248 COMPARATIVE ANATOMY. 



In the Pseudoneuroptera botli pairs of wings ai-e very similar in 

 character. In the other four- winged orders they have undergone 

 considerable differentiations. In addition to the differences in size, 

 which in the Hymenoptera and Lepidoptera are generally due to the 

 increased size of the first pair, there are differences of structure, 

 which imply a difference in function. In the Orthoptera the first 

 pair is often merely a covering for the second pair; this is very 

 distinct in the Beetles where the second pair is often rudimentary. 

 The elytra have then become organs of defence for the subjacent 

 abdomen. In the Hemiptera there is a similar differentiation. 

 The Diptera have the anterior pair only, the hinder pair being 

 rudimentary, and represented by the so-called balancers (halteres). 

 On the other hand, the Strepsiptera possess the hinder pair of 

 wings, or those attached to the third thoracic segment, only. 



Integument. 



§ 191. 



The integument of the Arthropoda is more independent and less 

 connected with the muscular system than in the preceding divisions ; 

 it can always be divided into two layers. The cuticular layer, 

 secreted by a layer of cells which is often much modified, extends 

 over the whole surface of the body just as it does in many Vermes ; 

 it is also continued into the openings of the internal organs. Owing 

 to its strength it forms the most important part of the integument, 

 but it varies greatly in thickness and firmness. It is soft and flexible 

 between the segments of the body, where these parts are movably 

 connected with one another, but it is generally stronger on the 

 metameres themselves, and on the appendages. Its physical cha- 

 racters vary widely ; there are all kinds of transitions between the 

 soft investment of the body of the Spiders and most Insect larvae, 

 and the firm carapace which covers the body of most of the 

 Crustacea, the Myriapoda, Scorpions, and such Insects as the Cock- 

 roach. The variations in its firmness depend not merely on the 

 thickness of the cuticle, but on the chitinisation of its layers. Even 

 thick layers, when freshly formed, are soft, and only become resistent 

 when chemical change occurs. To increase the firmness of this 

 chitinous carapace calcareous salts are deposited in it, in many 

 Crustacea, and even in the Myriapoda. When this cuticle has be- 

 come firm it limits the growth of the body ; in these cases therefore 

 so long as growth is going on the cuticle is periodically cast 

 (Ecdysis). 



In consequence of its mode of origin the cuticular layer is every- 

 where distinctly laminated. As a rule it is traversed by pore-canals, 

 into which processes of the subjacent matrix are continued. The 

 relatively thin '''matrix" of the cuticular layer is homologous with 



