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Flies exhibit circadian rhythms in their locomotor activity and in the sensitivity of the visual system to light. In their visual system, circadian rhythms 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 lamina axons. In principle, these two circadian rhythms, in the fly's behaviour and in structural changes in the lamina, could both be controlled by the same oscillator or by different oscillators. To examine these two alternatives we have studied both rhythms in the blow fly Calliphora vicina. The locomotor 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 cross-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 larger sizes correlated with the activity of the flies. Moreover, arrhythmic flies showing a high activity level had larger cells than less active flies. In DD, however, both circadian rhythms became out of phase. The results indicate 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 generated by different circadian clocks. |
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