SummaryWe provide an inventory of all named cytoforms of the Simulium damnosum complex (including those which are now considered invalid), along with all inversions that have been recorded (including synonyms and homonyms). There are 55 valid and distinct cytoforms known from the S. damnosum complex making it the largest sibling species complex of any vectors, and probably of any insect or other animal. All cytoforms are listed along with their fixed and diagnostic inversions and country distribution. There are 183 inversions known from the complex as a whole, of which 49% are fixed and ⁄ or diagnostic between cytoforms, and the fixed ⁄ diagnostic inversions seem to occur disproportionately on chromosome arm 2L.

Cytotaxonomic analysis of the polytene chromosomes from larvae of the Simulium damnosum Theobald complex from the island of Bioko in Equatorial Guinea is reported, and a new endemic cytoform is described. Chromosomally this cytoform is close to both S. squamosum (Enderlein) and S. yahense Vajime & Dunbar, but is not identical to either. However, it is morphologically and enzymatically identical to S. yahense. The Bioko form was also found to differ from other cytoforms of the S. damnosum complex in West Africa in the copy number or RFLP pattern of several different repetitive DNA sequences. It is clear that the Bioko form is genetically distinct from other populations of the S. damnosum complex, and whilst it is closest to S. yahense, it shows features that suggest a high degree of geographical and genetic isolation. Such isolation is an important consideration in the assessment of the potential for onchocerciasis vector eradication on Bioko.

A new cytotype of Simulium squamosum Enderlein (Diptera: Simuliidae) (here named S. squamosum'D') is described from around Mount Cameroon in south-west Cameroon on the basis of sex-chromosome differences on the short arm of chromosome I. Two polymorphic inversions, IS-To (a new inversion) and IS-2, are partially Y linked. These inversions usually occur together, although IS-To has a higher frequency and is more strongly Y linked than IS-2. This sex-chromosome system has not been previously described and the occurrence and evolution of sex-chromosome systems in S. squamosum is discussed.