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

Abstract

1. The physiological control of pupal diapause has been studied on a total of approximately 1200 pupae of the giant silkworms, Platysamia cecropia, Telca polyphemus, Samia walkeri, and Callosamia promethea. 2. The dormancy of diapausing pupae can be terminated readily by grafting them to activated (previously chilled) pupae. The two animals in each parabiotic combination grow together and some factor necessary for adult development passes from the activated to the dormant animal so that both develop simultaneously. This factor is not species- or genus-specific. 3. By implantation experiments the source of the factor terminating diapause is shown to be the brain and in this function a lack of species- and genus-specificity of brains is demonstrated. 4. In these species the well-known action of low temperatures in facilitating escape from diapause results from the effect of cold in rendering the brain competent to terminate domancy. Actual termination of dormancy is accomplished only after the previously chilled brain has been exposed to a developmental temperature for an average of two weeks. The earliest indications of adult development then become evident and the brain, thereafter, is no longer required for the completion of metamorphosis. 5. Therefore, the effect of low temperatures 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 temperature. 6. No evidence was found to support the theory that diapause results from the presence of inhibitory factors. In this regard, the functions of the corpora allata are considered in some detail. 7. It is concluded that diapause in these species results from an interruption in the normal processes of development by virtue of a failure of the brain to supply a non-species-specific factor necessary for adult differentiation. Diapause is terminated when this factor is provided. 8. The significance of the brain in the development of diapausing pupae is considered in relation to its other functions, as reported in the literature. Notwithstanding a certain amount of evidence to the contrary, it is probable that even in the absence of diapause the brain plays a vital role in adult formation.