Predation in Gerris (Hemiptera): Reactive distances and locomotion rates.

Two of the parameters which determine the rate at which prey are encountered by a predator, i.e. the distance at which a predator responds to a prey and its rate of movement relative to the prey's, were determined for all the stages of five species of Gerris using gerrids and Drosophila as prey. These parameters allowed calculation of the swath, or "encounter path", a gerrid would cover as it moved across the water surface. Gerris species prefer to attack live prey in front of them, and tend to ignore prey if the attack requires a turn of more than 100°. Hunger was found to affect the responsive angle required to clicit an attack by G. remigis, and regardless of species, smaller gerrids required the prey to be closer before an attack was initiated. The rate of movement in Gerris was measured as a function of stride length and the number of strides made per unit time. Stride length varied according to the length of the mesothoracic leg, and the frequency of movement was observed to be species specific. G. remigis, a stream species, moved 4-6 times as often as the four other species studied, all of which are characteristically found on non-moving water surfaces. Within a species, gerrid size had no significant effect on the frequency of movement, although there was a tendency for smaller gerrids to move less.