The Effect of Water Depth on the Predatory Behavior of the Water Stick Insect, Ranatra-Dispar (Heteroptera, Nepidae)

Abstract

The effect of water depth and prey density on the capture success (as measured by the number of prey eaten) of the aquatic, sit-and-wait predator, Ranatra dispar, was examined in the laboratory. When water depth was held constant at 5, 10, 20 or 30 cm, prey densities of 10, 15, 30 or 40 prey per container had no effect at the two shallowest depths, where very few prey were caught or eaten at any density. At 20 and 30 cm, density had a significant effect with progressively more prey being consumed as density increased. When prey density was held constant at 1, 2, 4 or 8 prey per litre, water depth significantly affected the proportion of prey eaten. At each density, predators held in 5 cm of water and, to some extent, those in 10 cm consumed significantly fewer prey than those in 20 or 30 cm. It is suggested that fewer prey are caught in shallow water because: (1) the predator cannot put itself in the position which most favours an efficient strike; (2) the absence of a significant 'vertical' component in the environment reduces the effectiveness of the sensory organs in locating prey. The lower capture success in shallow water may cause adult R. dispar to avoid shallow water, thus providing a partial refuge in which small R. dispar can avoid competing with adults for prey of similar sizes.