An Introduction to the Invertebrates, Second Edition - tiera.ru

WHAT IS DISTINCTIVE ABOUT INSECT LIFE CYCLES?20115.4 What is distinctive about insect life cycles?The egg is often very resistant and may be an important resting stage.Its early development is described in Chapter 19. Very often thereis division of labour within the life cycle, made possible as incrustaceans by metamorphosis: many insects have a feeding larvadifferent in form and food supply from the reproductive adult. Unlikecrustaceans, however, there is often a winged adult, and this ratherthan the larva is the agent of dispersal.15.4.1 ExopterygotesThese are the insects that have a gradual metamorphosis from thelarva (sometimes called a ‘nymph’) into the adult (‘imago’). Wing budsgrow externally on the larval thorax (hence ‘Exopterygota’). Manyexopterygote adults and larvae, such as those of cockroaches, lookvery similar, live in the same environment and presumably competefor resources. Aquatic larvae (as in stoneflies and dragonflies) differgreatly from the adults in feeding mechanism and appearance. Inmayflies the aquatic larva lives for up to three years and the veryshort-lived aerial adult has no gut and cannot feed at all.15.4.2 EndopterygotesThese have ‘complete metamorphosis’, with total reorganisationof the body within the pupa. From the outset the larva containsthe rudiments of adult organs (the ‘imaginal discs’). These remaininactive during larval life but at the final moult, when, in the absenceof juvenile hormone, the pupa is formed, the adult rudimentsdevelop and the larval tissues are resorbed. Endopterygote larvaetypically look very different from the adults, have different feedingmethods and exploit different niches.15.4.3 DiapauseStages of inaction in the life cycle are often necessary to match ananimal to its seasonal environment. The pupa of endopterygoteinsects may appear to be a resting stage while there is great activityinside, but then it may become entirely quiescent and providea resting stage in the life cycle, additional or alternative to the egg.‘Diapause’ is a period of arrested development, characterised by lowoxygen consumption. The cue to enter diapause is most frequentlya change in day length (‘photoperiod’), rather than in temperatureor humidity, which may be very variable. In the cabbage whitebutterfly (Pieris rapae), for example, the shorter day length at the endof summer switches off brain hormone in the final larval stage.The pupa can be formed, but cannot moult, and therefore goes intodiapause. After an extended period of chilling (winter) the brain hormoneswitches on again, the pupa moults and the butterfly emerges.