Drosophila pachea, which breeds only in the stems of senita cactus (Lophocereus schottii) throughout the Sonoran Desert, requires the cactus as a dietary supplement when reared on laboratory media. Delta(7)-Stigmasten-3beta-ol, isolated from the cactus or synthesized, can replace the cactus in the diet of flies reared nonaseptically or axenically. Delta(7)-Cholesten-3beta-ol and Delta(5, 7)-cholestadien-3beta-ol could be substituted for the cactus sterol; Delta(5, 7)-stigmastadien-3beta-ol produced infertile females. Cholesterol, 4alpha-methyl-Delta(7)-cholesten-3beta3-ol, beta-sitosterol, stigmasterol, ergosterol, and Delta(7)-ergosten- 3beta-ol did not support larval growth.

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