Waterstriders are predators that hunt in steams. Often, prey is more abundant in the faster flowing water than in the still water at the edges and waterstriders will hunt in the current. The current displaces the waterstrider downstream and could potentially move it out of a desirable habitat. To compensate, waterstriders will make periodic rapid jumps in the upstream direction such that it maintains its average position in the stream. To do this, a waterstrider must compensate for the speed of the current. As the speed increases a waterstrider must increase either the length or the frequency of its jumps. Studies have found that the length of the jump remains constant but the frequency increases as the water flow increases. How does a waterstrider modulate its jump?
Experiments in artificial streams find that a waterstrider, Gerris paludum, can use a single discrete landmark to trigger its jump. As the waterstrider drifts downstream, the position of a landmark shifts in the field of vision. The insect can use the direction and speed of the landmark in its visual field to determine the direction and flow of the current. A landmark will trigger a jump when its image passes a critical point in the field of vision. The jump carries the waterstrider back upstream where it continues its drift downstream. Use of visual landmarks as triggers allows a waterstrider to maintain its position in a stream.
W. Junger. 1991. Waterstriders (Gerris paludum) compensate for drift with a discontinuously working visual position servo. J Comp Physiology A. 169:633-639.