This research attempts to identify causal factors responsible for divergence within the black fly Simulium arcticum complex. Earlier analysis at the landscape level suggested that sex-chromosomes of larvae of the S. arcticum complex were more similar to those of larvae in the same river corridor even when sites in different corridors were closer in Euclidian distance. However, this was not always the case; some sites in the same corridors had larvae with very different sex chromosomes. To investigate these phenomena at the local level, sex chromosomes of larvae of S. arcticum at new sites were analyzed and compared to those of previously analyzed sites. Twelve of the 14 comparisons in the present study had larvae with either different sex chromosomes or larvae that had different frequencies of sex chromosomes, or both. These observations suggest that while there may be a river corridor effect, gravid female black flies may also be choosing oviposition sites that are ecologically and physically appropriate for survival of their offspring. Additional studies of ecological and physical parameters at the microhabitat level are encouraged.