PHYSIOLOGY OF INSECT DIAPAUSE: THE ROLE OF THE BRAIN IN THE PRODUCTION AND TERMINATION OF PUPAL DORMANCY IN THE GIANT SILKWORM, PLATYSAMIA CECROPIA

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Abstract
The physiol. control of pupal diapause has been studied on 1200 pupae of the giant silkworms, Platysamia cecropia, Telea polyphemus, Samia walkeri, and Callosamia promethea. The dormancy of diapausing pupae can be terminated readily by grafting them to activated (previously chilled) pupae. The 2 animals in each parabiotic combination grow together and some factor necessary for adult development passes from activated to dormant animal so that both develop simultaneously. This factor is not species[long dash]or genus-specific. By implantation expts. the source of this factor is shown to be the brain. In these spp. the well-known action of low temps. in facilitating escape from diapause results from the effect of cold in rendering the brain competent to terminate domancy. Actual termination is accomplished only after the previously chilled brain has been exposed to a developmental temp. for 2 weeks. The earliest indications of adult development then become evident and the brain, thereafter, is no longer required for completion of metamorphosis. Therefore, the effect of low temps. on the brain must consist in some physical or chemical alteration in its substance whereby the latter is rendered competent to produce or release an imaginal-differentiation factor after return to a developmental temp. No evidence was found to support the theory that diapause resulted from the presence of inhibitory factors. In this regard, the functions of the corpora allata are considered in some detail. It is concluded that diapause in these spp. results from an interruption in the normal processes of development by a failure of the brain to supply a non-species-specific factor necessary for adult differentia- tion. Diapause is terminated when this factor is provided.