Components of predatory behaviour underlying density-dependent prey-size selection by Notiophilus biguttatus F. (Carabidae, Coleoptera).

Prey-size selection by the carabid beetle Notiophilus biguttatus F. was studied, using prey belonging to two-size-classes of the spring-tail species Orchesella cincta (L.) (Collembola). The highest values for attack readiness and success ratio of the attacks were obtained for the smaller prey animals. A model for prey-size selection was constructed in which these two parameters are included, predicting an increasing chance of incorporating large prey in the diet with declining total prey density. This prediction was tested experimentally and qualitatively confirmed. In a quantitative sense, however, the results deviated from the expectation. This was caused by a greater attack readiness of the beetles in a semi-natural environment (in which the prediction was tested) than in the artificial environment in which the parameters had been estimated. General relevance of the results, their bearing on optimal foraging theory and their ecological implications are discussed.