In the wild type (Canton-S) and period mutant flies of Drosophila melanogaster, we examined the effects of light and temperature on the circadian locomotor rhythm. Under light dark cycles, the wild type and per(s) flies were diurnal at 25°C. However, at 30°C, the daytime activity commonly decreased to form a rather nocturnal pattern, and ultradian rhythms of a 2 ~ 4 h period were observed more frequently than at 25°C. The change in activity pattern was more clearly observed in per(o) flies, suggesting that these temperature dependent changes in activity pattern are mainly attributable to the system other than the circadian clock. In a 12 h 30°C:12 h 25°C temperature cycle (HTLT12:12), per(o) flies were active during the thermophase in constant darkness (DD) but during the cryophase in constant light (LL). The results of experiments with per(o)
eya flies suggest that the compound eye is the main source of the photic information for this reversal. Wild type and per(o) flies were synchronized to HTLT12:12 both under LL and DD, while per(S) and per(L) flies were synchronized only in LL. This suggests that the circadian clock is entrainable to the temperature cycle, but the entrainability is reduced in the per(S) and per(L) flies to this particular thermoperiod length, and that temperature cycle forces the clock to move in LL, where the rhythm is believed to be stopped at constant temperature.