Photoperiodic time measurement in insects and mites: a critical evaluationof the oscillator-clock hypothesis

The validity of the oscillator-clock hypothesis for photoperiodic time meas urement in insects and mites is questioned on the basis of a re-interpretat ion of available experimental evidence. The possible role of the circadian system in photoperiodism in arthropods is critically reviewed. Apart from t he outcome of kinetic experiments, based on diel and non-diel light/dark cy cles. evidence from various genetic and physiological experiments is discus sed in relation to the oscillator-clock hypothesis. The conclusion is that photoperiodic time measurement in insects and mites is performed by a non-c ircadian 'hourglass' clock. Experimental evidence suggests a non-clock role for the circadian system in the photoperiodic mechanism of insects and mit es. (C) 2001 Published by Elsevier Science Ltd.