1. The "indirect" thoracic muscles of adult dipterous and hymenopterous insects consist of a unique type of muscle characterized by the presence of numerous spherical, intracytoplasmic bodies termed "sarcosomes."
2. When the muscle is teased or ground, the sarcosomes are liberated as a turbid suspension of bodies ranging from 1 to 4 µ in diameter. A method is described for the isolation of sarcosomes by a simple differential centrifugation.
3. The cytochemical, chemical, and enzymatic properties of sarcosomes were examined for the purpose of appraising their relation to the cytoplasmic bodies of other tissues.
4. Fresh sarcosomes are slowly but selectively stained by the mitochondrial reagents, Janus green B and pinacyanol. Fixed sarcosomes give a positive reaction with Regaud's mitochondrial stain.
5. Chemical analyses show that approximately 29 per cent of the dry weight of sarcosomes consists of lipids and 60 per cent of protein. Microbiological assay indicates the presence of about 1 gamma of riboflavin per milligram of nitrogen. These values resemble those reported for isolated mitochondria of vertebrate liver and kidney.
6. When examined spectroscopically the sarcosomes, like the vertebrate mitochondria, show a high titer of cytochromes a, b, and c.
7. The titer of cytochrome oxidase varies systematically with the adult age of the insect. A similar relation is observed for the enzyme catalase.
8. Isolated sarcosomes show significant titers of succinoxidase, α-glycerophosphate dehydrogenase, malic dehydrogenase, and pyruvic dehydrogenase. The following dehydrogenases could not be demonstrated: xanthine, phenylalanine, glycine, lactic, choline, glutamic, and alcohol. These results are compared with those previously reported for vertebrate mitochondria.
9. In view of their manifold points of biochemical similarity, it is concluded that the sarcosomes are the mitochondria of this highly specialized muscular tissue.