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Abstract

The adaptive importance of case constructions for the crawling behaviour of three coexisting Trichoptera with scraping larvae, Agapetus fuscipes (Glossosomatidae), Apatania fimbriata and Drusus annulatus (Limnephilidae), from a first-order stream was studied with respect to larval biomass, case composition, substratum roughness and crawling velocity. Larval biomass differed among the species, with A. fuscipes carrying a relatively heavy case compared to A. fimbriata and D. annulatus. The organic portion of the mineral case, mainly silk, was lower in A. fuscipes than in the two limnephilid species. The number of particles per case was lower, and the size range of the sand grains selected was higher in A. fuscipes than in the limnephilid larvae. Large sand grains were not selected by the limnephilid larvae, but contributed to the relatively high mass of the A. fuscipes cases. The energetic costs of case construction (silk : biomass) were not significantly different between species. Species-specific crawling velocity on substrata without biofilms correlated positively with larval size, and negatively with the relationship between case and larval mass. Crawling velocity and crawling distance were significantly less on rough than on smooth tiles, especially with A. fuscipes and A. fimbriata. These two species searched more intensively for food on rough than on smooth tiles. The species-specific differences in case construction and crawling behaviour seem to be important for resource partitioning and habitat use in these scrapers, enabling them to coexist.