Evolutionary Ecology of Case Architecture in the Snailcase Caddisfly, Helicopsyche borealis

Larvae of the caddisfly Helicopsyche borealis (Trichoptera: Helicopsychidae) build portable cases of sand grains which are spiraled and resemble snail shells. Distributional, architectural and experimental data were examined to determine the role of the helical case in the microdistribution of this species. Both crushing resistance and hydrodynamics were probable selection pressures in the evolution of the coiled case architecture. This evolution probably took place in open-canopied streams where H. borealis larvae were grazers on periphyton-covered rock surfaces. In this habitat, the spiral shape of the case prevented larvae from being crushed during spates. Hydrodynamic aspects of the case shape allowed the larvae grazing access on surfaces exposed to current.