EVOLUTION 331 



wings suggests that early in the history of the class arose a 

 general tendency of hindwings to become broader than 

 forewings, with the development in many groups of a folding 

 anal area ; this is well seen in Orthoptera, Plecoptera, 

 Trichoptera,and many Lepidoptera,the forewings tending to 

 become elongate and relatively narrow. In the Orthoptera 

 and many Hemiptera the forewings are firmer in texture 

 than the hindwings, for which, in the resting position, they 

 form protective sheaths. This tendency is carried to its 

 extreme in the Coleoptera, most of whose families have the 

 forewings (elytra) hard and horny like the body-sclerites, 

 and devoid of all trace of the typical wing-neuration. The 

 varying relations of the different orders in which these 

 wing modifications are displayed suggests that they have 

 arisen independently along several lines of evolution in the 

 history of insects, and this viev/ receives confirmation from 

 the differences observable in the detailed structure of the 

 wings themselves. In the Orthoptera the fore wing is 

 elongate, narrow and firm in texture, but it displays the 

 typical series of nervures. This is also the case in some 

 Hemiptera, but in the large sub-order of the Heteroptera 

 the forewing is, as a rule, sharply divided into a larger basal 

 and median area, which is firm and horny and shows the 

 course of a few only of the nervures, and a smaller apical 

 membranous area often traversed by the full series of 

 nervures with their characteristic branches. In the beetles, 

 or Coleoptera, as already mentioned, the hardened fore- 

 wings retain no trace of the nervures ordinarily characteristic 

 of insect wings, so that it has been doubted if they should 

 be regarded as wings at all, and a suggestion was at one time 

 put forward that they correspond to the tegulae. But the 

 demonstration in the elytra of beetle pupae of the typical 

 tracheation that prefigures the wing-nervures has established 

 beyond doubt that these structures are indeed greatly 

 modified forewings, and some recent discoveries among 

 fossil insects, to be mentioned later in this chapter, throw 

 light on the course of their modification. 



The scheme of insect classification reminds the student 



