588 



ARTHROPODA. 



so that the wings not only act as organs of flight, but at the same 

 time assist in the process of respiration. Normally, two pairs of 

 wings are present, but one or other may be wanting. 



The arrangement of the " nervures " of the wings is definite and 

 characteristic in different groups of the Insects. In the most typical 

 forms of wing, as in the Orthoptera (fig. 442, a), there are six prin- 

 cipal veins, which arise, in groups of three, from two principal roots, 



one anterior and the other 

 posterior. According to 

 the nomenclature followed 

 by Heer and Scudder, 

 these six principal veins 

 are termed, from before 

 backwards, the marginal, 

 mediastinal, scapular, ex- 

 te?'no?nedian, internomed- 

 ian, and anal veins. The 

 general arrangement of 

 these veins (fig. 442, a) is 

 as follows : " The margi- 

 nal Vein (ma) is placed at, 

 or close to, the anterior 

 margin of the wing; and 

 " the ' mediastinal ' and 

 ' scapular ' veins, which 

 are superior (i.e., part from 

 the main vein on the upper 

 or anterior side), terminate 

 upon the anterior margin. 

 The ' internomedian ' and 

 'anal-' take the opposite 

 course, and their branches 

 are inferior, or, at least, 



Fig. 441. — Diagram of the external anatomy of an 

 Insect, a, Head carrying the eyes (o) and antennae (an) ; 



b, First segment of the thorax, with the first pair of legs ; 



c, Second segment of the thorax, with the second pair 

 of legs and the first pair of wings ; d, Third segment of 

 the thorax, with the third pair of legs and the second 

 pair of wings ; e, Abdomen, without limbs, but carrying 

 terminal appendages concerned in reproduction; f, 

 Femur ; /, Tibia ; ta, Tarsus. 



posed between these two sets, 



nomedian,' which is 

 terminates at the tip of the 



directed towards the inner 



margin ; while the ' exter- 



inter- 



wing, 



and branches indifferently on either side " (Scudder). In some 

 groups of the Insects there may be a suppression of certain of 

 these six primary veins, and the above general arrangement is liable 

 to characteristic modifications in different cases. The value of the 

 characters derived from the neuration of the wings is, however, im- 

 paired by the fact that entomologists have not adopted a uniform 

 nomenclature of the nervures in different orders of the Insecta. 

 In the Coleoptera (Beetles) the wings of the anterior pair become 



