SHORT COMMUNICATION PEERING - A LOCUST BEHAVIOUR PATTERN FOR OBTAINING MOTION PARALLAX INFORMATION

School of Biological Sciences, University of Sussex,Brighton BNi gQG, Sussex, U.K.(Received 12 April 1978)Motion parallax is probably the most important method available to insects forjudging the distance of objects, yet with one exception very little is known about theways in which insects exploit this cue. Almost twenty years ago Wallace (1959)proposed that the side-to-side peering movements of juvenile locusts are performedspecifically to obtain parallax information. Leg movements cause the whole body topivot about the abdomen, shifting the head laterally about 0-5 cm and displacing theretinal image of an object in front of the insect. Provided that the locust knowshow far (or fast) it moves its head and that it can measure image displacement (orvelocity) it can compute the distance of the objec 1t a). (Fig Th.e following suggestslocusts do indeed do something of this kind. When an object on to which a locust isabout 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 objectis moved in the opposite direction to that in which the locust peers, so enhancingimage movement, the locust jumps short (Wallace, 1959).Two features of peering are analysed here which suggest that the behaviourpattern is carefully designed for extracting parallax information.(1) If the head were to rotate during peering there would be added an unwantedcomponent to movement of the retinal image. This would be independent of thedistance of objects in the environment, and would distort the simple relation-betweenimage displacement and distance shown i 1n (a). Fig Film. s of head movement takenduring peering show that the angular orientation of the head remains constantdespite rotation of the body.(2) The precision with which distance can be estimated during peering is limitedby the amplitude of head movement and by the insect's horizontal acuity. Theaccuracy also decreases as the square of the distance between locust and target(e.g. Horridge, 1977). It is shown here that the locust increases its amplitude ofpeering when object distance is raised and so to some extent is able to compensatefor the inherent decline in accuracy with distance.Observations were made on 4th-insta Schistocerca gregariar obtained from alaboratory culture at the University of Sussex. To measure hea (^n^dd orientatio) na 1 cm length of thin needle was stuck to the top of the locust's head and peeringrecorded on 16 mm film at 25 frames.s"