The qualitative nutritional requirements of insects, though they may belong to different orders and families, and ingest different foods, are very uniform and similar to those of mammals. A conspicuous exception is the requirement by insects for dietary sterol. This is an indispensable nutrient for insects, because they lack the capacity for de novo biosynthesis of the steroid nucleus. Most species of aphids have two types of prokaryotic endosymbiotes. One is round-oval coccoid and the other is rod-shaped bacilliform. They are called the primary and secondary symbiotes, respectively. The secondary symbiotes are thought to have become associated with aphids later than the primary ones in evolutionary history. Although the metabolism of symbiotes and that of the insect host are undoubtedly highly integrated, the metabolic role played by the symbiotes in the insect-symbiote relationship may be elucidated by separating these two highly adapted organisms and maintaining them in isolation.