Distribution of the Simulium metallicum complex in Mexico in relation to selected environmental variables

Larvae of the Simulium metallicum complex (Diptera: Simuliidae) were collec ted from three foci of human onchocerciasis in Mexico. Specimens were separ ated into five different cytotypes, identified by morphological characteris tics (head pattern and body colour) or polytene chromosome features. Differ ences were found between foci in the cytotype composition of the S. metalli cum complex. Nearly all specimens were cytotype A in the Chamula (97%) and Soconusco (86%) foci, In the Oaxaca focus, however, cytotype I predominated (63%) with 14% cytotype A. Cytotype I comprised only 6% of specimens in th e Soconusco focus and was very rare in the Chamula focus. Cytotypes B and H occurred only rarely in all three foci. Cytotype X was found only in Oaxac a. Environmental variables were measured at the collection sites and canoni cal correspondence analysis (CCA) was used to investigate the associations of cytotype distributions with the following factors: stream size, amount o f shade, water clarity, pH, temperature and altitude. Members of the S. met allicum species complex were found to be differentially distributed accordi ng to stream conditions and there was a significant correlation between the ir distributions and the environmental variables. The most important factor in canonical axis 1 of the CCA was pH (t-value -4.38) with temperature (-2 .48) and altitude (2.19) having some influence, but the other variables wer e unimportant. In the second canonical axis, pH (-3.52) was the only variab le having a significant effect. Thus, cytotype A was associated with high p H, high temperature and low altitude; B was associated with similar sites b ut with higher temperatures and at lower average elevations. Cytotype H was found at the centres of the ranges of these variables, I at sites with the lowest temperatures and highest altitudes and X in rivers with the lowest pH.