Adjustment of flight speed of gregarious desert locusts (Orthoptera: Acrididae) flying side by side

In tethered flying locusts, optomotor thrust responses induced by translatory pattern motion within the lateral visual fields were studied under closed-loop conditions. By modulating thrust in a compensatory manner, locusts counteracted a bias motion superposed on the thrust-related motion. This way, pattern speed was kept at 0° s^–1, indicating the set point of the respective optomotor control circuit. Though the quality of bias compensation varied greatly, it was largely independent from pattern characteristics. It might indicate that the gain of behavior not only is controlled by an automatic mechanism but also is affected by spontaneous modulations. Compensation of bias motion was critically dependent on the relation between self- and bias-generated motion: Locusts did not take control over pattern motion if self- and bias-generated motion differed greatly. Instead, locusts adopted a constant, supposingly preferred, thrust value. Therefore, flight speed is assumed to be controlled by two systems: the optomotor and a preferred thrust system. In free flight, an equalization of the flight speed of locusts within a swarm might result from similar behavior. In combination with a presumed coordination of the locusts' course direction, this may explain the continued cohesion of swarms in the field.