Circadian Rhythms in the Insect Photoperiodic Clock

Abstract

THE parasitic wasp, Nasonia vitripennis, is a long day insect which produces developing larvae in the summer months and diapausing larvae when the day-length falls below the critical value of 15.25 h light in 24 h (ref. 1). There is now strong circumstantial evidence that the “time measuring” ability, and thus the photoperiodic “clock”, of this species is based on a circadian rhythm of sensitivity to light which is itself phase set by the cycle of light and dark². In particular, night interruption experiments produce two peaks of diapause inhibition and support the coincidence model proposed by Pittendrigh and Minis^3,4, and daily chilling during the light or dark of cycles close to the critical day-length can reverse their photoperiodic effect⁵. This communication describes experiments in which females of N. vitripennis were subjected to night interruption experiments (in which the dark component of the cycle is systematically scanned by supplementary light pulses) together with a daily period of chilling in an attempt to explain these photoperiodic reversals.