Density-Dependent Prey Selection in the Water Stick Insect, Ranatra linearis (Heteroptera)

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Abstract
(1) To test some aspects of foraging theory, all stages of the water stick insect Ranatra linearis L. were presented with a full range of size classes of water fleas (Daphnia) at different relative and absolute densities, under controlled conditions in the laboratory. (2) Selection proved to be density-dependent. First instar nymphs showed a preference for small Daphnia; after the first moult the choice changed and as the predators grew, their preference for big Daphnia increased. Ranatra were selective, first when relative densities for the preferred size class were low, then when absolute densities were high. The choice was generally made before striking, but selection was reinforced before rostral exploration. Size of prey, size of predator and prey density influenced the duration of a meal. (3) Adults increased their selectivity during a test in the presence of high prey densities. Adults performed more complex predatory movements towards large prey items than towards small ones, and more towards both size classes in low densities than in high ones.