The insect groups with the greatest number of species all have complete metamorphosis. This is as true with aquatic insects as terrestrial insects. The group with the largest number of aquatic species is the Diptera (Flies). Although dragonflies and mayflies are conspicuous, there are more species of caddisflies than mayflies and dragonflies and stoneflies combined.
Unlike their terrestrial relatives, the Lepidoptera, Caddisflies have larva and pupa stages that are aquatic. Some caddisfly larvae build protective cases from rocks sticks and other debris. When it is time to pupate, a caddisfly larva will attach its case to a large rock or other solid object and seal the opening. Inside its protective case, a pupa will move to create a flow of oxygen rich water into the case. When adults emerge, they climb onto the land and fly away.