Retinal and extraretinal pathways for entrainment of the circadian activity rhythm in the blow fly, Protophormia terraenovae

The role of the compound eyes in entrainment of the circadian activity rhythm was examined in Protophormia terraenovae (Diptera: Calliphoridae). In intact females, the freerunning period of the rhythm under constant darkness was about 25.0 h. The rhythm entrained to light–dark (LD) cycles with activity restricted to the photophase. When the compound eyes were completely covered with silver paint and black synthetic resin paint, the rhythm freeran under LD cycles with the photophase of intensities 1.4×10−3 as well as 1.4 W/m2. Control flies showed complete entrainment. When the compound eyes and ocelli were surgically removed, the rhythm freeran under 1.4×10−3 W/m2 LD cycles but entrained to 1.4 W/m2 LD cycles. Flies subjected to a sham operation, in which the ocelli were removed, showed complete entrainment to LD cycles. When the compound eyes were removed and then the operated region was covered with the paint, some flies showed entrainment while others showed freerunning rhythm under 1.4 W/m2 LD cycles. The results show that P. terraenovae uses both extraretinal and retinal pathways for rhythm entrainment, and that the extraretinal receptors receive light passing principally through the compound eyes. It appears that entrainment is mediated primarily by signals from the retinal receptors at low light intensities.