CHANGING RISK OF PREDATION FOR A FILTER-FEEDING INSECT ALONG A CURRENT VELOCITY-GRADIENT

Flume experiments were carried out to examine whether larval blackflie s (Simulium ornatum complex, Diptera: Simuliidae) use microhabitats wi th a high-velocity current to reduce the fisk of predation by some of their main predators, viz. larvae of the stoneflies Isoperla grammatic a and Diura nanseni (Plecoptera: Perlodidae), and the caddis-fly Rhyac ophila nubila (Trichoptera: Rhyacophilidae). We exposed blackfly larva e to four different current velocities and measured their feeding rate using dye particles. The maximum feeding rate was recorded at interme diate velocities (18.8 and 36.2 cm/s), whereas at low (7.3 cm/s) and h igh (53.3 cm/s) velocities, the feeding rate was reduced. In separate experiments, we investigated the behaviour and attack success of the d ifferent predator species. The two perlodids showed a similar hunting behaviour, which was significantly less successful at higher velocitie s. Drift lf the perlodids from the experimental arena resulted in redu ced encounter and attack rates, especially in I. grammatica, which had completely lost efficiency at 36.2 cm/s. R. nubila had a slower mode of hunting and was unaffected by current speed within the velocity gra dient studied. Drift in Rhyacophila was rare. Observations on the beha viour of blackfly larvae were performed in the same experiments. The l arvae showed no apparent ability to sense the presence of the predator s except when these disrupted the flow pattern or were in physical con tact, which often resulted in aggressive defence, though without effec t on the predators. Escape of blackfly larvae by drift did occur, but this was no more common than being captured. In a current velocity gra dient, blackfly larvae showed a weak preference for increasing velocit ies. Thus, at velocities between 7 and 54 cm/s, blackfly larvae appear to select microhabitats with high current velocities, despite a reduc tion in feeding optimality, thereby easing the predation impact from p erlodids, though not from Rhyacophila. The study demonstrates the impo rtance of microhabitat selection by blackfly larvae both for efficient feeding and predator avoidance.