Built-in behavior is the set of patterned repetitive activities done by insects. Let us analyze the female Aedes mosquito egg-laying behavior. The Aedes mosquitoes are orphaned by their parents as soon as the eggs are laid. After they hatch, each mosquito is strictly on its own. They have no opportunities to learn the special kind of egg-laying behavior; yet all Aedes female mosquitoes behave in this way: they lay their eggs just above the water line of a pond rather than in the water; second, they do not put all their eggs in one basket, but distributes them in clusters in several locations. Their behavior has several results. The eggs will not hatch until water level rises. This pattern of behavior seems quite "intelligent" and "purposeful", for the Aedes female mosquitoes insure that their offsprings will have the best chances of survival. This behavior is repeated exactly from generation to generation.
As the offsprings do not have any chance of learning, as we human do, we can say that this survival-geared behavior is genetically inherited and the control of it is built into the insect's nervous system. Thus, the nerve cells of an Aedes mosquito, or of a bee worker, which is totally specialized in sucking the nectar out of flowers, are wired up in such a way that they will automatically display exactly the same patterns of behavior as their ancestors have done for millions of years. These inherited behavioral patterns are as much part of insects as the color of their bodies.