Insect photoperiodism: Bunning's hypothesis, the history and development of an idea

In insects, the photoperiodic system comprises a linked sequence of events from photoreception to fi nal seasonally-appropriate phenotypes such as overwintering diapause. The fi rst and last of these events are reasonably well known, but central phenomena such as those distinguishing short from long days (time measurement) and the nature, accumulation and transfer of this information through development, metamorphosis and sometimes across generations remains obscure. Bünning's intuitive suggestion that photoperiodic time measurement was a function of the circadian system, made eight decades ago, however, has provided a framework for numerous studies investigating these connections. This review examines the development of Bünning's hypothesis from its origin in plants to the physiology of diapause in insects. Despite considerable inter-species differences, a close and probably causal relationship between circadian rhythmicity and photoperiodism is indicated.