For some insects it’s part of their life cycle. They spend the majority of their life in a larval stage after being born in the water. Then one day they rise from the water as adults, fly around and mate , finally returning to the water surface to lay their eggs and expire. This is the basis for fly fishing. The angler has to present a fly that imitates the insect in the particular life stage that the fish are eating. Mayflies, midges, caddis and stoneflies are the most common insects. Nymphs, pupae, emergers, dun, spinners are the flies imitating the life stages.