Hemimetabolism

Hemimetabolism or hemimetaboly, also called incomplete metamorphosis, is a term used to describe the mode of development of certain insects that includes three distinct stages: the egg, nymph, and the adult stage, or imago. These groups go through gradual changes; there is no pupal stage. The nymph often resemble the adult somewhat, as they have compound eyes, developed legs and wing stubs visible on the outside.

Examples of hemimetabolous insects

 * Hemiptera (scale insects, aphids, whitefly, cicadas, leafhoppers and true bugs)
 * Orthoptera (grasshoppers and crickets)
 * Mantodea (praying mantids)
 * Blattaria (cockroaches)
 * Dermaptera (earwigs)
 * Odonata (dragonflies and damselflies).

Previously used nomenclature
Hemimetabolous insects used to be further divided into two categories: paurometaboly and heterometaboly. In paurometabolous insects, the nymph and the adult would live in the same environment (water, air, soil, etc.). This is the case of the Orthoptera (grasshoppers and crickets) and some Hemiptera (true bugs) are examples of this. The nymph and adult of heterometabolous insects live in different environments. For example, Odonata naiad live in the water and cicada nymph underground, whereas the adults are aerial.

Terminology of aquatic entomology
In aquatic entomology, different terminology is used when categorizing insects with incomplete metamorphosis. Paurometabolism refers to insects whose nymphs occupy the same environment as the adults, as in the family Gerridae of hemiptera. The hemimetabolous insects are those whose nymphs, called naiads, occupy aquatic habitats while the adults are terrestrial. This includes all members of the orders Plecoptera, Ephemeroptera, and Odonata. Aquatic entomologists use this categorization because it specifies whether the adult will occupy an aquatic or semi aquatic habitat, or will be terrestrial. This classification system is similar to previously used nomenclature in terrestrial entomology.