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Influence of Plecopteran and Megalopteran Predators on Hydropsyche (Trichoptera:Hydropsychidae) Microdistribution and Behavior

In laboratory stream channels we investigated the influence of two predaceous aquatic insects on the substratum particle size preference and drift of a prey species. Predators were the stonefly Acroneuria lycorias and the dobsonfly Corydalus cornutus. The prey organisms were Hydropsyche bronta. In the absence of predators, no significant substratum preference was noted except for the strong avoidance of fine sand particles 0.5-1.0 mm in diameter. In the presence of predators, however, small substratum (13-33 mm) was significantly preferred over large substratum (53-175 mm). By choosing small substratum, Hydropsyche found some refuge from predation. This conclusion is based on our finding that mortality resulting from predation was lower on small than on large substrata. The presence of Acroneuria caused increased drift of Hydropsyche; however, Corydalus did not induce Hydropsyche drift. Drift was used by Hydropsyche both as an escape mechanism and as a method to redistribute at the microhabitat level. The major influence of the predaceous aquatic insects on Hydropsyche habitat selection probably occurred during the prey's active colonization period, during which Hydropsyche became redistributed in a new microhabitat based on substratum size and presence or absence of predators.