Circadian rhythms in behaviour and in the visual system of the blow fly, Calliphora vicina

Flies exhibit circadian rhythms in their locomotor activity and in the sens itivity of the visual system to light. In their visual system, circadian rh ythms have been detected not only in the retina but also in the first optic neuropile, the lamina. In the lamina two classes of interneuron, L1 and L2 , which receive photoreceptor input, exhibit daily size changes in their la mina axons. In principle, these two circadian rhythms, in the fly's behavio ur and in structural changes in the lamina, could both be controlled by the same oscillator or by different oscillators. To examine these two alternat ives we have studied both rhythms in the blow fly Calliphora vicina. The lo comotor activity rhythm of C. vicina was monitored in light/dark (LD 12:12) and in constant darkness (DD). After this, each fly was fixed and the cros s-sectional areas of L1 and L2's lamina axons were examined during the day and night in LD, and in the fly's active and rest periods in DD. In LD, L1 and L2's axons were larger during the day than during the night and the lar ger sizes correlated with the activity of the flies. Moreover, arrhythmic f lies showing a high activity level had larger cells than less active flies. In DD, however, both circadian rhythms became out of phase. The results in dicate a correlation between both circadian rhythms, which run in-phase in LD, larger cell sizes corresponding to higher activity in the flies. In DD, however, they run independently, so that these rhythms are probably genera ted by different circadian clocks. (C) 2001 Elsevier Science Ltd. All right s reserved.