1. The relationship between morphology and site-occupancy provides opportunities to infer differences in dispersal and flight ability, but empirical data for aquatic insects is limited.
2. In this study, 17 species of caddisflies from 10 families were collected from springs, streams and lakes, and total body mass, relative thorax mass, relative wing area (wing loading), and the aspect ratio of the fore and hind wings (combined) were measured.
3. Partial least-squares regression analysis of two independent distributional data sets produced significant models within which total body mass, relative thorax mass and wing loading were positively associated with site-occupancy, whereas aspect ratio was negatively associated with site-occupancy.
4. These results suggest that the faunal composition of streams is influenced by species dispersal abilities.