Compensation of Visually Simulated Wind Drift in the Swarming Flight of the Desert Locust (Schistocerca Gregaria)

Abstract

ABSTRACT The visual control of translatory movements in the desert locust Schistocerca gregaria was investigated under open-loop conditions. When locusts were flown tethered in a wind tunnel, wind drift, visually simulated by ground pattern motion either in line with or transverse to their long body axis, induced a modulation of yaw-torque, thrust and lift correlated with the reversal of the direction of motion. Yaw-torque and thrust responses were independent of each other. Spon-taneous modulation of amplitude and differences in the time course of these responses indicate that a gain control mechanism is involved in the conversion of the visual stimuli to a behavioural response. Two opposing types of response were observed for each flight parameter and they were found equally often. They were elicited by either transverse or longitudinal pattern motion. The polarity of yaw-torque, thrust or lift responses was thus either positively or negatively correlated with the direction of pattern motion and was preserved throughout an experiment or reversed repeatedly. The yaw responses revealed a tendency for locusts to orient either upwind or downwind under the same stimulus situation. Modulations of thrust and lift confirm that locusts compensate for deviations of the retinal image flow from a preferred value by adjusting both air speed and altitude in free flight. They either speed up or slow down and either increase or decrease flight altitude under the same stimulus situation. The visually induced turning tendency often interacts with a variable internal turning tendency. The internal turning tendency might be responsible for the orientation menotactic to wind seen in the field. The threshold of optomotor responses in the visual control of translation is below 0.15°s-1 for both transverse and longitudinal pattern motion, indicating that wind-related orientation can occur at altitudes of several hundred metres. The orientation behaviour of locusts subjected to visually simulated wind drift depended on the transverse and longitudinal components of pattern motion and on internal factors. The observed variability of response is assumed to result from the locust’s ability to modulate independently the gain and sign of the optomotor responses for yaw-torque, thrust and lift.