ABSTRACT
Motion parallax is probably the most important method available to insects for judging the distance of objects, yet with one exception very little is known about the ways in which insects exploit this cue. Almost twenty years ago Wallace (1959) proposed that the side-to-side peering movements of juvenile locusts are performed specifically to obtain parallax information. Leg movements cause the whole body to pivot about the abdomen, shifting the head laterally about 0·5 cm and displacing the retinal image of an object in front of the insect. Provided that the locust knows how far (or fast) it moves its head and that it can measure image displacement (or velocity) it can compute the distance of the object (Fig. 1 a). The following suggests locusts do indeed do something of this kind. When an object on to which a locust is about to jump is moved horizontally in a frontal plane, in synchrony with peering, locusts are fooled into misjudging the object’s distance. For instance, if the object is moved in the opposite direction to that in which the locust peers, so enhancing image movement, the locust jumps short (Wallace, 1959).
