PHOTOPERIODIC CONTROL OF A PHYSIOLOGICAL RHYTHM

Abstract

1. In mature diapausing larvae of the European corn borer, Ostrinia nubilalis (Hübner), the epithelial cells of the anterior portion of the proctodeum were observed to show evidence of a regular cycle of synthesis and secretion. The cycle involved progressive accumulation and subsequent disappearance of paraldehyde-positive, autofluorescent cytoplasmic inclusions. For the purposes of this study, the secretory cycle was divided into a series of 5 arbitrary secretory stages. 2. The proctodeal secretory activity was shown to constitute a rhythmic function. The rhythm was found to be non-circadian, in that it displayed an approximately 8-hour period, such that three complete secretory cycles occurred during each 24-hour day. 3. The proctodeal secretory rhythm was shown to be phase-set by photoperiod. The release of secretions (loss of intracellular granules) occurred in response to the beginning of the scotophase (lights-off stimulus) and endogenously every 8 hours thereafter. 4. A thermoperiod that was superimposed on the photoperiod, so that a low temperature was concurrent with the scotophase, had the effect of intensifying the physiological response to the photoperiod. 5. From two to ten days were required for the secretory rhythm to re-synchronize with a four-hour change in the clock time of the lights-off signal. Because of great individual variability, the number of transient phase responses could not be determined.