Effects of X rays on metamorphosis of the abdominal epidermis in the flesh fly, Sarcophaga peregrina, and age dependency of radiosensitivity were studied. The imaginal epidermis of abdomen is formed from the histoblast nests, which are composed of undifferentiated tiny cells lying between large larval epidermal cells. There were two types of effects of X rays: (1) the arrest of metamorphosis including degeneration of larval epidermal cells and histogenesis of imaginal epidermis. (2) partial deficit of imaginal epidermis at the final stage of development. It was suggested that the second type of effect was brought about by a decrease in the number of abdominal histoblasts caused by X rays. Age dependency of radiosensitivity on the second type of effect was examined in detail, and it was shown that the most sensitive stage occurred just before transition to a highly radiation-resistant period.

Arrest of metamorphosis induced by X irradiation at prepupal stage was studied histologically, and age dependency of radiosensitivity with regard to this effect was examined. Prepupae did not cease their development soon after irradiation, but continued to develop and evaginated the head and the thorax. At this point, development came to a stop. In these animals, not only the histogenesis of imaginal tissues but also the histolysis of larval tissues was arrested. Since the arrest of development was not observed after irradiation at the pupal stage, the effect was considered to result from inhibition of initiation of postpupation development. A possible mechanism of the arrest of postpupation development in the irradiated animals was discussed in connection with the neuroendocrine control of insect development.