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In alternating light and darkness at 25° C., Ptinus tectus shows a diurnal rhythm of locomotory activity with maximum activity occurring in the dark period. The rhythm is continued for a few days in continuous light. In continuous light, no inherent 24 hr. rhythm is apparent, but in subsequent alternating light and darkness, within 1 day, activity becomes practically confined to the dark period. The rhythm can be reversed by reversing the hours of light and darkness and the reversed rhythm is similarly continued in continuous light. In conditions of alternating light and darkness with fluctuating temperature (10-20° C. with low temperature in the dark periods) Ptinus shows greater activity than at 25° C., and maximum activity occurs in the cold, dark period. In constant light and daily fluctuating temperature (17-23° C.), the period of greatest activity occurs when the temperature is falling. After transfer to constant temperature, this period still occurs at the same time of day for a few days.

1. In an aktograph at 25.5°C., at upwards of 75% relative humidity and with food present, the average locomotory activity of the cockroach per day does not depend on whether there is continuous light for weeks, or continuous darkness, or a daily alternation of light and darkness. 2. When temperature and humidity do not vary during the day and other factors are kept as constant as possible, the cockroach's activity can be largely concentrated into any desired half of the day, simply by suitably adjusting the time of onset of the half-day's darkness. A rhythm can thus be set up, so that the main activity occurs at the same hours each day. 3. This activity rhythm persists for some days in continuous light or continuous darkness, but eventually activity becomes much more evenly spread over the whole day, leaving only a slight residual rhythm which is unrelated to the previous conspicuous one. A new conspicuous rhythm can then be started at once by alternation of light and darkness. 4. There are indications that animal responses to physical stimuli may depend to a considerable extent on whether the animal is in the active or the inactive phase of its daily cycle. A method is suggested for making it possible to study the nocturnal phase during the daytime.