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Insects eat a wide variety of organic materials. Their diet ranges from dry dead wood to the living tissues of mammals and reflects the hundreds of thousands of species and the diverse habitats of this remarkable class of animals. What an insect-eats and how much it eats largely determine its ecological role and its economic importance. If the Colorado potato beetle had not begun eating the leaves of the cultivated potato it would still be an insect of restricted geographical range and of interest only to coleopterists.

What an insect eats is not always as obvious as one might at first suppose. Larvae of the fruit fly. Drosophila, live in decaying fruit, but their diet consists of the yeasts and other microorganisms growing in their fruit. The meadow grasshoppers (Conocephalinae) were thought to eat primarily the leaves of grasses and other plants, but observations on the crop contents of these Orthoptera show that they actually subsist on insects, pollen, flowers, and seeds of grasses, and hardly ever eat leaves. Anobiid beetles which burrow in wood feed on the fungi that grow along the walls of the burrows.

Moreover, not everything that an insect ingests as food can serve as a source of nutrients. Various wood-eating beetles, such as the power-post beetle, Lyctus, are unable to digest cellulose, the main constituent of their food, and utilize only the starch, sugars, and nitrogenous materials also present in the wood. Similarly, most leaf-feeding insects do not digest and hence obtain no nutritional value from the cellulose which constitutes a considerable proportion of their natural food.

A knowledge of what an insect eats and of what substances in its diet it is able to assimilate constitutes but the initial steps toward an understanding of its fundamental nutritional physiology.