Metamorphosis

Metamorphosis is a dramatic reorganization of an animal's body as it changes from a juvenile form into an adult. It is best known in insects, but frogs, sea squirts and many marine invertebrates also go through it. Insects show two main kinds. In complete metamorphosis, seen in beetles, flies, moths, butterflies, bees and ants, development passes through egg, larva, pupa and adult. The larva, such as a caterpillar or maggot, looks nothing like the adult; inside the pupa, most larval tissues are broken down and the adult body is built from groups of cells set aside earlier. In incomplete metamorphosis, seen in grasshoppers, cockroaches, dragonflies and true bugs, there is no pupa: the young, called nymphs, resemble small wingless adults and change gradually with each molt.

Hormones control the process. Ecdysteroids trigger each molt, while juvenile hormone keeps the insect in its immature form; when juvenile hormone falls in the last larval stage, the next molts produce the pupa and adult. Because larvae and adults of fully metamorphosing insects often eat different foods and live in different places, they do not compete, which may help explain why these groups are the most species-rich of all insects.