Insects are incapable of de novo sterol synthesis and therefore require a dietary or exogenous source of sterol for their normal growth and development. The sterol requirement of insects is, in most cases, satisfied by cholesterol. This is the principal sterol in insects and serves the role of structural component of cell membranes and as biogenetic precursor of the moulting hormone ecdysone. A brief treatment of fucosterol epoxide with Lewis acid, e.g. boron trifluoride etherate, afforded among other compounds desmosterol at 35% yield. It was hypothesized that a similar reaction would also occur in the insect. Inhibition of sterol metabolism has proven valuable for studying metabolism of plant sterols in insects. When the imine or the allene II were administered in the silkworm diet in combination with sitosterol or cholesterol, the growth and development of B. mori were markedly retarded. Insect sterol analysis revealed that the imine is a sitosterol dealkylation inhibitor, and brings about accumulation of the unchanged sitosterol.