ReviewInsect photoperiodism: Measuring the night

In studies of photoperiodic induction of over-wintering diapause, independent variation of the light (L) and dark (D) components of the daily (LD) cycle shows, with few exceptions, that the duration of the night (D) is more important than that of the day (L). Extensions of D to give cycle lengths up to 3 days or more in so-called Nanda-Hamner (NH) experiments suggest that night length is measured repeatedly in the extended night, with peaks of high diapause incidence occurring at intervals close to 24 h. This indicates a circadian involvement in night length measurement. The circadian oscillation revealed in NH experiments is shown to take its principal time cue from the beginning of the night - at a phase close to Circadian time, CT 12 - in series of such experiments with increasing light (L) components, in a manner comparable to other circadian oscillations such as that controlling the adult eclosion rhythm. It is considered that the photoperiodic circadian oscillation is causally involved in the discrimination between short (summer) and long (autumnal) nights, although further 'downstream' actions of the circadian system on the outcome of time measurement are also likely. Therefore Bünning's original hypothesis - or development of it - is considered to offer the most likely explanation for the photoperiodic mechanism.

► Independent variation of light (L) and dark (D) indicates that D is more important than L. ► With L constant and D up to 72 h, diapause incidence occurs rhythmically with a circadian period. ► When L and D are both varied, circadian rhythmicity starts at light-off. ► Data suggest that night length is measured by the circadian system. ► Circadian system may also modulate night length measurement downstream of the clock.