Circadian clock and prothoracicotropic hormone secretion in relation to the larval-larval ecdysis rhythm of the saturniid Samia cynthia ricini

Observations on the timing of ecdysis and neck ligation experiments on larvae of Samia cynthia ricini under various light-dark conditions show that an endogenous circadian clock controls the timing of larval ecdysis and prothoracicotropic hormone (PTTH) secretion preceding it. The clock, upon reaching a specific phase point, causes the brain to secrete PTTH provided that the brain has acquired the secretory competence. This time may vary, in relation to a previous ecdysis, according to the light-dark conditions by which the clock phase is specifically determined, but is fixed relative to a subsequent ecdysis. Thus, in the case of the ecdysis to the 5th instar, PTTH is secreted [15+nτ] hr (τ: free-running period, slightly less than 24 hr) after the clock has started when the rhythm is free-running, and in the second and third nights of the 4th instar under a photoperiod of 12 hr light and 12 hr dark. Full secretion of ecdysone occurs 6 hr after PTTH secretion and ecdysis ensues 34 hr thereafter to complete the ultimate sequence of ecdysis.