Different tissues, especially muscles, from insects belonging to various groups were extracted and studied for their lactic dehydrogenase (LDH) and α-glycerophosphate dehydrogenase (GDH I) activities from the comparative point of view. In all cases assays of flight muscle extracts showed extremely low values of LDH activity whereas the GDH activities were surprisingly high. The activities in leg muscles were generally lower. In some insects, however, a very active LDH was found; in these cases the GDH activity seemed to be decreased. GDH I was rather active in the fat bodies. The presence of particulate glycerophosphate dehydrogenase (GDH II) was also demonstrated in insect muscles. These results were interpreted as indicating a system by which there is accomplished immediate and direct breakdown of metabolites to supply large amounts of energy during flight and other activities without the accumulation of intermediate substances.

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Abstract--1. Glycogen is the only significant substrate in the anaerobic metabolism of Upogebia pugettensis and Callianassa californiensis. Small quantities of aspartate are utilized in addition.2. L-lactate is accumulated as the main end-product and L-alanine and succinate are minor endproducts.3. Striking differences between both species were found in the amounts of glycogen stored, in the rate of lactate production and in the resistance of anoxia.4. Accumulation of lactate also occurs in the natural habitat, but the levels reached are rather low. 5. The results suggest that in crustaceans lactate formation is the characteristic mode of anaerobic metabolism as opposed to other invertebrates.