1. Talitrus saltator possesses a clear-cut locomotor activity rhythm, largely under the control of a persistent endogenous timing mechanism of circadian frequency. In dim red illumination the period of the rhythm is somewhat greater than 24 h; in continuous white light it is less than 24 h, provided that the animals are unable to avoid the light by burrowing. The rhythm is synchronized by light/dark fluctuations, and the animal is active during the dark period.

2. The timing mechanism is temporarily advanced by increases in temperature, but otherwise it possesses a large measure of temperature independence within the normal environmental range.

3. There is some evidence that, in groups, the animals influence one another and so reduce the rate of drift of the activity rhythm in constant conditions.

4. The adaptive significance of the endogenous timing mechanism of Talitrus is discussed.

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