In biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes gradual metamorphosis (hemimetabolism) before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult, except for a lack of wings (in winged species). In addition, while a nymph moults it never enters a pupal stage. Instead, the final moult results in an adult insect. Nymphs undergo multiple stages of development called instars.
This is the case, for example, in Orthoptera (crickets and grasshoppers), Hemiptera (cicadas, shield bugs, etc.), mayflies, termites, cockroaches, mantises, stoneflies and Odonata (dragonflies and damselflies).
Nymphs of aquatic insects, as in the Odonata, Ephemeroptera, and Plecoptera, are also called naiads, an Ancient Greek name for mythological water nymphs. In older literature, these were sometimes referred to as the heterometabolous insects, as their adult and immature stages live in different environments (terrestrial vs. aquatic).
In fly fishing with artificial flies, this stage of aquatic insects is the basis for an entire series of representative patterns for trout. They account for over half of the over all patterns regularly fished in the United States.