PREDATORY FORAGING BEHAVIOR AND ACTIVITY PATTERNS OF LARVAE OF 2 SPECIES OF LIMNEPHILID CASED CADDIS

Temporal feeding behaviour, locomotory activity patterns and functional responses of larvae of two species of cased caddis were investigated in the laboratory. The temporal pattern of locomotory activity of both Poramophylax cingulatus (Stephen) and Halesus radiatus (Curtis) was examined under constant conditions of 12 h light/12 h darkness, continuous light and continuous darkness. Larvae of both showed a similar endogenous rhythm of activity consistent with the species being nocturnal and were most active at night during a 4 h period (2000-2400 h). They also demonstrated a greater consumption of prey at night and under the continuous darkness conditions. This feeding activity pattern appears to be based primarily on intrinsic predator activity patterns rather than generated as a response to prey activity. The response of larvae of the two species to a range of prey densities (Baetis rhodani (Pictet) nymphs and Orthocladinae chironomid larvae) was monitored in the laboratory in a weak current under ambient temperature and light conditions. Fourth and fifth instars of H. radiatus and fifth instars of P. cingulatus demonstrated sigmoid Type Ill functional responses. For both species a threshold density (15/20 prey/arena -56cm2) was evident above which the predator was able to increase its predation rate considerably, H. radiatus showed a higher attack rate. Fifth instar H. radiatus also showed a trend for higher attack rate than fourth instars. Both species are generally considered detritivorous, but they demonstrated strong predatory behaviour in their penultimate and final instars in the laboratory. The Type III functional responses shown by the two species are theoretically potentially stabilising in nature, thus these caddis larvae could play a more significant role in macroinvertebrate communites than generally considered.