Control of Moulting and Metamorphosis in the Tobacco Hornworm, Manduca Sexta (L.): Growth of the Last-Instar Larva and the Decision to Pupate

The dynamics of growth and the timing of release of the brain ‘s pro-thoracicotropic hormone (PTTH) in final instar larvae of Manduca sexta are consistent with the following hypothesis. When a 5th-stage larva reaches a critical weight of about 5 g an unidentified process is initiated which requires 24 h to be completed. At the completion of this process the brain is rendered competent to release PTTH. The actual release of PTTH is gated by the photoperiod and occurs when the gate opens during the very next photophase.