Group size and anti-predator strategies in a marine insect

Abstract Predator avoidance is a primary function of the flotillas of the marine insect Halobates robustus (Hemiptera: Gerridae). With fish predators whose approach is unseen by the insects, Halobates that live in groups acquire a double protection: a lower probability of attack, as a result of a simple dilution effect, and a higher probability of escape if attacked. With visible predators, such as surfacefeeding fish and birds, this protection is augmented by predator-induced avoidance behaviour, the nature of which is related to the size of the group. In large groups, the highly aroused individuals respond quickly during a predator's approach and do not disperse, but maintain a high level of confusion behaviour within the aggregation. In small groups, the individuals are not highly aroused and respond only when the predator approaches very closely, whereupon they rapidly and synchronously disperse. Predator avoidance behaviour is postulated to result from a balance between the advantages of suppressing conspicuous escape movements until the last possible moment (the small group strategy) and the advantages of early warning and conspicuous confusion behaviour (the large group strategy).