Insect growth is discontinuous, at least for the sclerotized cuticular parts of the body, because the rigid cuticle limits expansion. Size increase is by molting — periodic formation of new cuticle of greater surface area and shedding of the old cuticle. Thus, for sclerite-bearing body segments and appendages, increases in body dimensions are confined to the postmolt period immediately after molting, before the cuticle stiffens and hardens (section 2.1). Hence, the sclerotized head capsule of a beetle or moth larva increases in dimensions in a saltatory manner (in major increments) during development, whereas the membranous nature of body cuticle allows the larval body to grow more or less continuously.
Studies concerning insect development involve two components of growth. The first, the molt increment, is the increment in size occurring between one instar (growth stage, or the form of the insect between two successive molts) and the next. Generally, increase in size is measured as the increase in a single dimension (length or width) of some sclerotized body part, rather than a weight increment, which may be misleading because of variability in food or water intake. The second component of growth is the intermolt period or interval, better known as the stadium or instar duration, which is defined as the time between two successive molts, or more precisely between successive ecdyses (Fig. 6.1 and section 6.3). The magnitude of both molt increments and intermolt periods may be affected by food supply, temperature, larval density, and physical damage (such as loss of appendages) (section 6.10), and may differ between the sexes of a species.
In collembolans, diplurans, and apterygote insects, growth is indeterminate — the animals continue to molt until they die. There is no definitive terminal molt in such animals, but they do not continue to increase in size throughout their adult life. In the vast majority of insects, growth is determinate, as there is a distinctive instar that marks the cessation of growth and molting. All insects with determinate growth become reproductively mature in this final instar, called the adult or imaginal instar. This reproductively mature individual is called an adult or imago (plural: imagines or imagos). In most insect orders it is fully winged, although secondary wing loss has occurred independently in the adults of a number of groups, such as lice, fleas, and certain parasitic flies, and in the adult females of all scale insects (Hemiptera: Coccoidea). In just one order of insects, the Ephemeroptera or mayflies, a subimaginal instar immediately precedes the final or imaginal instar. This subimago, although capable of flight, only rarely is reproductive; in the few mayfly groups in which the female mates as a subimago she dies without molting to an imago, so that the subimaginal instar actually is the final growth stage.
In some pterygote taxa the total number of preadult growth stages or instars may vary within a species depending on environmental conditions, such as developmental temperature, diet, and larval density. In many other species, the total number of instars (although not necessarily final adult size) is genetically determined and constant regardless of environmental conditions.