CHAPTER 8 - THE PHOTOPERIODIC COUNTER

Abstract

This chapter describes the concept of the photoperiodic counter. Superimposed on the photoperiodic clock is a second mechanism called the photoperiodic counter, which accumulates successive long- and/or short-day cycles to a point at which diapause or development are determined. Both long and short days are added up, although the effects of the former are frequently more pronounced. In at least three species, Nasonia vitripennis, Acronycta rwnicis, and Sarcophaga argyrostomd, the summation of photoperiodic cycles has been shown to be a temperature-compensated process. In these species, the induction of diapause is controlled by an interaction between the length of the sensitive period and the number of short-day cycles needed to raise the proportion of diapause in a days' batch to 50%. These two components have different temperature coefficients with the latter showing a high degree of temperature-compensation. Insects raised at high temperature, therefore, reach the end of their sensitive period before seeing a sufficient number of inductive cycles; consequently, they develop without arrest.