EFFECTS OF JUVENILE HORMONE ON ECDYSONE-DEPENDENT DEVELOPMENT IN THE TOBACCO HORNWORM,<i>MANDUCA SEXTA</i>

Abstract

1. Juvenile hormone delayed or prevented the onset of metamorphosis by neck-ligated fourth-instar tobacco hornworm larvae and by fifth instars neck-ligated prior to the cessation of feeding. 2. After the onset of the wandering period J H had precisely the opposite effects in that it accelerated the onset of metamorphosis. This was the case both for wandering larvae and for pupae, irrespective of whether they were intact or subjected to brain removal or neck ligation. 3. These findings point to a previously unsuspected shifting role of JH in the control of metamorphosis. 4. Fourth-instar larvae underwent no further development after brain removal unless they were also effectively allatectomized by neck ligation. Evidently the secretion and inhibitory action of JH persists in brainless fourth-instar larvae. 5. Brainless fifths, by contrast, were often able to initiate metamorphosis despite the continued presence of their CA. Development could be prevented by daily application of JH. The disappearance of effective levels of JH, which normally occurs in the late fifth instar as a necessary prelude to metamorphosis, can evidently take place in the absence of the brain. This implies that the brain is not prerequisite for the decline in JH. 6. Since the ability of JH first to delay and subsequently to accelerate the development of intact individuals also persists after brain removal, the brain and its PTTH cannot be the sole targets for these effects of JH, if in fact they are targets at all. 7. The shifting effects of JH on development appear to be ecdysone-dependent. Four possible sites of JH action are identified.