摘要：We examined seasonal and diel variation in the diet and prey selection (Chesson's ) of field-collected predatory stoneflies (Acroneuria abnormis). In the laboratory, we investigated how stoneflies feed on two morphologically and behaviorally distinct prey types (Baetis sp. and Hydropsyche sp.) commonly found in the diet of A. abnormis. One A. abnormis was presented with either: (1) 20 Baetis sp., (2) 20 Hydropsyche sp. added 30 hours prior to stonefly addition, thus having time to construct retreats, or (3) 20 Hydropsyche sp. added just prior to stonefly addition, thus having no time to construct retreats. Stonefly behavior was videotaped at night during 30 minute trials in circular, plexiglass streams illuminated by infrared light. Numbers of prey encounters, attacks, and captures were recorded, and handling time was determined. Field-collected A. abnormis primarily consumed Baetidae, Heptageniidae, Hydropsychdae, and Chironomidae in spring, and Isonychiidae, Hydropsychidae, and Philopotamidae in fall. In spring, baetids ( =0.20) and chironomids ( =0.21), and in fall, isonychiids ( =0.32) and hydropsychids ( =0.29), were consumed preferentially. In spring, the number of baetids per stonefly gut was greatest at 2400. In fall, the number of isonychiids per stonefly gut was greatest at 0300. In the laboratory, A. abnormis encountered and consumed Hydropsyche sp. only when caddisflies were not given time to make retreats. We detected no statistically significant differences in the number of encounters per 30 minutes, attacks per encounter, captures per attack, and number of prey consumed between the Baetis sp. and Hydropsyche sp. (no retreats) treatments. Handling time for Hydropsyche sp. was much greater (212 ± 60 seconds, mean ± SE) than for Baetis sp., which was so short that it could not be measured. Our results show that prey abundance was important in prey selection by A. abnormis in the field, and that retreat-making by Hydropsyche sp. in the laboratory reduced their vulnerability to foraging A. abnormis.
