10.1. Taxonomic distribution and terminology
The orders of insects that are almost exclusively aquatic in their immature stages are the Ephemeroptera (mayflies; Box 10.1), Odonata (damselflies and dragonflies; Box 10.2), Plecoptera (stoneflies; Box 10.3), and Trichoptera (caddisflies; Box 10.4). Amongst the major insect orders, Diptera (Box 10.5) have many aquatic representatives in the immature stages, and a substantial number of Hemiptera and Coleoptera have at least some aquatic stages (Box 10.6), and in the less speciose minor orders two families of Megaloptera and some Neuroptera develop in freshwater (Box 10.6). Some Hymenoptera parasitize aquatic prey but these, together with certain collembolans, orthopteroids, and other predominantly terrestrial frequenters of damp places, are considered no further in this chapter.
Aquatic entomologists often (correctly) restrict use of the term larva to the immature (i.e. postembryonic and prepupal) stages of holometabolous insects; nymph (or naiad) is used for the pre-adult hemimetabolous insects, in which the wings develop externally. However, for the odonates, the terms larva, nymph, and naiad have been used interchangeably, perhaps because the sluggish, non-feeding, internally reorganizing, final-instar odonate has been likened to the pupal stage of a holometabolous insect. Although the term “larva” is being used increasingly for the immature stages of all aquatic insects, we accept new ideas on the evolution of metamorphosis (section 8.5) and therefore use the terms larva and nymphs in their strict sense, including for immature odonates.
Some aquatic adult insects, including notonectid bugs and dytiscid beetles, can use atmospheric oxygen when submerged. Other adult insects are fully aquatic, such as several naucorid bugs and hydrophilid and elmid beetles, and can remain submerged for extended periods and obtain respiratory oxygen from the water. However, by far the greatest proportion of the adults of aquatic insects are aerial, and it is only their nymphal or larval (and often pupal) stages that live permanently below the water surface, where oxygen must be obtained whilst out of direct contact with the atmosphere. The ecological division of life history allows the exploitation of two different habitats, although there are a few insects that remain aquatic throughout their lives. Exceptionally, Helichus, a genus of dryopid beetles, has terrestrial larvae and aquatic adults.