Complete metamorphosis is one of the three ways that insects reach the adult stage. Most insects, about 87%, undergo it. Butterflies and moths are the best known example of complete metamorphosis.
Primitive insects hatch as miniature adults, and change only by growing larger. They molt, splitting their skin and crawling out each time it becomes too tight. They undergo no metamorphosis at all.
Incomplete metamorphosis imposes a second stage between egg and adult. The creature that comes out of the egg is a nymph, which closely resembles the adult but lacks certain adult characteristics, for example wings. The dragonfly undergoes incomplete metamorphosis.
Complete metamorphosis happens in four stages: egg, larva (caterpillar), pupa (chrysalis), and imago (adult). The larvae usually look very different from adults. They eat different foods, have different habits, and live in different niches.
Insect eggs are very tough, though they may not be hard. Many can freeze and still produce larva. They are nevertheless produced in quantity by most species (though not all) in order that some survive. Within the egg, embryo cells are shaped either for roles they will play in the larva, or in the adult. One pattern will be put to use immediately, and one will be held latent.
Larvae generally have chewing mouth parts, while imagoes generally have sucking or piercing mouths. Thus, some caterpillars live on leaves, and chew them ragged, while the adults live on nectar, on rotting fruit as the Black Witch moth does, or sometimes on nothing. Larva may lack compound eyes. Maggots, for example, have primitive, nearly non-existent eyes, while fly eyes are compound. Larvae do not have wings.
The role of the larva is to eat and grow, and when it has eaten and grown enough, it becomes a pupa.
As a pupa, in a fuzzy cocoon or papery sheath, or in only its skin, the insect undergoes its greatest change. Larval cells die. A caterpillar loses most of its stumpy crawling legs, and extends six to the shock-absorbing pivots of a flying creature. A mosquito pupa, called a tumbler, grows wings. Mouths change function or lose it, and eyes are utterly transformed. Powdery or glassy wings develop, folded in the confining space of the chrysalis or cocoon. Reproductive organs grow.
The role of the adult insect is to reproduce. It flies to find a suitable mate, although sometimes the female stays near the shed chrysalis and gives off pheromones, and the male flies to find her. Sometimes he does not even eat while he searches, and she does not eat while she waits.
When the imago emerges, it is not ready to fly. Generally, the creature will crawl up a twig or blade of grass. Slowly, the wings unfold and unfurl. They color even more slowly, sometimes taking weeks to gain their brightest shades. When it is ready, an imago tries its wings and flies. It will not grow any more.
Some insects die right after mating, or after the eggs are deposited safely. Some live to mate twice in a summer, laying eggs that hatch in summer's warmth and others that wait (in diapause) until spring. Some overwinter, to mate and die in spring. Lifespans vary. A particular butterfly may lay five hundred eggs and die, but a termite queen may live for years, and lay 500 million eggs.
Insects that undergo complete metamorphosis pass through four life stages. Eggs are hidden, caterpillars stay down and safe, pupae are camouflaged and protected, but imagoes fly and look for mates. An egg becomes a larva, which becomes a pupa and then an imago. The imagoes mate, and produce more eggs, and the cycle begins again.