ABSTRACT.Alkanes, diethyl ether, and various other organic solvents proved to be potent stimulants of development in diapausing pupae ofSarcophaga crassipalpisMacquart and ofManduca sexta(Johansson). Topical application of 2μl or vapour exposure for 1–2h was sufficient stimulation for the flies, but the solvent had to be injected to elicit the response in the hornworms. In flies, oxygen consumption increased nearly 100‐fold within 15 min of hexane application, and thereafter persisted at non‐diapause levels. Sensitivity of fly pupae to hexane remained high throughout diapause but acetone sensitivity dropped sharply after the second day in diapause. Acetone applied the day before the onset of diapause averted diapause in flies, and topical application to hornworm larvae 3 days before pupation likewise prevented pupal diapause. Debrained pupae failed to respond to solvent treatment, and we conclude that it is the brain, rather than the prothoracic gland, that responds directly to stimu