Ted Gaten 1 , Stephen J. Huston 2 , Harold B. Dowse 3 & Tom Matheson 1

1. Ted Gaten1, Stephen J. Huston2, Harold B. Dowse3 & Tom Matheson1 • 1. Department of Biology, University of Leicester, Leicester, LE1 7RH, UK. • 2. Caltech Mail Code 139-74, 1200 E California Blvd, Pasadena, CA 91125, USA. • 3. School of Biology and Ecology, University of Maine, Orono, ME 04469, USA. 1. The Animals. Desert locusts (Schistocercagregaria) exist in one of two distinct phases. • 2. The System. The descending contralateral movement detector (DCMD) is a key collision detection system in locusts. • responds to looming stimuli • characteristic train of spikes • peak response just before collision • recorded extracellularlyover a 24 h period under constant light conditions Solitarious: cryptic colouration avoid other locusts fly only at night Circadian rhythmicity in the response of a collision detecting interneuron differs in solitarious and gregarious locusts Gregarious: brightly coloured form swarms fly during the daytime Time of peak firing Response of DCMD to a looming stimulus Upper trace: smoothed firing rate 3. Results. From the extracellular recordings we can measure the number of spikes per stimulus (A, B), the peak firing rate (C, D) and the time at which this peak firing rate occurs (E, F). • Middle trace: train of spikes recorded from DCMD • circadian rhythm in spike frequency & peak firing rate • minimum around expected dawn • gregarious maxima in mid-afternoon • solitarious maxima around expected dusk • no rhythm in the time at which peak firing rate occurs • The dotted line represents the collision time • Lower trace: angular size of the approaching object • 4. Conclusions. • The changes seen are not the result of a simple phase shift as the data for gregarious and solitarious locusts are congruent over most of the 24 h period. • Fourier analysis reveals the presence of a significant 12 h peak in the gregarious (but not solitarious) data. When superimposed on the 24 h curve, this results in an earlier peak. • This suggests that a separate clock is involved which is only activated by crowding. • Time of peak firing does not vary rhythmically, thus providing a consistent cue to initiate avoidance behaviour. • The remarkable phenotypic plasticity seen in locusts is shown to extend to finely matching the collision avoidance system to the flight times. Gregarious Solitarious Gregarious Solitarious Solitarious Gregarious Circadian rhythms in DCMD responses in gregarious (left column) and solitarious (right column) animals under constant light. Each point is the mean ± standard deviation of 6 animals and the data are fitted where appropriate with a four parameter sine function. A – relative number of spikes in gregarious animals (R2=0.7871). B – relative number of spikes in solitarious animals (R2=0.9221). C – relative peak firing rate in gregarious animals (R2=0.7003). D – relative peak firing rate in solitarious animals (R2=0.6697). E, F – time of peak firing for gregarious and solitarious animals. Vertical arrows mark the maxima of the fitted sine curves. Shaded areas represent time periods when the locusts would have anticipated darkness. Funded by