Some of the best known cases of metamorphosis occur in insects, a class of the Arthropoda phylum. There are about a half million known species of insects, and great diversity in the way different insects develop. According to one classification scheme based on metamorphosis, insects are classified as Ametabola, Hemimetabola, or Holometabola.
The Ametabola do not undergo metamorphosis. This is an evolutionary primitive condition and is exemplified by insects such as the bristletails and springtails. During development, these insects increase in size, but do not undergo distinct changes in form. In general, the Ametabola do not have wings.
The Hemimetabola undergo gradual metamorphosis. This is exemplified by insects such as the dragonflies, termites, roaches, and grasshoppers. In the Hemimetabola, a form called the nymph hatches from the egg. Nymphs lack wings, but have compound eyes and otherwise resemble the adult form, except they are smaller. The wings of the Hemimetabola grow gradually during a series of molts, developmental periods in which the cuticular exoskeleton is shed, allowing for growth.
The Holometabola undergo complete metamorphosis. This is exemplified by insects such as moths, butterflies, wasps, and flies. In the Holometabola, a worm-like larva with short legs, no wings, and simple eyes, hatches from the egg. As in the Hemimetabola, the larva increases in size through a series of molts. Eventually, the larva develops into a pupa inside a cocoon. The pupa is often considered a resting stage and can often survive in unfavorable environments. Eventually, the pupa metamorphoses into an adult. In this process, the pupa resorbs larval organs and uses them as nutrients while special groups of cells, called imaginal discs, form and reshape the insect. The adult typically has wings, compound eyes, legs, antennae, and sexual organs.