A Threshold Size for Metamorphosis in the Tobacco Hornworm, Manduca Sexta (L.)

Under standard laboratory rearing conditions, caterpillars of Manduca sexta invariably go through 5 larval instars before pupating.2. Evidence is presented that the head capsule width of a given instar is proportional to the weight that the individual had attained at the time that the molt to that instar occurred. Fifth-instar larvae with a large range of head capsule sizes were produced by temporarily starving 3rd and 4th instars, thus inducing them to molt at subnormal weights.3. Further observations on such larvae revealed that individuals with head capsules wider than 5.1 mm proceeded to pupate at the following molt whereas larvae with smaller head capsules underwent a supernumerary larval molt.4. It was concluded that larvae of Manduca simply continue to grow and molt until they reach or exceed a sharply defined threshold size (corresponding to a head capsule size of 5.1 mm). The instar in which this threshold size is attained is then the final larval instar during which the corpora allata will be inactivated.5. This threshold size for metamorphosis is absolute and does not depend on the prior growth history of the larva. An allometry model is presented for a mechanism by which an animal could "measure" its own absolute size.