Seasonal size variation in females of the Simulium damnosum complex in the Ivory Coast.

The thorax lengths of monthly samples of female S. damnosum s.l. caught between August 1975 and February 1977 at Danagoro in the Ivory Coast were measured. The blackflies could be separated into three species groups according to their external characters. Flies attributable to S. damnosum/S. sirbanum and S. soubrense/S. sanctipauli were common. S. yahense was occasionally found. The flies of the S. soubrense/S. sanctipauli group were on average consistently larger than those of the S. damnosum/S. sirbanum group within any one month. The mean sizes of these two groups varied in parallel from month to month and this variation was related to the rainfall in and around the drainage basin. This relation is discussed with respect to possible changes in the discharges of local rivers and their effects on larval development.