Variation and distribution of forms of Simulium soubrense and S. sanctipauli in West Africa

Examination of specimens from savanna areas of Togo and Benin attributable to Simulium soubrense and S. sanctipauli according to the criteria of Vajime and Dunbar (1975) revealed hitherto undescribed features. Cytological examination showed many larvae with the arrangement IIL-4.6/4.6.7, heterozygous for the 'diagnostic' inversion and 86% of the males possoessed a previously undescribed inversion IIS-6b. The occurrence of such larvae in Togo and Benin was associated in adult males with records of a type of scutal pattern which is described and termed Type IV. This pattern is one extreme of a range of scutal pattern variation in the males of the S. soubrense/S. sanctipauli sub-group, which was separated into four arbitrary classes and specimens were scored as Type I, II, III or IV. The distribution of these different types of male scutal pattern, based on collections made in West Africa between 1963 and 1981, is presented. Type I and II patterns predominated in forest areas and Type IV in savanna. Many female flies which occurred together with the latter possessed pale wing tufts. In the light of the above and additional evidence, derived from the progeny of individual females, and because the 'diagnostic' inversion for S. soubrense and S. sanctipauli together with a previously undescribed sequence IIL-6 occurred in Hardy-Weinberg equilibrium, it was concluded that the populations in Togo and Benin are a single interbreeding group. This group is described as a new form, named the Beffa form, of the S. soubrense/S. sanctipauli subgroup of the S. damnosum s.l. complex and its possible taxonomic status is discussed.