The characteristic smell or taste of a plant, to insects as well as to humans, depends on its chemical composition. Broadly speaking, plants contain two categories of chemical substances: primary and secondary. The primary substances, such as proteins, carbohydrates, vitamins, and hormones, are required for growth and proper functioning and are found in all plants. The secondary substances are a diverse and multitudinous array of chemicals that have no known role in the internal chemical processes of plants' growth or metabolism. Only a few of these substances occur in any one species of plant, but the same or similar ones tend to occur in related plants such as the various species that constitute a single family. It is these secondary substances that give plants their distinctive tastes and smells.

Insects appear to have played a major role in many plants' having the secondary substances they have today. Such substances undoubtedly first appeared, and new ones continue to appear, as the result of genetic mutations in individual plants. But if a mutation is to survive and be passed on to subsequent generations, it must pass the muster of natural selection—that is, it must increase the likelihood of the organism's surviving and reproducing. Some secondary substances are favored by natural selection because they are scents that attract pollinating insects to blossoms. Such scents signal the presence of nectar, which nourishes the insects without damage to the plants. Other secondary substances that arose by mutation were conserved by natural selection because they proved to be biochemical defenses against the enemies of plants, the majority of which are insects. Some of these defensive substances cause insects to suffer unpleasant symptoms or even to die. Still other secondary substances are not in themselves harmful to insects, but are characteristic smells or tastes that dissuade the insect from feeding by warning it of the presence of some other substance that is harmful.

For hundreds of millions of years there has been an evolutionary competition for advantage between plants and plant-eating insects. If insects are to survive as the plants they eat develop defenses against them, they must switch to other foods or evolve ways to circumvent the plants' defenses. They may evolve a way to detoxify a harmful substance, to store it in their bodies out of harm's way, or to avoid its effects in some other manner. Insects quickly come to prefer the plants whose defenses they can circumvent, and they eventually evolve the ability to identify them by their characteristic flavors or odors, or both. As the competition has progressed, fewer and fewer plants have remained as suitable food sources for any one species of insect; species of insects have thus tended to become associated with narrowly defined and often botanically restricted groups of plants.