Insects and related arthropods must pass through a series of developmental stages on their way to becoming sexually mature adults (Agrell and Lundquist 1973). These stages are all the more discrete because of the necessity of molting and growth in stepwise phases. The animal itself between molts is referred to as an "instar," the time period, "stadium." In virtually all insects, the first instar possesses the complete number of segments after hatching; in other groups, segments are added as development proceeds.

As the animal progresses toward maturity, it increases in size, and changes in internal and external form and proportions occur to a greater or lesser degree (Sehnal 1985). In most noninsects and primitive apterous insects, the immatures are fairly similar to the adults. Juvenile insects of the higher orders that possess wings, however, undergo a fair amount of body modification, called metamorphosis, primarily associated with the growth of the wings and exploitation of habitats different from the adult. Metamorphosis is said to be "gradual" (incomplete) in lower winged insects with externally developing wing buds; the single juvenile type is called a nymph (or sometimes naiad in aquatics). Nymphs generally have feeding and other habits similar to the adult; naiads live rather different lives because of their water habitats. Metamorphosis is "complete" in the higher winged insects. In these, there are two fundamental juvenile stages: a larva, which has several instars; it finally molts into a pupa, which eventually yields the adult. These early stages look totally unlike and live in ways very different from the adult and indeed diverge from them in almost every way. This has contributed to the evolutionary success of these insects through the dichotomous specialization of life functions (feeding and growth by immatures, dispersal and reproduction by adults). Divergence of body form and function has even taken a further step in many species with varying types of larvae (hyper-metamorphosis) such as found in the blister beetles (Meloidae), chalcidoid wasps, and others. Immatures of different insect groups are called by various names. For example, larvae of Lepidoptera are caterpillars; pupae of butterflies, chrysalids; larvae of muscoid flies, maggots; and beetle larvae, grubs. Pupae generally are protected by their location, underground in cells or in wood or other material or encased in a cocoon of silk spun by the prepupal instar.

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