In Lepidoptera, the eye and the leg imaginal discs form only in the final larval instar from imaginal primordia that make larval cuticle during the earlier instars but remain diploid. Starvation from the time of ecdysis prevents this formation, which can be restored by feeding in Manduca sexta（L.）larvae. By contrast, these discs form and grow slowly in starved allatectomized（CAX）larvae lacking juvenile hormone（JH）, and this formation is prevented by JH; JH acted directly on the primordia to suppress morphogenesis, but insulin induced by nutrients, which overcomes the inhibitory action of JH, is necessary for the morphogenesis to occur. Thus, disc growth in the final instar larvae is comprised of both morphogenetic growth under the suppressive control of JH and nutrient-dependent growth. Larvae must surpass critical weight before they can enter metamorphosis on a normal schedule that is under the control of JH. Normal larvae respond to starvation by extending their feeding period from 5 to 7 days to ensure that they begin metamorphosis above critical weight. By contrast, CAX larvae start metamorphosis 4 days after feeding regardless of larval size. This constant period between the start of feeding and the onset of metamorphosis suggests that larvae possess a molt timer that establishes a minimal time to metamorphosis.