In the moth Samia cynthia ricini, timing of the release of prothoracicotropic hormone is controlled by a circadian clock in the cephalic organ (brain); while this hormone release is necessary for gut purge, the final timing of this event is controlled by a circadian clock in the prothoracic glands that gates release of ecdysone. The photoreceptor of the prothoracic gland clock(s) is extraocular and evidently in the glands themselves. Demonstration that the clock and its photoreceptor are in the prothoracic glands is based on a mixture of localized illuminations and the transplantation of glands from one larva to another.