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The present work analyzes the mechanism of activation of the prothoracic glands in larvae and pupae of saturniid moths, and examines changes which occur in the prothoracic glands during the larval-pupal and pupal-adult transformation.The prothoracic glands of Samia Cynthia ricini and Antheraea polyphemus synthesize DNA in the fourth instar. DNA synthesis occurred to a lesser extent in the fifth instar and in prepupae, but was not found in diapausing pupae or developing adults. Injury to diapausing pupae did not stimulate DNA synthesis in the Prothoracic glands.The prothoracic glands of pupae of A. polyphemus injected with juvenile hormone extract did not synthesize DNA, nor did the prothoracic glands of pupae of Samia Cynthia walkeri after transplantation into larvae. Apparently the prothoracic glands lose their ability to synthesize DNA during the larval-pupal transformation.The prothoracic glands of larvae of S . Cynthia ricini synthesized RNA at a high rate throughout the fourth larval instar. In A. polyphemus the rate of RNA synthesis in the prothoracic glands of developing adults was several times greater than the rate of RNA synthesis in diapausing pupae. The injection of juvenile hormone extract into pupae of A. polyphemus caused a marked increase in the rate of RNA synthesis. Changes in the rate of RNA synthesis were correlated with cytological changes in developing adults and in pupae injected with juvenile hormone extract.Prothoracic gland cells were found in adult S. Cynthia ricini. Some of the cells were degenerating while the others appeared inactive.The relationship of RNA synthesis to activation and secretion, and the significance and control of DNA synthesis in the prothoracic glands are considered. Preliminary work on protein synthesis on the prothoracic glands and their activation in vitro is evaluated.

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