The photoperiodic clock in the flesh-fly, Sarcophaga argyrostoma

Abstract

Larval cultures of the flesh-fly, Sarcophaga argyrostoma, were raised in experimental light cycles with periods ( T) of 21 to 72 hr, each cycle containing a photoperiod of 4 to 20 hr of white light. This ‘resonance’ technique revealed periodic maxima (∼24 hr apart) of pupal diapause, thereby demonstrating an endogenous circadian component in the photoperiodic clock. The positions of these maxima of pupal diapause suggested that the oscillation, like that controlling the pupal eclosion rhythm in Drosophila pseudoobscura, is ‘damped out’ by photoperiods longer than about 11 to 12 hr, but restarts at dusk whereupon it runs with circadian periodicity in a protracted dark period. With photoperiods shorter than 12 hr, however, the two diapause maxima were less than 24 hr apart, suggesting that an additional component, possibly a ‘dawn hour-glass’, was modifying the position of the first peak. Both photoperiod and the period of the driving light cycle ( T) were shown to affect the length of larval development (the sensitive period) and the number of calendar days needed to raise the incidence of pupal diapause to 50 per cent (the required day number, RDN). Peaks of diapause induction were shown to be the result of an interaction between a long sensitive period (slow development) and a low RDN, whereas troughs in diapause induction were the result of an interaction between a short sensitive period (fast development) and a higher RDN. Larvae of S. argyrostoma are unable to distinguish (in a photoperiodic sense) between 12 and 18 hr of red light (600 nm).