Photoperiodic time measurement inSarcophaga argyrostoma: An attempt to use daily temperature cycles to distinguish external from internal coincidence

Abstract

1. Larvae ofSarcophaga argyrostoma raised in continuous darkness (DD) and in ‘square-wave’ temperature cycles or thermoperiods (Hot:Cold4:20 to20:4, whereH=25 °C and C=15 °C) entered pupal diapause at a rate equivalent to that in DD and a constant temperature equal to the arithmetic mean of the cycle. Therefore, unlike many other insects so tested,S. argyrostoma appears to show no thermoperiodic regulation of diapause induction. 2. A daily low temperature pulse (3 h at 5 °C; =HC21:3), however, had marked phase-dependent effects on diapause incidence when administered with a concomitant light cycle (LD14:10), diapause increasing when the cold pulse fell early in the night, but decreasing when late in the night. This result is interpreted in terms of the phaseresetting effects of the cold pulse on the circadian oscillations making up the clock. 3. These results are considered to be consistent with an ‘external coincidence’ model for the photoperiodic clock and appear to differentiate between the ‘internal’ and ‘external’ alternatives (forS. argyrostoma) by demonstrating an inductive requirement for light in addition to that for entrainment.