Insect that spends all or part of its life in water. Of the 29 insect orders, 11 members have some aquatic stages. Most of these have aquatic, immature stages, which usually take place in fresh water, sometimes in brackish water (very few species are truly marine); the adults are terrestrial, but in some orders there are species where all stages (egg, larva, and adult) live in the water.

Partially aquatic
Three orders, Ephemeroptera (mayflies), Odonata (dragonflies), and Plecoptera (stone-flies) have aquatic larvae, but the adults are terrestrial. In the orders Neuroptera (alder flies), Tricoptera (caddis flies), Lepidoptera (butterflies and moths), and Diptera (true flies), some species have aquatic larvae, but the adults of all species are terrestrial. Hymenoptera, the social insect order which includes the ants and bees, has some aquatic species of ichneumon fly: immature stages of Agriotypus are aquatic, and adults of Caraphractus and Prestwitchia.

Totally aquatic
The order Collembola (springtails) has two species in which all stages are aquatic: Hydropodura aquatica and Isotoma palustris. In Hemiptera (bugs), and Coleoptera (beetles), some members, for example the water bugs, spend all stages of the lifecycle in the water.