a1 Virus Research Unit, West African Council for Medical Research, Yaba, Lagos, Nigeria.
An account is given of a survey of the mosquitos of the Jos area of Northern Nigeria, during May–June 1960, particular attention being given to those species likely to be concerned with the transmission of arthropod-borne viruses. The area concerned is in the savannah belt at an altitude of about 4,000 ft., with numerous rocky outcrops. The dry season is fairly severe, and extends from about October to May; during this time, drinking water for the villages is obtained from deep wells. At the time of the survey, that is, at the beginning of the rainy season, the most abundant species were Culex univittatus Theo., Aëdes ingrami Edw., A. aegypti (L.), A. africanus (Theo.), A. luteocephalus (Newst.) and A. vittatus (Big.). The biting cycles of these species are illustrated.
A survey was carried out of the breeding habits of A. vittatus in rock holes; those most favoured were about a foot across and six to nine inches deep, and appeared to be of human origin. Larvae appeared rapidly in the holes after the first rains, and predators, particularly dragonfly nymphs and tadpoles, were common in the larger rock holes. Under natural conditions, the duration of the life-cycle was about five days. The survival of the early stages under adverse conditions was investigated in simple laboratory experiments. At normal temperatures, the pupae did not survive desiccation for six hours, and, though the resistance of larvae to drying was not tested fully, it is considered that they are probably no more resistant than are pupae. Eggs survived desiccation for 10 but not for 18 weeks. Water temperature in exposed rock holes was not a factor limiting development, but the temperature of the exposed surface of dry rock holes was such that it appeared unlikely that eggs in these exposed situations would be able to survive the dry season. It was suggested that there may be collections of water in sheltered tree holes or rock holes which support a small but more or less continuously breeding population; this, combined with the ability of the eggs of A. vittatus to withstand desiccation for at least ten weeks at normal temperatures would enable the species to survive the relatively prolonged dry season.