Life Cycle of Aquatic Insects
What is a nymph? How about larvae? Are your thinking about pixies and maggots? Well, you are partially correct. Chances are you already are familiar with the adult forms of many aquatic insects.
The aquatic insects comprise the bulk of benthic macroinvertebrate communities in healthy, freshwater streams. These insects are mostly in their immature form and live their adult life on land, sometimes for only a few hours. Most aquatic insects can be divided into two groups: ones that develop through complete metamorphosis, and ones that develop through incomplete metamorphosis.
Metamorphosis is the change that occurs during the organism's development from egg to adult. Some aquatic insects develop through complete metamorphosis, which consists of four stages. These immature insects are called larvae and the do not resemble the adults, and in fact, may look grossly different. During the pupae stage, the organisms inhabit a "cocoon-like" structure where the transformation from larvae to adult occurs.
Incomplete metamorphosis has three main stages of development (except for the mayfly that has two winged growing stages). These immature insects are called nymphs and they undergo a series of molts until the last decisive molt transforms the organism into an adult or imago in mayflies. There is no intermediate pupae stage where transformation occurs. The nymphs resemble the adults closely except for wing development.