Drosophila nigrospiracula

D, Drosophila longicornis] D. m. Drosophila mettleri D. n. Drosophila nigrospiracula. 

Drosophila pachea is limited to the senita cactus (Lopho-cereus schottii) by a biochemical dependency on sterols found in that cactus. Unlike most insects that can utilize such compounds as P-sitosterol (4) to synthesize ecdysones, flies of this Drosophila species must have the compound schot-tenol (37) in which a particular double-bond position is found (Fig. 23.18) (Harbome, 1982 Herout, 1970 Kircher, 1982). This cactus contains 3-15% dry weight of complex isoquinoline alkaloids that are toxic to both adults and larvae of Drosophila nigrospiracula, D. mojavensis, and D. mela-nogaster. 

Individuals of Drosophila nigrospiracula use saguaro (Carnegiea gigantea) on the mainland and cardon Pachy-cereus pringlei) in Baja California. The two cacti are morphologically and chemically similar (Frank and Fogleman, 1992). Both have low concentrations of isoquinoline alkaloids. 

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