A phase response curve (PRC) is a polynomial curve plotted phase shifts induced by changes of environmental signals given at certain circadian time against the time when the stimulus pulse onset of an organism. The study of PRCs to a Zeitgeber lead us to understand how the circadian pacemakers adjust their phases to environmental changes. Among the various Zeitgeber signals, including light, temperature or social factors, light stimulus is the most powerful one. The effects of 2 different Zeitgeber pulses, light (3600 lux) and sound of human activities, on the locomotor onset phase in two close-related cockroaches, Blattella bisignata and B. germanica, are investigated. Since both cockroach species are closely related, the PRC differences may reflect the various characteristics of circadian clock to the living environment. From the light induced PRCs, the amplitude of phase shift in B. germanica is much smaller than that of B. bisignata. This finding indicates the adjustability of the circadian clock of B. germanica is inferior to that of B. bisignata. However, the consequence of this inferior clock does not impair its timing of locomotion since B. germanica lives in a relatively stable domicile habitat. The PRCs of cockroach species show that their circadian clocks are more resisting the advance adjustment than the delay phase shifting. As the phase advance occurring, the circadian periods of B. bisignata are increased, and vice versa. On the contrary, negative correlations between circadian periods and phase shifts of B. germanica are found. The sound induced PRCs and free-running periods are compared with those induced by light pulses.

Index terms: PRCs, light pulse, locomotor activity, circadian rhythm, sound pulse.

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