1. Changes in total lipid and R.Q. show that female pupae of H. cecropia begin to catabolize lipid early in adult development. In males there is a conservation of lipid during adult development resulting in the male moth having about three times the lipid content of the female. In the adult moth both sexes utilize lipid as the major energy source.

2. Lipid is the available substrate as well as the preferred substrate in flight-muscle metabolism in male moths.

3. Flight-muscle homogenates show greater oxidative activity with fatty acids and citric acid cycle intermediates than with glucose or glycolytic intermediates, indicating that carbohydrate pathways are not prominent.

4. A fatty acid oxidizing system has been identified in flight muscle which requires ATP, magnesium and a citric acid cycle intermediate for optimum activity.

5. Experiments with radiotracers and metabolic inhibitors reveal that fatty acid oxidation in flight muscle proceeds via the citric acid cycle and the cytochrome chain.

6. Active fatty acid activating enzymes are present in flight muscle, and fatty acid oxidation in H. cecropia is discussed in relation to vertebrate and other invertebrate systems.

A portion of this work was reported at the XVI International Congress of Zoology, Washington, D.C., in Auguest 1963.