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Netless filter-feeding caddisflies in Europe

According to our present knowledge, European benthic filter feeders have evolved only in the insect groups of Ephemeroptera, Trichoptera and Diptera. Caddisflies have refined these techniques by net-spinning as shown by most families of Hydropsychoidea (Hydropsychidae, Ecnomidae and Polycentropodidae) as well as in Philopotamidae ; in addition, genera Allogamus, Brachycentrus and Drusus developed extraordinary sets of filtering bristles at femora, heads and sometimes sterna, sometimes combined with elongations of femora or with strange head morphologies. Dipterans use nets like genus Rheotanytarsus or a modified labrum with retractable head fans as in Simuliidae. This presentation focuses on the importance of filtering species among netless larvae of Trichoptera within stream communities. Within Brachycentridae only genus Brachycentrus has evolved bristles originating from elongated femora for optimizing filtering while the second genus Micrasema feeds on mosses and fine detritus and consequently shows no adaptations at all. Most limnephilids are shredding omnivores (with the exception of some grazers like Melampophylax) but the genus Drusus has developed a remarkable radiation in feeding types (grazers, shredders and filter feeders). As a genus should represent both an ecotype and a morphotype, these findings have important taxonomic consequences. In contradiction to the River Continuum Concept netless filter feeders among caddiflies cover the crenal parts of high altitude mountains from 2000 m a.s.l. down to epi- and metapotamal lowland rivers at 200 m a.s.l. in high densities. This phenomenon seems to be a European peculiarity as no netless filtering species of caddisflies are known in North America except family Brachycentidae (Wiggins, 1996). In Asia the genus Limnocentropus takes the role of filtering Drusus species and occupies a similar niche at high-current microhabitats of torrents. Its general morphology shows strong analogies to Drusus and Cryptothrix.

ecology; feeding types; caddisflies; Drusinae

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