Time-Measurement in Insect Photoperiodism

Abstract

Diapause in the European corn borer, Ostrinia nubilalis, has been shown to be induced and maintained by short-day photoperiods (12 hours light: 12 hours dark). Diapause development was found to be promoted by long-day photoperiods (16 hours light: 8 hours dark). The rate of diapause development appears to be determined by the interrelations among endogenous physiological rhythms that are sensitive to photoperiodic stimuli. A recently discovered endocrine function was shown to play a role in diapause development. The hormone proctodone, produced by specialized epithelial cells in the proctodeum, was found to stimulate the neuroendocrine system of the larval brain. The rate of diapause development was found to depend on interactions between proctodone and brain hormone production, both processes being influenced by photoperiod. The secretion of proctodone appears to be a rhythmic physiological process, with the timing of the rhythm being determined by the lights-off signal from the extrinsic photoperiod. The period of the proctodone-secretion rhythm was found to be 8 hours. Three such noncircadian cycles occurred during each 24-hour day. The lateral neurosecretory cells of the larval brain were also found to display a noncircadian activity rhythm having an 8-hour period. The phase-determining signal for the neurosecretory rhythm is apparently the lights-on stimulus. A theoretical explanation for diapause induction, maintenance, and termination was advanced. It was postulated that diapause would be induced and maintained when the larvae were continuously exposed to a short-day photoperiod, because such a photoperiod would result in neurosecretory and proctodeal rhythms being held in a physiologically unfavorable phase relationship (physiologically out-of-phase). A long-day photoperiod, on the other hand, would prevent diapause (or would hasten the completion of diapause development) because the two physiological rhythms would be held in a physiologically in-phase relationship. The possible significance of noncircadian rhythms in circadian and so-called time-measuring phenomena in insects and other organisms was discussed.