Physiology of insect rhythms. 3. The temporal organization of the endocrine events underlying pupation of the tobacco hornworm.

The moulting process of insects is governed by the interaction of three hormones: prothoracicotropic hormone (PTTH), juvenile hormone, and ecdysone (see Doane, 1973; Wyatt, 1972; Wigglesworth, 1971 for recent reviews). To initiate the moult the brain releases PTTH which causes the secretion of ecdysone from the prothoracic glands. Ecdysone in turn triggers the detachment of the epidermis from the old cuticle and the subsequent secretion of the new cuticle. Whether this new cuticle is larval, pupal, or adult depends on the titre of juvenile hormone at the time of ecdysone secretion. In larvae of the tobacco hornworm, Manduca sexta, the secretion of PTTH by the brain is regulated by a photosensitive circadian clock (Truman, 1972). The interaction of this clock with the environmental photoperiod serves to restrict hormone release to a specific portion of the light-dark cycle. Consequently, if a larva becomes competent to release hormone after the close of one of these temporal' gates', it must then wait for the opening of the gate on the following day. The gating of PTTH release facilitated a quantitative study of the temporal relationship of the activity of the endocrine organs which were involved in the moult to the fifth larval stage (Truman, 1972). The present paper considers the timing of the endocrine events which govern pupation in the tobacco hornworm.