The group size – flight distance relationship in water striders (Gerris remigis)

Abstract

When food availability is constant, the distance from which first instar water striders (Gerris remigis Say) initiate escape from approaching (potentially cannibalistic) adults initially rises and then falls with increases in group size. This result cannot be explained by the currently accepted many-eyes hypothesis, which predicts a monotonically increasing relationship between flight initiation distance and group size, resulting from the increased probability of predator detection by larger groups. The results are consistent with an alternative hypothesis, according to which the preferred flight distance results from a trade-off between the costs and benefits of flight and declines with increasing group size due to risk dilution. The gerrids are constrained from achieving their preferred flight distance when tested as individuals, and the adult is able to approach more closely before being detected.