During larval growth and molting in Lepidoptera, the wing imaginal discs and the imaginal eye primordia grow but do not differentiate, due to the presence of juvenile hormone (JH). In the final larval instar, they undergo morphogenetic growth dependent on nutrient input. During the molt to the final instar, the wing discs begin the process of pupal commitment by losing sensitivity to JH and acquiring a dependence on ecdysteroids for proliferation. In Manduca sexta, feeding on sucrose after ecdysis is necessary for the completion of pupal commitment, accompanied by the appearance of Broad (Br), a pupal specifying BTB/POZ transcription factor. Wortmannin, an inhibitor of insulin signaling, prevented this appearance of br mRNA in fed larvae. Conversely, injection of bombyxin, the insect insulin, or bovine insulin up-regulated br expression in starved larvae. When early 5th instar wing discs were cultured in vitro with or without low concentrations of 20-hydroxyecdysone (20E), br mRNA was induced in a dose-dependent manner. This induction was suppressed by JH, but could be reversed by bombyxin or bovine insulin. Therefore, we propose that bombyxin released due to sugar feeding is required to complete pupal commitment initiated by the high 20E of the final instar molt. (Supported by NSF)