LIFE-HISTORIES 247 



equivalent to the subclass Exopterygota, but this connection ob- 

 scures the important formal difference between nymph and naiad. 



The remaining insects are Holometabola. There is a true larva 

 in every sense of the word, and so a true metamorphosis. The 

 features in which the larva differs from the adult are its general 

 form ; the immaturity of its gonads ; the presence of internal 

 wing buds instead of wings ; the mouth-parts ; and the possession 

 of small lateral simple eyes or ocelli in place of compound eyes. 

 In addition aquatic larvae possess respiratory adaptations. The 

 Holometabola comprise all the typical members of the subclass 

 Endopterygota. There are several different types of larvae, such 

 as those which, possessing only thoracic legs, resemble the primi- 

 tive wingless insects and are called oligopod or campodeiform 

 (e.g. the water-beetle Dytiscus and other beetles), the caterpillars 

 of butterflies and others, which have abdominal legs as well as 

 thoracic, and are called polypod or cruciform, and the apodous 

 or limbless grubs or maggots of flies. In several insects more 

 than one type of larva is found in a single life-history, when 

 hypermetamorphosis is said to occur. 



Metamorphosis is concentrated into a restricted stage of the 

 life-history, that of the penultimate instar, which is called a pupa. 

 In most Holometabola the wings and legs have no secondary 

 attachment to the body and may be capable of some movement ; 

 the pupa has a form roughly between that of the larva and the 

 adult, and is caUed exarate. In butterflies and moths, some 

 beetles, and most two-winged flies, the limbs, though just visible 

 externally, are closely glued down to the body by the moulting 

 fluid of the previous ecdysis, such a condition being called obtect. 

 In the houseflies and their relatives the pupa is coarctate, that is, 

 enclosed in the last larval skin, called the puparium, and no limbs 

 are visible. The typical pupa is inactive, quiescent, and incapable 

 of feeding. Inside its skin a great deal of breakdown of the body 

 goes on, largely by phagocytosis (p. 192), and the form of the 

 imago is built up by the division and growth of persistent em- 

 bryonic cell masses called imaginal discs. The gut, limbs, many 

 muscles and many tracheae are often completely replaced in this 

 way. The nervous system persists throughout metamorphosis, 

 although there may be much growth and alteration, especially of 

 the brain ; the heart beats throughout pupation and undergoes 

 only slight changes. The gonads grow throughout life, and in a 

 few species even become functional in the larva. Finally the pupal 



