The Simulium ochraceum and S. metallicum species complexes in Mexico : identification, distribution and relation to onchocerciasis
Both S.ochraceum and S.metallicum, which are vectors of onchocerciasis, are now known to be species complexes (Hirai et in preparation; Conn, 1988). Larval collections were made in a variety of streams in the three onchocerciasis foci in Mexico and identified by cytotaxonomic criteria. S.ochraceum cytotype A was found in the Soconusco focus, cytotype B in the Oaxaca focus and cytotype C in the Chamula focus. Cytotype C was recorded in Mexico for the first time. S.metallicum cytotypes A, B, H and I were found in all three of the foci. Cytotypes B, H and I were recorded in Mexico for the first time. In addition a new cytotype, named X, was discovered in the Oaxaca focus. Attempts to analyse chromosomes of S.ochraceum adults were unsuccessful. The larval head patterns and body colouration of different cytotypes of S.metallicum were examined and variations were found between them, Cytotypes A, B, H and I could be distinguished in the majority of cases on these morphological differences. Cytotype X however resembled cytotype B. Cuticular hydrocarbon analysis by gas liquid chromatography, a technique to distinguish adult members of species complexes, was attempted on S.ochraceum and S.metallicum specimens. Some separation was achieved amongst S.ochraceum cytotypes A, B and C. The distributions and abundances of the larvae of different members of the S.ochraceum and S.metallicum species complexes and other Simulium species were investigated in relation to selected environmental variables, by multivariate analyses. Different cytotypes of each vector complex were found to be associated with characteristic stream conditions. The distribution of the different cytotypes of S.ochraceum and S.metallicum in relation to onchocerciasis is discussed.