The Nymphomyiidae are one of the most specialized and distinctive families of nematocerous Diptera. These flies usually colonize cool, pristine, headwater streams where all life stages may frequent currentexposed habitats (0.5-1.0 m/s). Larvae are collector-gatherers or grazers, feeding on the thin films of algae, bacteria and other organic matter (= periphyton) on currentexposed rocks. Adults possess wings at emergence, but few details of flight behavior exist. Data for several species suggest that adults mate soon after emergence, crawl beneath the water in copula, and the female attaches eggs to the coupled bodies (Courtney 1994. Smithsonian Contributions to Zoology 550: 1-41). Adults of at least some species die in copula. In fact. A', walkeri (Ide) was for several years known only from "apterous" (i.e. dealate) adults, most as copulating pairs (e.g. Ide 1965. The Canadian Entomologist 97: 496-507; Cutten and Kevan 1970. Canadian Journal of Zoology 48: 1-24; Mingo and Gibbs 1976. Entomological News 87: 184-185). It is now assumed that the wingless condition is related to oviposition behavior. The vestigial mouthparts and poorly developed digestive tract suggest an ephemeral adult life. but adults of some species can survive several days in the laboratory (Courtney 1994).