Chapter 9 - Photoperiodism and Seasonal Cycles of Development

Abstract

This chapter describes the principal photoperiodic phenomena that are observed in the insects. In classical photoperiodism, insects are able to distinguish the long days (or short nights) of summer from the short days (or long nights) of autumn and winter, and respond with seasonally appropriate switches in metabolism. Some species are active during long days; others are active during short days. Photoperiodic switches control diapause induction, diapause termination, seasonal morphs (polyphenism), growth rates, migration strategies, and a variety of associated physiological states. Diapause involves the temporary inactivation or alteration of the endocrine system, triggered by an appropriate photoperiodic stimulus acting on the brain. Larval-pupal diapause generally involves an inactivation of the brain-prothoracic gland system with a resulting low titer of ecdysteroids; adult, or reproductive, diapause occurs as an inactivation of the brain-corpus allatum system, a lack of juvenile hormone (JH), and gonotrophic dissociation. Some larval diapauses (in the Lepidoptera) are controlled by particular titers of JH, while some embryonic (egg) diapauses are imposed on the embryo by a maternally produced diapause hormone (DH). Insects at higher latitudes may enter diapause (hibernation) as winter approaches in response to short days (long nights) acting as a token stimulus. In areas with a hot, dry summer, insects may enter a summer (aestivation) diapause in response to long days. Within the tropics, insects may also enter diapause, often during a dry season, although day length might not be the environmental trigger.