Responsiveness to photoperiodic changes in the circannual rhythm of the varied carpet beetle, Anthrenus verbasci

The circannual pupation rhythm of Anthrenus verbasci is entrained to an environmental cycle by changes in photoperiod. Exposure of larvae reared under short-day conditions to 4 weeks of long days can induce phase-dependent phase shifts. In the present study, we examined the range of photoperiodic changes effective for phase shifts at 20°C. For larvae under light/dark (LD) 12:12 conditions, 4-week exposure to LD 14:10 caused a clear phase delay, as great an extent as that brought about by exposure to LD 15:9 and LD 16:8. In contrast, the delay brought about by exposure to LD 13:9 was slight. For larvae under LD 10:14, exposure to LD 14:10 and LD 16:8 for 4 weeks induced a phase delay, but exposure to LD 12:12 did not. These results indicate that a clear phase delay is induced when the photoperiodic change exceeds a critical value in the photophase between 13 and 14 h, regardless of the amplitude of the change. Although phase advances were smaller than phase delays, they depended on the amplitude of photoperiodic changes rather than the absolute photophase duration in contrast to the case of the phase delay.