The most obvious change in metabolism during the more advanced type of insect metamorphosis is the change in the integral metabolic activity. After pupation there is first a marked decrease and later an increase of, for instance, the oxygen consumption. The respiratory metabolic curve is U-shaped. The cause of this change is a corresponding variation in the activity of oxidative enzyme systems. If one compares the variation of the spontaneous Mb-reduction and of the oxygen consumption in the fly Calliphora, one finds that the two curves have almost the same course (Agrell, 1947b). This shows an important co-operation of the dehydrogenase systems. The cytochrome system also shows a similar U-shaped variation during the period of metamorphosis, according to Wolsky (1938), Williams (1948) and Sacktor (1950). One limiting factor is the protein part of the enzymes, which is first broken down and later rebuilt (Agrell, 1946). Another limiting factor is the prosthetic heme group in the cytochromes (Williams, 1951).