PHYSIOLOGY OF INSECT DIAPAUSE. IV. THE BRAIN AND PROTHORACIC GLANDS AS AN ENDOCRINE SYSTEM IN THE CECROPIA SILKWORM

Abstract

1. The pupation and adult development of the Cecropia silkworm are under the control of a hormonal system consisting of the brain and prothoracic glands. The adult development of the pupa is controlled by the same "brain hormone" and "prothoracic gland hormone" which, at an earlier stage in the life history, control the pupation of the larva.2. Though the brain gives leadership to the endocrine events that preside over pupation and adult development, it apparently does so by supplying a tropic hormonal stimulus for the prothoracic glands. The latter's "growth and differentiation hormone," secreted under stimulation of the brain hormone, then reacts with the larval tissues to promote pupation or with the pupal tissues to promote adult development.3. The pupal diapause is a state of endocrine deficiency resulting from a temporary failure of the brain in secreting its tropic hormone following pupation. What the pupal tissues require is the prothoracic glands' growth and differentiation hormone. But this factor becomes available only after the brain recovers its secretory ability and triggers the function of the prothoracic glands.4. Evidence is presented that the prothoracic glands can be triggered also by the prothoracic gland hormone itself. This device, it is suggested, serves to couple the paired glands into a functional unity and assure their simultaneous response when the brain hormone is released.5. Evidence of a more circumstantial character indicates that larval growth and moulting are also promoted by the prothoracic glands' growth and differentiation hormone, acting within the immature insect in conjunction with a conservative factor secreted by the corpora allata. The corpus allatum hormone plays no positive role in the pupation or adult development of the Cecropia silkworm.6. The endocrine system of Cecropia is compared to that of bivoltine and non-diapausing Lepidoptera. In Cecropia, both the brain and prothoracic glands become inactive after providing the endocrine stimulus for pupation. Prompt, non-diapausing development becomes possible in those species where either the brain or the prothoracic glands retain their endocrine activity within the newly formed pupa.7. The pupal diapause therefore results from the delayed function of a normal endocrine mechanism shared with non-diapausing species.