No abstract

No abstract

Rivers cease to flow for several months each year in many parts of Africa where Simulium damnosum Theo. occurs, and the means by which it survives this season has long been a subject of speculation. In many areas it is still impracticable to control this insect, which is the main African vector of Onchocerca volvulus and is therefore indirectly responsible for much blindness. It is to be hoped that dry-season studies will reveal a weak link in the annual cycle of this black-fly. If it were found that the insect survives in only one of its stages, it might be possible to devise suitable control measures and to determine the best time to apply them. have discussed the question, and De Meillon (1957) suggested the possibility of pupal diapause. It has been thought that eggs or adults might survive the dry season or that 8. damnosum could breed in isolated streams and spread from them in the rainy season. Crisp (1956, pp. I l l , 115) searched for dormant larvae and pupae in pools in Ghana without result, and he and Edwards (1956) examined many samples of sand from river-beds but found no pre-adult stages. Lamontellerie (1963) considered that in Ghana during the dry season S. damnosum existed below the confluence of the three Voltas, and that in the rains it spread thence past the Bolgatanga area into Upper Volta (fig-1).In our studies, made in northern Ghana, we use the term ' dry season ' in a special sense for the season during which running water is absent from rivers or is insufficient for the development of a significant number of S. damnosum from egg to adult, and which the insect must survive by some means at present largely unknown. Fig. 3 shows that at Shishi, for example, biting adults disappeared in early February 1962 when the river stopped flowing. These flies had presumablv emerged when the river was still flowing fast enough for pupae of S. damnosum to live, and long after the rains had ceased. Adults were again seen in mid-April, long after the next rainy reason had begun, but before the rivers had started to flow again. The ' dry season ' then, corresponds neither with the season of no rain nor with the period when the rivers are not flowing.J It varies from year to year and place to place.