Postembryonic Development

Abstract

Among the evolutionary trends that may be seen in insect postembryonic development are (1) increasing separation of the processes of growth and accumulation of reserves (functions of the juvenile stage) from reproduction anddispersal,which are functions of theadult stage; (2) the spending of a greater proportion of the insect’s life inthe juvenile stage; and (3) an increasing degree of difference between larval and adult habits and form. The latter has been accompanied by modification of the final larval instar into a pupa in which the considerable changes from larval to adult form can occur. Insects may be arrangedinthree basic groups in terms of the pattern of postembryonic development that they display. Apterygotes are ametabolous; that is, thechanges from ju- venile to adult form are very slight. Adults continue to molt, and the number of instars is both large and variable. Almost all exopterygotes are hemimetabolous. Juveniles broadly resemble adults and undergo only a partial metamorphosis. The number of instars is gener- ally four or five and constant for a species. Themajor event in exopterygote metamorphosis isthe full development of wings and genitalia. Internal organs grow progressivelythrough larval life. Endopterygotes and averyfew exopterygotes are holometabolous. Juveniles and adults are normally strikingly different and major changes (complete metamorphosis) occur in the pupa. In primitive endopterygotes most organs growprogressively during larval life, and metamorphosis consists mainly of the development of the flight mechanism. In most endopterygotes considerable differentiation of adult tissues occurs during metamor- phosis, often from imaginaldiscs, groups of cellsthat remainembryonicthroughlarval life, probably because of the hormonal milieu in juvenile instars. Several types of endopterygote larvae may be distinguished: oligopod (including scarabaeiform and campodeiform larvae), which are most primitive; polypod (eruciform); and apodous (including eucephalous, hemicephalous, and acephalous larvae). Pupae may be decticous (having functional mandibles) and exarate (appendages not sealed against the body), or adecticous and exarate or obtect (appendages sealed against the body). For protection, a pupa maybe enclosed within a cocoon or cell,ormaybe heavily sclerotized and/or camouflaged, or in cyclorrhaph Diptera remain inside the cuticle of the last larval instar (the puparium). Adult emergence (eclosion) isachievedbyswallowing airto increase the bodyvolume and thus splitthe pupal cuticle.When an insect pupates in a cocoon, etc., it may chewor tear its way out, either as a pharate adultorafter eclosion, using mandibles, spines, or cocoon cutters. The cocoon of some species is equipped with an escape hatch. Hormones regulate development. A molt cycle is initiated with the release of protho- racicotropic hormone from the brain, which stimulates release of ecdysone from themolt glands. The nature of amolt is determinedbythe concentration of JH at a critical period inthe stadium.When the JH concentration isaboveathreshold value, a larval-larval molt follows; at below-threshold concentrations the molt is larval-pupal or pupal-adult. Polymorphism is the existence of several distinct forms of the same stage of a species. It may have a genetic basis (as in transient andbalanced polymorphism) or be inducedby changing external conditions (polyphenism), whose effects are manifest viatheendocrine system, specificallythe concentration of JH at critical periods. Examples of polyphenism are caste differentiation in social insects, seasonal polymorphism in aphids, and the phases of locusts.