1. The low values of the Respiratory Quotient (0.65) during the pupal period are most readily explained on the theory that some constituent of the tissues is only partially oxidised and remains, in part at least, as a permanent constituent of the body instead of being oxidised to products such as CO2 and H2O which are eliminated. 2. There is definite evidence of a very marked synthesis of glucose during pupation. If this glucose were all oxidised to CO2 and H2O, the expected Respiratory Quotient would be about 0.7 or 0.8 depending on whether the glucose is formed from protein or fat. Actually the Respiratory Quotient seldom rises as high as 0.7 and is frequently below 0.6. In certain instances it may be much lower than this. It is improbable then that the glucose formed is entirely used in respiration; some must almost certainly be used in building up the growing imaginal tissue substance. 3. The synthetic processes which form glucose are active throughout the whole pupal period, though not uniformly so. The alcohol-ether soluble constituents of the body do not diminish in quantity during the first part of the pupal period. During this period therefore the glucose formed cannot be derived from fat. There is no storage (in the body of mature larvae) of glycogen or any other higher carbohydrate convertible by acid hydrolysis into glucose (or other reducing sugar). Therefore, during the earlier part of the pupal period the glucose must be derived from a protein source. The protein is used in such a way that no nitrogen is lost by the body. 4. During the latter half of the pupal period the fat content of the body continually diminishes and during this period there may be a fat to carbohydrate metabolism. The shape of the respiration curves shows a gradual diminution of respiration during the early part of the pupal period and a gradual increase during the latter half. 5. There is no reliable evidence which indicates that the metabolic processes of the two sexes are different from each other.