1. Fibre diameter and enzyme content were studied in the flight muscles of butterflies exhibiting various capacities for flight.
2. The flight muscles of the better fliers are composed of narrow fibres, while those of the poor fliers are composed of larger fibres. The fibres in any one muscle are uniform with regard to the size and the content of a few enzymes studied, viz. lipase, acid and alkaline phosphatases, ATPase and succinic dehydrogenase.
3. Quantitative estimation of the lipase activity in the flight muscles of different butterflies showed a remarkable relationship with the insect's ability to fly and the concentration of lipase in its flight muscles. Good fliers are equipped with larger quantities of lipase than are poor fliers.
4. It is suggested that, like birds and locusts, butterflies also utilize fat for energy during sustained flight.