Localization of the clock controlling circadian rhythms in the first neuropile of the optic lobe in the housefly

The visual system of a fly expresses several circadian rhythms that have be en detected in the photoreceptors of the compound eye and in the first neur opile, the lamina, of the underlying optic lobe. In the lamina, axons of tw o classes of interneuron, L1 and L2, exhibit cyclical size changes, swellin g by day and shrinking by night. These rhythmic size changes may be generat ed by circadian oscillators located inside and/or outside the optic lobe. T o localize such oscillators, we have examined changes in the axonal cross-s ectional areas of L1 and L2 within the lamina of the housefly (Musca domest ica) under conditions of 12 h of light and 12 h of darkness (LD12:12), cons tant darkness (DD) or continuous light (LL) 24 h after the medulla was seve red from the rest of the brain. After the lesion, the axon size changes of L1 and L2 were maintained only in LD conditions, but were weaker than in co ntrol flies. In DD and LL conditions, they were eliminated. This indicates that circadian rhythms in the lamina of a fly are generated central to the lamina and medulla neuropiles of the optic lobe. Cyclical changes of light and darkness in LD conditions are still able, however, to induce a weak dai ly rhythm in the axon sizes of L1 and L2.