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DYNAMICS OF JUVENILE HORMONE ACTION IN LARVAE OF THE TOBACCO HORNWORM, MANDUCA SEXTA (L.)

The Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138; and Department of Zoology, University of Washington, Seattle, Washington 98195

1. The half-life of endogenous juvenile hormone (JH) in the hemolymph of early 5th (final) instar larvae of Manduca sexta is approximately 25 minutes. Two hours after ligation JH is no longer detectable in isolated abdomens.

2. The morphogenetic action of JH as well as its inhibitory action on the secretion of the prothoracicotropic hormone persist for several days after molecular JH has become undetectable in the hemolymph and in blood-free carcasses.

3. Evidence is presented which suggests that the action of JH is mediated by long-lived covert effects and that these covert effects of JH are swiftly eliminated toward the end of larval life, apparently through the action of an unidentified blood-borne factor.

4. Two contrasting hypotheses can explain the persistent covert effect of JH. One suggests that the covert effects are stable biochemical changes in the target tissues induced as a normal part of the molecular action of JH. Alternatively, it is possible that molecular JH persists in an unextractable form, lightly bound to the target tissues. In the latter case, the blood-borne factor that causes the elimination of the covert effects could simply be a JH-esterase.