Behavior of zygopteran nymphs in a simulated weed bed

The results are reported of some laboratory experiments intended to (I) gather baseline behavioral observations of zygopteran nymphs in a simulated rush bed, (2) test a hypothesis to account for switching by nymphs between alternative prey in the laboratory, and (3) quantify some components of nymph predation on cladoceran prey. The predators were Ischnura verticalis nymphs in the 2nd day of their final instar; their movements (strikes, captures, ignores, discards) were measured during 1-hr observation periods using event recorders. The experiments were designed as a factorial with 3 main treatments (hunger: starved 24 hr, fed; — illumination: light, dark; — prey: Daphnia, Simocephalus, none) and 7 replicates of each treatment combination. Analyses of variance and orthogonal contrasts performed on the data indicate that nymphs move around more when hungry, in the light, and in the absence of prey, and that predation rates are higher for starved nymphs, in the light and with Daphnia present. The data are consistent with the switching hypothesis, but a "dispersal” interpretation is offered as an alternative explanation. These results and other evidence also imply that vision can be important in prey capture; that actively swimming prey are much more vulnerable to this classic sit-and-wait predator; and that prey ingestion depends more strongly on the frequency of predator-prey encounters than on predation efficiencies.