Metamorphosis, Insect

Metamorphosis is a temporal process of development involving the interaction of hormones triggered atparticular stages of growth. Metamorphosis of greater or lesser degree is found in most organisms where there is a developmental transition over time in body form between the egg and adult. Insects undergo a particularly noticeable metamorphosis involving distinct stages of development that often occur in different habitats or utilize different food resources. Developmental transitions occur between different juvenile stages and are terminated when the adult stage is reached.

with a relatively inflexible outer integument or exoskeleton, insect growth is only possible through periodic shedding or molting of cuticle between each instar followed by a rapid expansion of a soft, [Page 856]new cuticle until it hardens. This expansion facilitates further growth during each developmental stage or instar. At emergence from the egg, most insects are structurally different from their adult counterparts. This difference may be slight or pronounced. Juvenile stages are usually characterized by feeding, growth, and development of external and internal structures such as wings and reproductive organs that are not fully developed until the final molt into the adult. In many insects metamorphosis is confined to a series of instars during a single season or year for those species with an annual life cycle.

Metamorphosis between instars may be confined to a matter of days in species with rapid life cycles (such as insects feeding on ephemeral fungal fruiting bodies) or spread out over many years in long-lived species with an extended juvenile growth period. There can also be considerable variation within a single species. Juvenile development in the wood-boring ghost moth Aenetus virescens, for example, may vary from as little as 9 months between egg and adult to as long as 4 years within a single population. The number of instars is also variable between species, and sometimes within species. Many insects, especially those that develop through their life cycle each season, have relatively few instars, with four to five stages being common.

There are several distinct patterns of insect metamorphosis with contrasting developmental patterns that contribute to the structural diversity of a group of organisms that may otherwise have had a more homogenous appearance. Insects that never evolved wings, where juveniles resemble adults and adults also continue to molt, are called ametabolous (or aptergota = without wings). This development pattern occurs in five primitive insect orders that include springtails and the common silverfish. Most insect orders are bemimetabolous (also called exop-terygota for their externally visible wing development) with a dimorphic life history divided into a series of nymphs that molt through several instars and adults that do not molt. In these insects, wings develop gradually as external wing pads in the older juveniles, and only the adult has fully functional wings (with the unique exception of mayflies, where the final juvenile instar has fully developed wings). Hemimetabolous insects include grasshoppers and their close relatives, such as stoneflies, and true bugs.

Insects with the most distinctive stages of metamorphosis are bolometabolous, where there are three main stages of development: larva, a pupa, and adult. In this developmental sequence the larva is structurally and behaviorally different from the adults. Compound eyes are usually absent in the larval and pupal stages (where the eyes are otherwise absent limited to several single lenses). The holometabola are also referred to as endop-terygotes, because the wings and other features develop internally until the pupal stage, when they become everted and visible externally, although they are not fully expanded to the adult structure. Evolution of holometabolous development is widely regarded as a key evolutionary innovation contributing to the comparatively diverse speciation within 11 orders that compose about 75% of all insect species.

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