Physiology of Insect Rhythms

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 neurosecretory hormone from the brain acts on the nervous system to trigger the behaviour involved in emergence (Truman, 1971 ; Truman & Sokolove, 1972).