The immature insect, or larva, escapes from the egg and acts essentially as an eating machine, gathering nutrients. The larva has no wings or reproductive organs and has a form which does not resemble the adult of the species, but appears to be more like its ancient pre-insect ancestor. The larva's body is generally wormlike and may have no legs or may have extra legs to support the long body. As the larva eats, it grows extraordinarily quickly, molting a specific number of times before it pupates. The larvae of beetles are commonly called grubs, the larvae of flies maggots, and the larvae of butterflies and moths caterpillars.
When the larva has grown large enough and ingested enough food, it enters a stage of apparent dormancy as a pupa. The larva usually protects itself, either in a secure hiding spot, within a shelter of its own construction, or inside of a cocoon spun of silk from a gland near its mouth, as it prepares to pupate. It appears to be resting, but in fact, nothing could be further from the truth. It is during this phase that the metamorphism occurs. The insect must completely rearrange its internal and external structure and create some entirely new structures such as reproductive organs and wings.
The mature insect represents the true form of the species: it is the larva which departs from the characteristic body plan. The adult emerges head-first from the skin of the pupa, fills its wings with blood and, immediately or within a few hours, it flies away. In all insect species, only the adult can reproduce. Indeed, in many species the adult's sole purpose is reproduction; many adult moths lack a feeding apparatus entirely and live only a few days, just long enough to mate, whereas houseflies, for example, are well known for their adult eating habits.
Other insects undergo what is termed incomplete metamorphism. The immature individuals of the species, called nymphs if they live on land and naiads if they live in water, closely resemble the adult form, but with significant differences. Such features as coloration and the shape of the body may differ between the nymph and the adult, but the most significant difference is that the nymph has no wings and cannot reproduce. All of these insects go through a procedure called molting numerous times as they grow. When the idividual becomes too large to fit comfortably inside of its skeleton, a hormone is produced which triggers molting. The immature insect then develops a soft protective layer underneath its hard skeleton, splits this shell and pulls itself out of it, and lets its new, soft skeleton harden. In all incompletely metamorphosing winged insect species, the wings only appear with the final molt, as they would otherwise be shed with the skeleton and they cannot be replaced.