Chapter 12 - The Photoperiodic Counter

Abstract

This chapter examines the formal properties of the photoperiodic counter in detail, particularly its interactions with temperature, latitude, and diet. Superimposed on the photoperiodic clock is a mechanism called the photoperiodic “counter” that serves to accumulate successive long nights (short days) or short nights (long days) to a point at which diapause or diapause-free developmental pathways are determined. Both long nights and short nights may be accumulated, although the effects of the latter may be physiologically stronger. In several species, the accumulation of photoperiodic cycles during the sensitive period is a temperature compensated process. Induction of diapause or a seasonally appropriate morph is frequently effected by an interaction between the length of the sensitive period (SP) and the number of long or short nights needed to produce a 50% response (the required day number, or RDN). These two components have different temperature coefficients, the SP being temperature sensitive, but the RDN having a high degree of temperature compensation. Insects raised at high temperature, therefore, may reach the end of the sensitive period before seeing a sufficient number of inductive cycles. On the other hand, at a lower temperature, they may see a sufficient number of inductive cycles before the end of the SP, and the alternate pathway is followed.