48.
The incidence of diapause in the spider mite
Tetranychus urticae was predicted for various photoperiodic regimes, according to the external coincidence model of photoperiodic time measurement. A phase response curve was constructed for the hypothetical photoperiodic oscillator in these mites: entrainment of this photoperiodic oscillator to a variety of ‘complete’ and ‘skeleton’ photoperiods was calculated using a transformation method for circadian rhythms. The external coincidence model proved adequate to describe experimental results with
T. urticae in ‘complete’ photoperiods (
T = 24 hr), symmetrical ‘skeleton’ photoperiods (
T = 24 hr), asymmetrical ‘skeleton’ photoperiods (
T = 24 hr) (night-interruption experiments), and ‘resonance’ experiments, in which the light component of a light/dark cycle was held constant at 8 hr and the dark component was varied over a wide range in successive experiments, providing cycles with period lengths up to 92 hr. The external coincidence model proved inadequate to explain results obtained in a ‘
T-experiment’ with
T. urticae comprising 1 hr pulses of light in a cycle of
LD
1:17.5 (
T = 18.5 hr) with the first pulse of the train starting at different circadian phases. The validity and limitations of the external coincidence model as an explanation of photoperiodic time measurement in
T. urticae are discussed in view of the above results.
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