Sex chromosome variation and cytotaxonomy of the onchocerciasis vector Simulium squamosum in Cameroon and Nigeria

On the basis of sex chromosome variation, three cytotypes of Simulium squam osum (Enderlein) (Diptera: Simuliidae) are described from Cameroon and Nige ria. Simulium squamosum A is the typical form as originally described by Va jime & Dunbar (1975) with chromosome I as the sex chromosome. It occurs thr oughout most of Cameroon and south-east Nigeria. A second cytotype, S. squa mosum B, is described from the river Sanaga (Cameroon). It also has chromos ome I as the sex chromosome, but the nature of the sex differential region is different. Simulium squamosum C has no sex-linked chromosomal rearrangem ents. It is widespread in Nigeria and occurs near Mount Cameroon, where it seems to hybridize with S. squamosum A.