Aquatic insects

Insects include the greatest number of species of any class of animals. They have been divided into thirty Orders.
However only four Orders of insects consist of species whose larval forms are always aquatic, while another nine Orders contain some species with either aquatic larvae or which are aquatic throughout larval and adult stages.
Insects are characteried by a hard, segmented, exoskeleton and by a three segmented thorax, each bearing a pair of legs and usually with two pairs of wings attached to the second and third thoracic segments. In the Diptera and some mayflies (Ephemeroptera) the wings are reduced to a single pair, while in primitive insects such as springtails and silver fish, wings are absent. They have also been lost from some species of more advanced orders of insects.