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Physiology of Insect Rhythms


1. Late in the fifth instar, Manduca sexta larvae cease feeding and become ‘wandering larvae’ which are morphologically characterized by an ‘exposed heart’ and the appearance of a pink pigment along the dorsal midline. Two days later ocellar retraction signals the beginning of the prepupal period and 3 days thereafter the pupal ecdysis occurs.

2. The timing of the endocrine events which are responsible for these changes was determined by ligaturing animals of the appropriate developmental stage at various times of day. The times of prothoracicotropic hormone (PTTH) release by the brain were determined by neck ligations. Estimates of the times of prothoracic gland activity were obtained through the isolation of abdomens.

3. It was found that the fifth stage larva releases PTTH on two occasions. The first release lasts approximately 3.5 h and triggers the transformation to the wandering stage. The second release occurs two days later, lasts at least 7 h, and provokes the onset of the pupal moult.

4. The prothoracic glands are involved in triggering the same two changes. In the first instance the glands apparently require the continuing influence of the brain and consequently secrete for about 3.5 h. During the stimulation of the pupal moult the prothoracic glands become ‘turned-on’ and continue to secrete for at least 10 h after the time when the brain is no longer required. In this latter instance the total time of prothoracic gland activity may be as long as 17 h.