Locusts Schistocerca gregaria take on a range of forms between two morphologically and behaviourally distinct extremes, the solitarious and gregarious phases. An identified visual interneurone, the descending contralateral movement detector (DCMD), has different response properties in the two phases (Matheson et al. 2003), which may be related to known differences in visually mediated behaviours. DCMD responds maximally to objects looming on a direct collision course. During flight or walking, most looming objects will appear in the frontal visual field as the animal moves forwards. When the locust is stationary on the ground, predators could approach from any direction. We tested the responses of DCMD to objects looming from different angular positions within the visual field.

DCMD in gregarious locusts (Fig. 1A) was excited more strongly by objects that approached from just above or below the horizontal plane (-15° to +15° elevation) than by objects that approached from ± 45 or ± 70°. For objects approaching at an elevation of +15° or -15° the sensitivity of DCMD was maintained for azimuths of 30-150° but fell off for angles of 0° and 180°, where 0° is straight ahead (Fig. 1B). DCMD in solitarious locusts generally responded with fewer spikes per approach than did DCMD of gregarious animals (Fig. 1A, B). The difference between phases was greatest for elevations of +15° and -15°, so that the tuning curve (in elevation) of solitarious animals was flatter than that for gregarious animals (Fig. 1A). The greatest relative difference in number of spikes per approach was at azimuth 0°, where DCMD of solitarious locusts produced 18 spikes and DCMD of gregarious locusts produced 37 spikes (Fig. 1B; a difference of 206 %). Overall, there was a significant effect of phase on number of spikes per approach across the visual field (GLM, F _{1, 222} = 5.34, P = 0.022). DCMD of gregarious locusts is relatively more sensitive than DCMD of solitarious locusts to objects looming from just above the horizon, particularly on a frontal trajectory. This difference may reflect adaptation to different behaviourally relevant stimuli. It may permit gregarious locusts to better avoid collisions when flying in swarms or marching in bands.

This work was supported by the BBSRC.