Set point of retinal velocity of ground images in the control of swarming flight of desert locusts

Summary

• 1. The visual control of ground speed in the flight of gregarious desert locusts, Schistocerca gregaria, was investigated under open- and closed-loop conditions. Locusts were flown tethered in a wind tunnel, and ground pattern motion in line with their longitudinal body axes was used to simulate the visual effects of headwind and tailwind while the locusts' flight thrust and yaw-torque were monitored.

• 2. In open-loop experiments, a correlation between thrust and pattern speed modulation was observed only when pattern speed modulation crossed 0°/s, (i.e. when the pattern direction of movement reversed). Otherwise thrust was kept constant at one of two extreme levels, depending on whether patterns moved progressively or regressively. When pattern motion was stopped intermittently, thrust production approached a mean level between the two extremes, which was assumed to be a “preferred” value.

• 3. In closed-loop experiments, a regressive bias of pattern motion used was counteracted by a compensatory modulation of thrust. Pattern speed was in this way minimized. Quality of bias compensation differed both between experiments and during an experiment. In some of the experiments, the residual mean slip speed, though variable, was proportional to the strength of the bias. In other experiments the pattern was stopped, except for a small amount of positional jitter, over a wide range of bias modulation.

• 4. Compensation for the pattern motion bias was restricted to a limited range of bias modulation. Beyond this range, thrust was always strongly increased, independent of whether the biased motion was too slow or too fast for compensation by thrust modulation.

• 5. The thrust response was often combined with a modulation of yaw-torque, especially when the biased pattern speed was beyond the range of compensation by thrust.

• 6. The results from both open- and closed-loop experiments indicate that in visual control of ground speed locusts tend to minimize the retinal velocity of ground images. It is suggested that this may be a general characteristic of the visual flight control system in swarming desert locusts.