Anopheles gambias giles and anopheles melas Theobald in a coastal area of Liberia, West Africa☆

Abstract 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.