Environmental factors associated with the distribution of Simulium perflavum (Diptera : Simuliidae) among streams in Brazilian Amazonia

To investigate the aquatic distribution of Simulium perflavum Roubaud, 97 s treams were sampled (1995-1996) over a wide (12 degrees 57' S-04 degrees 25 ' N; 67 degrees 42'-59 degrees 56' W) geographic area of Brazilian Amazonia . Simulium perflavum was collected in 40 (41.2%) of the streams sampled and reported for the first time from the states of Rondonia and Acre. Last ins tar larvae of S. perflavum were analyzed at the cytotaxonomic level, giving added certainty to the species-level identification. At each sampled site, 10 variables were measured (temperature, water velocity, depth, width, dis charge, streambed-particle size, riparian vegetation, canopy cover, pH, pre sence of upstream impoundments). Principal component analysis (PCA) and log istic regression were used to determine significant associations between st ream-site conditions and larval distribution. Based on stream-site characte ristics the occurrence (presence/absence) of S. perflavum larvae could be c orrectly predicted 75.3% of the time. Larval distribution showed a signific ant (p < 0.001) positive association with small, slow, sandy bottomed, open , streams near impoundments. Results agree with similar work in the Nearcti c region, suggesting that the general response of black fly distribution to environmental conditions might be universal in nature.