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…
Source: Living With Insects Blog