Physiology of Insect Rhythms

Abstract

1. The larval ecdyses of the tobacco hornworm, Manduca, and of the oak silkmoth, Antheraea, occur during specific times of the day. These times are instar-specific and species-specific and depend upon the photoperiod and temperature conditions. 2. The larval ocelli are not required for the synchronization of larval ecdysis with the photoperiod. 3. The synchrony observed in larval ecdysis does not arise from a gating of the ecdysial event per se by a biological clock. Rather, it is an outcome of the gating of the endocrine events which initiate the moulting process. 4. Application of neck ligatures to fourth-instar hornworm larvae at various times of day showed that prothoracicotropic hormone (PTTH) secretion occurred only during a gate in the early to middle portion of the night. The times of the gates for second- and third-instar larvae were similar. 5. When neck ligatures were applied to fourth-instar larvae within the first 1-5 h after PTTH release, the larvae underwent precocious metamorphosis rather than Moulting to the Fifth Instar. A Consideration of the Number of Larval-Pupal Intermetiates Produced Indicated that with in Span of 15 min Sufficiant JH is Liberatd from the Corpora Allata to Ensure Full Larval Differentiation. 6. Larval abdomens which were isolated during the period from 3 to 6 h after PTTH release underwent a mosaic larval moult-i.e. they became a patchwork of new and old cuticle. Thus, the epidermal structures of the abdomen vary as to the time that each becomes independent of the prothoracic glands. The first structures which continue fifth-instar differentiation in the absence of the prothoracic glands are the crochets. These are then followed in order by the prolegs, the ventral midline, the intersegmental areas, and, by 6 h after PTTH release, the remainder of the abdomen. During this 3 h period the prothoracic glands are presumably secreting ecdysone.