Adult blow flies (Calliphora vicina) present robust circadian rhythms of locomotor activity which persist (“free-run”) in continuous darkness and constant temperature with an average period (?) of about 22.5 h. This period is temperature compensated between 15 and 25?C (Q10 0.98 – 1.03) but lengthens in continuous dim light to become overtly arrhythmic in light above about 0.03 ? W cm -2 . The rhythm becomes entrained to a daily light-dark cycle with locomotor activity confined to the light portion of the cycle. It also entrains to LD cycles with periods from 20 to 30 hours, showing phase relationships dictated by the phase response curve. Entrainable rhythms persist in lobectomised flies indicating that the photoreceptor(s) and the relevant ‘clock’ are both brain centred. The overt rhythm is a product of several constituent circadian oscillators, probably the coupled output of oscillators in individual clock neurons in the lateral part of the brain. Whilst the circadian clock is in operation, a separate but parallel circadian-based mechanism (the photoperiodic ‘clock’) is ‘measuring’ and ‘accumulating’ successive day lengths to regulate the diapause/non-diapause status of the larval progeny.

Index terms: Circadian rhythms, locomotor activity, photoperiodism

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