21 Postembryonic Development 1. Introduction During their postembryonic growth period insects pass through a series of stages until they become adult, the time interval occupied by each instar being terminated by a molt. Apterygotes continue to grow and molt as adults, periods of growth alternating with periods of reproductive activity. | 21 Postembryonic Development 1. Introduction During their postembryonic growth period insects pass through a series of stages instars until they become adult the time interval stadium occupied by each instar being terminated by a molt. Apterygotes continue to grow and molt as adults periods of growth alternating with periods of reproductive activity. In these insects structural differences between juvenile and adult instars are slight and their method of development is thus described as ametabolous. Among the Pterygota which with rare exceptions do not molt in the adult stage two forms of development can be distinguished. In almost all exopterygotes the later juvenile instars broadly resemble the adult except for their lack of wings and incompletely formed genitalia. Such insects in which there is some degree of change in the molt from juvenile to adult are said to undergo partial incomplete metamorphosis and their development is described as hemimetabolous. Endopterygotes and a few exopterygotes have larvae whose form and habits by and large are very different from those of the adults. As a result they undergo striking changes complete metamorphosis spread over two molts in the formation of the adult holometabolous development . The final juvenile instar has become specialized to facilitate these changes and is known as the pupa see also Chapter 2 Section . In insect evolution increasing functional separation has occurred between the larval phase which is concerned with growth and accumulation of reserves and the adult stage whose functions are reproduction and dispersal. Associated with this trend is a tendency for an insect to spend a greater part of its life as a juvenile which contrasts with the situation in many other animals. Thus in apterygotes the adult stage may be considerably longer than the juvenile stage. Furthermore feeding in the adult serves to provide raw materials both for reproduction and for growth. In exopterygotes and primitive .
