THE CLASS AND ORDERS OF INSECTS 177 



the absence of the gonapophyses and in the absence of scales, 

 which form a close covering to the body and limbs in most 

 members of the order. They pass, therefore, through a slight 

 but definite transformation. 



Order 2. Protura 



The Protura are very small wingless insects without feelers 

 or cerci. The jaws are modified into piercers and the creatures 

 feed by suction. The abdomen has a pair of short unjointed 

 limbs on each of the first three of its eleven segments. 



Order 3. Collembola 



The Collembola or spring-tails have short feelers with four 

 or six segments, no compound eyes but groups of ocelli (usually 

 eight on each side of the head), the mouth inpushed so that 

 the jaws seem retracted within the head. The abdomen 

 consists of six segments only, the limbs of the first segment 

 are united to form an adhesive ventral tube, those of the 

 third are minute, forming a catch (retinaculum) which holds 

 the spring consisting of the limbs of the fourth abdominal 

 segment. The air-tube system is usually wanting, breathing 

 being carried on through the thin body-wall. 



Young spring-tails closely resemble their parents and 

 undergo no transformation. 



Sub-ClaSS II. EXOPTERYGOTA 



The great majority of these insects are winged ; the wingless 

 condition of some is clearly secondary. Wing-rudiments 

 appear on the second and third thoracic segments at an early 

 stage in the life-history, and increase in size until the penulti- 

 mate stage, after which wings become fully developed in a 

 short time so that they can be used for flight. The newly- 

 hatched young of these insects may resemble their parents 

 very closely, except for the absence of wings (ametabolous 

 insects) or may differ so markedly from them that they have 

 to undergo a considerable transformation (hemimetabolous 



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