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 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, 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 have thus tended to become associated with narrowly defined and often botanically restricted groups of plants.
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