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 rhyt hm was examined in Protophormia terraenovae (Diptera: Calliphoridae). In in tact females, the freerunning period of the rhythm under constant darkness was about 25.0 h. The rhythm entrained to light-dark (LD) cycles with activ ity restricted to the photophase. When the compound eyes were completely co vered with silver paint and black synthetic resin paint, the rhythm freeran under LD cycles with the photophase of intensities 1.4x10(-3) as well as 1 .4 W/m(2). Control flies showed complete entrainment. When the compound eye s and ocelli were surgically removed, the rhythm freeran under 1.4x10-3 W/m 2 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 regio n was covered with the paint, some flies showed entrainment while others sh owed freerunning rhythm under 1.4 W/m2 LD cycles. The results show that P. terraenovae uses both extraretinal and retinal pathways for rhythm entrainm ent, and that the extraretinal receptors receive light passing principally through the compound eyes. It appears that entrainment is mediated primaril y by signals from the retinal receptors at low light intensities. (C) 2001 Elsevier Science Ltd. All rights reserved.