five nights per week in three man-baited verandah-trap huts. Three other verandah-trap huts served as controls. Smoke from 004–0–3% pyrethrins coils deterred 24–84% of A. gambiae and 71–93% of M. uniformis from entering treated huts, and that from 7–3–130% DDT coils deterred 91 % of A. gambiae and 92% of M. uniformis.In pyrethrum-treated huts 62–76% A. gambiae were unfed (33–67% in the controls) and 80–99% were in the exit traps (43–79% in the controls). In DDT-treated huts 85% A.gambiae were unfed (48% in the controls) and 100% were in the exit traps (55% in the controls). With M. uniformis, 61–84% in the pyrethrum-treated huts were unfed (38–54% in the controls) and in the DDT-treated huts 71 % (69% in the controls). The effects of the smokes on the egress of M. uniformis could not be assessed since 82–100% were in the exit traps in the treated and in the control huts. Overall mortalities never exceeded 22% of A. gambiae and 12% of M. uniformis, but some dead mosquitoes may have been removed by ants. With pyrethrum coils, feeder-survivor indices were 15–84% for A. gambiae and 90–98% for M. uniformis, and with the DDT coils, 94% and 96%, respectively. When caged A. gambiae females were exposed all night in the huts, 24-h delayed mortalities caused by 019–0–30% pyrethrins coils were 38–56%, by 009% pyrethrins coils 12–27%, and by 004% pyrethrins coils 3–5%. DDT coils produced 99–100% mortality in 24 h, but this may have been due to contact with contaminated mosquito netting.

During studies on yellow fever in Bwamba county, Western Uganda, five non-stop series, each of six 24-h catches, were carried out in an uninhabited area of dense rainforest. All these yielded very large numbers of mosquitoes belonging to the Anopheles gambiae Giles complex. Measures of the central tendency, applied to summed results, show a “typical” pattern with a peak in the second half of the night. Examination of the results in detail reveals great variations in the biting pattern from day to day. These cannot be correlated with local weather conditions and their existence shows that considerable caution must be used in this type of analysis, except in well synchronised biting patterns.

A survey of Anopheles gambiae in Brazil was carried out in May, June, July, and the first week in August, 1939, months representative of the climate of the country at the close of the wet, and well into the dry, seasons. We were enabled to obtain in a comparatively short time a great deal of first-hand information about A. gambiae in Brazil and its effect upon the population as we were supplied with full facilities for transportation and field laboratories. I. DETAILED STUDY OF A FRONTIER Opportunity for a full study of the advance of a species of anopheles into new territory is not often offered to the investigator, and is a matter of much sanitary importance. We have therefore described in some detail the frontier shown on the map, representing a portion of the Jaguaribe River above and below Iguatu in Ceara.

Summary Owing to the position of Wadi Halfa on the Nile route, the control of A. gambiae is important to prevent it from travelling northward. Conditions affecting this and other species are described at some length because the Aswan Dam may be raised and produce further problems. The mosquito fauna is discussed in relation to the zoogeographical position of the area. Notes on each species and on seasonal changes in distribution of A. gambiae are given. It is probable that this species has occurred at Wadi Halfa for many years and that it winters in the Second Cataract. Methods of control, particularly of A. gambiae , are described, with special reference to the possible effect of the prevailing wind.