MONOPOLAR CELL AXONS IN THE FIRST OPTIC NEUROPIL OF THE HOUSEFLY, MUSCA-DOMESTICA L, UNDERGO DAILY FLUCTUATIONS IN DIAMETER THAT HAVE A CIRCADIAN BASIS

Two types of monopolar cell interneurons, each with a single represent ative in every unit cartridge of the first optic neuropil, or lamina, of the housefly's optic lobe, have axons that undergo cyclical changes in diameter. The axons are largest during the beginning of day in a n ormal LD light cycle and smallest during the middle of the night, chan ges that were however significant only for one of the cells (L2). The axon cross-sectional area and its cyclical change for both L1 and L2 w ere both larger in the proximal lamina. The changes are not a simple c onsequence of relative osmotic change. Dehydration paradoxically incre ases axon size, and also fails to alter the day/night rhythm of axon s ize changes. Under conditions of constant darkness, both axons decreas e in size, and one of the cells (L2) retains its cyclical size changes , being larger in the subjective day than in the subjective night. Und er conditions of constant light, both axons increase in size, and L2 a gain shows a cyclical sire change, just as under conditions of constan t darkness. These changes seen under constant conditions are, by defin ition, circadian in origin. The effects of exposure to light or darkne ss can partially reset these circadian changes. One extra hour of ligh t during the day increases the size of L1 and L2, whereas 1 hr of extr a dark during the night does not decrease their size. It takes 13 hr o f light to reverse the rhythm in size. The mechanism for all these cha nges is unclear but may involve ionic fluxes, possibly that are second ary to osmotic shifts and probably that involve at least two independe nt processes.