120 TEXT-BOOK OF ENTOMOLOGY 



d. The wings and their structure 



The insects differ from all other animals except birds in possessing 

 wings, and as we at the outset have claimed, it is evidently owing to 

 them that insects are numerically so superior to any other class of 

 animals, since their power of flight enables them to live in the air 

 out of reach of many of their enemies, the greatest destruction to 

 insect life occurring in the wingless larval and pupal stages. 



The presence of wings has exerted a profound influence on the 

 shape and structure of the body, and it is apparently due to their 

 existence that the body is so distinctly triregional, since this feature 

 is least marked in the synapterous insects. The wings are thin, 

 broad leaf-like folds of the integument, attached to the thorax and 

 moved by powerful muscles which occupy the greater part of the 

 thoracic cavity. The two pairs of wings are outgrowths of the middle 

 and hinder part of the thorax, the anterior pair being attached to the 

 mesothoracic and the hinder pair to the metathoracic segment. The 

 larger pair is developed from the middle segment of the thorax. 

 The differentiation of the tergites into scutum, scutellum, etc., is 

 the result of the appearance of wings, because these sclerites are 

 more or less reduced or effaced in Avingless insects, such as apterous 

 Orthoptera and moths, ants, etc. 



The size of the hinder thoracic segments is closely related to that 

 of the wings they bear. In those Orthoptera which have hind wings 

 larger than those of the fore pair, the metathorax is larger than the 

 mesothorax. In such Neuroptera as have the hind wings nearly or 

 quite as large as the anterior pair, or in the Trichoptera and in the 

 Hepialid;e, the metathorax is nearly as large as the mesothorax, 

 while in Coleoptera the metathorax is as large and often much larger. 

 In the Ephemeridse, Diptera, and Hymenoptera, which have either 

 only rudimentary (halteres) or small hind wings, the metathorax is 

 correspondingly reduced in size. 



The wings morphologically, as their development shows, are simple 

 sac-like outgrowths of the integument, i.e. of the free hinder edge of 

 the tergal plates, their place of origin being apparently above the 

 upper edge, of the epimera or pleural sclerites. Calvert 1 however, 

 regards the upper l;miin;i of the wing as tergal, and the lower, pleural. 



The wings in most insects are attached to the thorax by a mem- 

 brane containing several little plates of chitin called by Audouin 

 articulatory epidemes. 



1 Trans. Amer. Eat. Soc. xx, p. 168. 



