The life cycle of insects that undergo incomplete metamorphosis (or hemimetabolous insects) takes place in three stages: egg, nymph and adult. When they hatch from the egg, nymphs often already look like adults. Some people call the nymphs of hemimetabolous insects larvae. In scientific literature, the term "naiad" is also used to describe the aquatic nymph of certain species, such as dragonflies and mayflies.
Usually, nymphs are just as mobile as adults, except that they cannot fly. They can live in the same environments as adults or in very different environments. Their diet may be the same as or different from the adult’s diet. After moulting a certain number of times, the insect metamorphoses directly into an adult, also called an imago.
The scenario is almost the same for dragonflies and mayflies, but the differences between nymphs and adults are more pronounced. Nymphs live in the water and breathe using gills, while adults live in the air and breathe using tracheas.
Mayflies present a phenomenon that is unique in the insect world. Once its life in the water is over, the nymph swims to the surface of the water and transforms into an individual that, although winged, is not entirely an adult. It is then called a subimago or pre-adult. Its last moult follows a few hours later, and it is then that the insect reaches adulthood.