Physiology of Insect Rhythms I. Circadian Organization of the Endocrine Events Under- Lying the Moulting Cycle of Larval Tobacco Hornworms

In insects the development of a rigid exoskeleton requires that for growth and metamorphosis to occur a new cuticle must be periodically formed and the old one cast off. This process (the moult) is initiated with the release of the prothoracicotropic hormone (PTTH) from the brain. Then follows the secretion of a second hormone, ecdysone, by the prothoracic glands, the apolysis of the epidermis from the cuticle, the secretion of a new cuticle, and, finally, the shedding or ecdysis of the old cuticle. The type of cuticle formed is regulated by a third hormone, the juvenile hormone (JH), produced by the corpora allata (see Wigglesworth (1971) and Wyatt (1972) for current reviews of the hormonal control of moulting). Recently, a study of the eclosion of adult silkmoths revealed that the last step in the moulting process is also under endocrine control (Truman & Riddiford, 1970). In this case a neuro-secretory hormone from the brain acts on the nervous system to trigger the behaviour An interesting feature of adult eclosion is its association with a Orcadian clock. This underlying clock is sensitive to photoperiod and restricts emergence to temporal 'gates', which occur during a specific period of the day (Pittendrigh, 1966; Skopik & Pittendrigh, 1967). However, in addition to the act of eclosion, other steps in the moulting process may show a sensitivity to photoperiod cycles. Williams & Adkisson (1964) clearly demonstrated that, in diapausing pupae of Antheraea pernyi, long-day photoperiods act directly on the brain to promote the secretion of PTTH. Also, a possible gating of PTTH release has been inferred from histological studies on Drosophila (Rensing, 1971). The events beginning with PTTH release, proceeding through apolysis and cuticle deposition, and ending with ecdysis of the old cuticle are considered here to comprise the moult. This paper examines two aspects of the moulting process in lepi-dopteran larvae. The first involves the question as to whether the mechanism which controls adult eclosion is also used to regulate the time of larval ecdysis. The second examines the possible involvement of a circadian clock in the control of PTTH release and the temporal relationship of the release of this hormone to that of ecdysone and JH.