Phase shifting the circadian rhythms of nocturnal insects by temperature changes

The behavioural rhythms of three species of nocturnal insects were studied: locomotion in a cockroach, luminescence in a firefly and luminescence in a glow‐worm. The behaviour was released at the change from light to dark in all cases. In continuous dim light, a circadian rhythm was apparent with a period of from 22.5 to 27 h in the cockroach, and from 19 to 23.5 h in the glow‐worm. In constant dim light, the cockroach shifted the phase of its rhythm when subjected to a change of temperature. A temperature step down some time prior to the expected onset of locomotion generated a phase advance, whereas a temperature step up generated a phase delay; the greater the step up, the greater the phase delay. By advancing the change from light to dark, it was found that glow‐worms could synchronize immediately to advances of up to about 6 h only. At greater advances an increasing phase angle difference between onsets of darkness and activity occurred. However, if a temperature step down was applied at the light change, immediate synchronization occurred up to 9–12 h before the normal onset. A theory is propounded which assumes that the onset of rhythmic behaviour is determined by the interaction of a circadian process of sensitization and a threshold for release of the behaviour. The threshold is high at high temperatures and light intensities, and low at low values of these. A temperature step up raises the threshold and delays the activity while a step down lowers the threshold and advances the activity.