Observations were carried out, chiefly between March and June 1962, in an area centred on Bolgatanga in northern Ghana in the hope of finding out how Simulium damnosum Theo. survives the ‘ dry season ’. The term ‘ dry season ’ is used here for the unfavourable season when running water in the rivers is evidently absent or is insufficient or perhaps unsuitable for the development of any, or more than a negligible number of, individuals of S. damnosum from egg to adult. The incidence of this season varies from year to year and does not correspond to the season without rain. It may usefully be divided into reduced-flow, main dry, pre-flow and early-flow periods.
Search was made for larvae and pupae before rivers began to flow, without result. Observations were made on the numbers of flies biting, general appearance, wing measurement, antennal colour, mid-gut, fat-body, ovaries and parasites, chiefly at the beginning of the rainy season, in an attempt to trace the history of the species during the preceding weeks. Flies appeared before the rivers began to flow but did not give evidence of having lived for months. On the days when two rivers began to flow, considerable numbers of flies were caught, and most of those dissected proved to be parous and had evidently been in the adult stage for some days. A nematode parasite of the Malpighian tubes was found only in April, with one exception up to 10th May, and was probably associated with the dry-season biology of the host.
The possibility that S. damnosum migrates from permanent foci, or exists near its breeding places in one or more stages at different times, is discussed. Migration from afar seems very unlikely. The species survives in the adult stage for at least part of the ‘ dry season ’, but more information is required to complete the picture.
Survival is apparently not dependent to a significant extent on perennial breeding locally, and there is, therefore, no easy solution to the problem of control. Operational research might show if there is a carry-over generation and if control after the rains and towards the end of the breeding season would prevent it from developing. There is some indication that the risk of infection with Onchocerca tends to be low at the end of the ‘ dry season ’, which is a time when control would be particularly difficult.
The results of observations on wing measurements, antennal colour and the peritrophic membrane are given in an Appendix.