Time-Measurement in Insect Photoperiodism

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Abstract
Diapause development in larvae of the European corn borer, Ostrinia nubilalis, was shown to be rate-determined by photoperiod, with the highest rates being promoted by long-day photoperiods (16 hr light, 8 hr dark per day). The effects of different photoperiodic conditions on the rate of diapause development showed that rhythmic physiological processes underly such development. The secretion of the hormone proctodone was shown to be a rhythmic phenomenon, with the time relations of the rhythm being determined by the lights-off signal of the extrinsic photoperiod. The period of the proctodone-secretion rhythm was 8 hours, and 3 such noncircadian cycles occurred during each 24-hour day. The lateral neurosecretory cells of the larval brain were also found to display an 8-hour rhythm. The phase-setting signal for the neurosecretory rhythm was the lights-on signal. It was postulated that diapause would be induced and maintained when the larvae were continuously exposed to a short-day photoperiod (12 hr light, 12 hr dark per day), because the proctodeal and neurosecretory rhythms would be held in a physiologically out-of-phase relationship. A long-day photoperiod would tend to prevent diapause and hasten diapause development, because the photoperiod would enforce phase relationships that would promote development.