Time measurement in insect photoperiodism: reversal of a photoperiod effect by chilling.

Spectacular reversals of the photoperiodic control of diapause are obtained if females of Nasonia vitripennis are chilled for 4 hours in certain lightdark cycles. Experiments in which chilling is combined with short light breaks (night interruptions) show that the first peak of diapause inhibition moves in response to the chilling. This result provides an explanation for the photoperiodic reversal; it is also circumstantial evidence for the participation of circadian rhythms in photoperiodism.