Larval Specialization and Phenotypic Variation in Arctopsyche Grandis (Trichoptera: Hydrospsychidae)

Abstract

Life history, trophic dynamics, abundance, and microdistribution of Arctopsyche grandis (Banks) were investigated in the Flathead River Basin, Montana. Two morphologically and ecological distinct larvae (Type I, with a head stripe and Type II, without a head stripe) were found throughout the drainage except in lower order streams. Type II larvae grew more rapidly and attained a larger size in final instar than Type I larvae. In areas where A. grandis biomass was greatest, Type I larvae were >10 times as abundant as Type II larvae. Type II larvae selected microhabitats characterized by larger interstitial spaces; Type I larvae were more common in tightly compacted substrata. Food items consumed by both larval phenotypes varied between sites, indicating a natural variability in the environment. Significant differences in foods ingested were also observed between larval types within particular riverine locations, suggesting phenotypic differentiation in food habits. Larvae of both phenotypes were reared in laboratory streams. Type I were both male and female, but all Type II were female. We concluded that the presence of Type II larvae increased resource utilization and species fitness.