364 ZOOLOGY. 



wings ; three pairs of thoracic legs ; often a pair of jointed 

 abdominal appendages, besides the ovipositor or sting which 

 morphologically represents three pairs of legs. 



There are eight orders, forming two series : first, the 

 Heterometabola, comprising -the Thysanura, Neuroptera, 

 Orthoptera, Hemiptera, and Coleoptera, in which the meta- 

 morphosis is either incomplete or complete ; the body 

 shows a tendency to become flattened ; the prothorax is 

 usually large and flattened ; the mouth-parts (except in the 

 Hemiptera) adapted for biting. There are a number of 

 wingless forms. 



The second series of orders, or Metabola, comprising the 

 Diptera, Lepidoptera, and Hymenoptera, usually have a 

 cylindrical body, with the prothorax small, the mouth-parts 

 often adapted for sucking or lapping up liquid food ; with 

 a complete metamorphosis, the larva being worm-like, 

 cylindrical, very unlike the adult. 



Order 1. Thysanura. The spring-tails (Podura) and 

 bristle-tails (Lepisma) represent this group. They are 

 wingless, with some affinities to the Myriopods, and the 

 typical form Campodea (Fig. 324) is regarded as the ances- 

 tral form of the six-footed insects, as it is a 

 generalized type, and forms like it may have 

 been the earliest insects to appear. 



In Podura, the spring-tail, and also 'n 

 Smynthurus (Smynthurus quadrisignatus 

 Pack., Fig. 322), the characteristic organ is 

 a forked abdominal appendage or " spring," 

 held in place by a hook ; when released the 

 spring darts backward, sending the insect 



Pig. V&.-Smyn- u , . ,, 

 thurus, a spring-tail, high in the air. 



Our commonest Poduran is Tomocerus 

 plumbeus Linn. (Fig. 323), found all over the northern 

 hemisphere in North America and Europe. The snow-flea, 

 Achorutes nivicola Fitch is blue-black, and is often seen 

 leaping about on the snow in forests. 



The Podurans belong to the suborder Collembola ; the 

 higher forms, which bear a greater resemblance to the larvae 

 of Neuropterous insects and to the young cockroach, are 



