Controversial aspects of photoperiodism in insects and mites

This review examines several controversial aspects of photoperiodism in insects and mites including the role of the circadian system in night length measurement, the nature of apparent hourglass-like responses, and whether or not the circadian component in photoperiodism is the same as that in overt behavioural rhythms. These aspects of the phenomenon are discussed in terms of the entrainment of circadian oscillations by cycles of light and temperature. There is considerable variety of photoperiodic response within the insects (and other arthropods) to show, inter alia, circannual rhythms, internal and external coincidence night length timers, and in some species, non-circadian hourglass-like devices. Many apparent hourglass-like responses, however, could be circadian 'clocks' of the external coincidence type involving oscillations that dampen below threshold in extended periods of darkness. The review also concludes that there is little evidence in favour of the "Hourglass clock-oscillator counter" model proposed for the mite Tetranychus urticae by Vaz Nunes and Veerman (1982a). The responses of this species to complex light and temperature cycles may also be interpreted in terms of a damped oscillator version of external coincidence.