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Abstract
Laboratory experiments on prey selection by Chaoborus larvae show that predator choice as well as differential encounter rates with prey determine the composition of the diet. As the larval crop and midgut become filled with food during a feeding bout, the predator avoids eating some available Daphnia and specializes on Diaptomus. After 3 days of starvation at 15°C, the gut system is empty and Chaoborus attacks prey indiscriminately. Then, daphnids are overrepresented in the diet because the predator encounters them more frequently than copepods of equal size. Daphnia swims about twice as fast as Diaptomus and “encounters” a stationary “electric eye” at twice the rate measured for cop epods. The strike efficiency of larvae for encountered prey is the same for both species.Since feeding selectivity is inversely proportional to larval hunger state, prey selection varies with the abundance of prey and season. In general, larvae collected during autumn have lower feeding rates and are more selective than larvae collected during summer. When food increases, previously opportunistic larvae may become selective within a few hours; but satiated larvae take several days to relax their preferences under a lowered food regime.