Previous studies on the morphological identification of adult female Simulium damnosum s.l. have involved use of colour characters. To determine the degree to which these qualitative characters are influenced by the physical environment, a portable rearing system was constructed. Wild-caught female flies and flies emerging from pupae maintained at ambient temperatures or above showed entirely pale antennae, fore-coxae, wing-tufts and hairs on the scutellum and ninth abdominal tergite, as is normal for S. sirbanum. Adults reared from larvae at below ambient temperatures were also pale, except for the antennae which were dark in all specimens. Adult females emerging from pupae maintained at below ambient temperatures were entirely pale up to the fourth day of emergence and mostly with dark antennae. The biological significance and the implications to the morphological identification of the savannah species of the S. damnosum complex in West Africa are discussed.