378 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



be functional as in the Coleoptera (beetles). In no 

 case are they developed except on the second and 

 third segments of the thorax (rneso- and meta- 

 thorax). 



A wing consists essentially of two flattened 

 A 



81 



Fig. 160. Skeleton of Butterfly's Wing. 



A, Costalvein ; B, subcostal ; mi m*, median branches ; ri 7-2, radial ; si s5, subcostal 

 branches; c, median vein ; D, submedian vein ; dc. discocellular veinlet ; 

 in, internomedian veinlet ; B, internal vein. (After Butler.) 



membranes, the presence of which is due to the organ 

 arising in the form of a sac, which gradually becomes 

 elongated and flattened out ; through its substance 

 there pass blood-vessels and air tubes, the walls of 

 which are strengthened by chitin ; the chitin may 

 invade the rest of the wing, and convert it into a more 



