It is important to remember that most insects spend most of their time as juveniles. Juveniles are also more common, as most will not reach adulthood.
It is often the juvenile stages that are most economically important.
Most insects hatch from eggs (oviparity).
The exoskeleton of insects is rigid, so to increase in size, they must moult and lay down a new exoskeleton (see cuticle and moulting) until they are adults.
Each of these juvenile stages between moults is called an instar. (The time that is spent in a particular instar is called the stadium).
Most species of insects go through a particular number of instars although it may vary between the sexes and be shortened according to climate or food supply.
Insects develop by one of 3 types of metamorphosis or change which defines their type of life cycle.
This is essentially no change. Young emerge from the egg looking very much like a small adult. With each instar, the insect increases in size and eventually gains sexual maturity. This is the life cycle of primitive insects such as silverfish (Thysanura). Such primitive insects may continue to grow and moult even after becoming adult.
Here there is a gradual change. Although juveniles often still have the same form as the adult, there is a development of wings through the instars and often changes in shape and colour. There are 3 distinct stages in the life cycle - egg, nymph (juvenile of any instar) and adult.
The larva is usually a caterpillar, grub or maggot and bears little resemblance to the adult. During these instars, the juvenile is actively feeding and growing.
The pupa is usually an inactive stage while the insect reconstructs itself into the adult form.
Flies (Diptera), beetles (Coleoptera), wasps (Hymenoptera) and butterflies (Lepidoptera), have holometabolic life cycles.
Some aquatic hemimetabolous insects breathe through gills and appear quite different to their adults. Some entomologists call these juveniles larvae, although strictly speaking, they are nymphs.
Larval forms can be named by the number and type of legs they possess. The larval form (or pupal form) is important in the identification of the order and family of insect.