Studies on Anopheles gambiae and A. melas in and around Lagos

The differences between the eggs of A. gambiae and A. melas form an absolutely reliable diagnostic character for distinguishing the two species in the Lagos area. Identification of eggs laid by wild-caught females has formed the basis of work on the seasonal incidence of gambiae and melas adults in houses in different localities in and around Lagos. In the area as a whole both melas and gambiae are abundant in most localities, but the greater part of Lagos town on Lagos Island is an almost pure gambiae area. The sporozoite rate of gambiae —10·0 per cent.—is consistently much higher than that of melas —3·5 per cent.—in all catching stations. Fluctuations in sporozoite rate in relation to population density have been studied in both species. The composition of the mosquito population in houses, and in outside resting places has been studied in relation to the movements of mosquitos into and out of houses. Melas makes use of outside resting places to about ten times the extent of gambiae . In African village houses about 90 per cent. of the blood feeding by these two species takes place after midnight, the peak of activity being the hour or two before dawn. An improved design of experimental hut and window trap is described. It has been used in studying the exodus of mosquitos from houses after feeding, and on experiments on host selection by Anopheles . Swarms of male “ gambiae ” in nature have been described. In captivity both melas and gambiae have been persuaded to mate by using artificial light. By cross fertilisation between melas and gambiae a generation of healthy hybrid adults can be obtained. Of these the females are evidently normal sexually, while most if not all of the males are sterile. The breeding places of melas and gambiae in and around Lagos have been studied in relation to control. In this lagoon area, where conditions are very different from those of Freetown Estuary, melas breeds under a variety of conditions, and Avicennia mangrove is only of limited value in indicating major breeding grounds.