1. The increase in weight of flies of both sexes is followed on several diets. Unfed flies and flies fed on water alone or on meat and water lose weight. Males fed on meat, sugar and water, or on sugar and water, increase in weight rapidly during the first day of adult life and then lose weight until they die. Females fed on sugar and water increase in weight rapidly for the first day and then more slowly until the twelfth day, when they commence to lose weight until death occurs. A similar rapid increase in weight on the first day occurs in females fed on meat, sugar and water, but a second period of rapid increase occurs between the fourth and sixth days. The first period corresponds to the growth of the imaginal fat-body and the second to the growth of the ovaries. The percentage dry weight of flies fed on meat, sugar and water and on sugar and water increases rapidly at first but then decreases slowly until death occurs.

2. The disappearance of the pupal fat-body cells and the growth of the imaginal fat-body on several diets is described. It is suggested that the inclusions of the pupal fat-body cells are transferred to the rapidly growing imaginal fat-body.

3. The hypothesis is put forward that the abdominal air-sacs in flies of the Muscid type have but little respiratory function and are chiefly concerned in

(a) preserving the increased volume of the newly-emerged fly ;

(b) providing ample space within the abdomen for growth of the imaginal fat-body in the male and for the fat-body and ovaries in the female, both of which organs increase greatly in size.