Wings 



27 



and hyaline, stiffened only along the front or outer 

 edge (costa) ; shorter and much broader than the fore- 

 wings (fig. 21). In all the wings a set of radiating 

 and cross nervures can be observed. The arrange- 

 ment of these nervures varies greatly in the different 

 groups of insects. In the Cockroach's hind-wing, 

 five principal radiating or longitudinal nervures can 

 be distinguished j these are the sub-costal, radial, ulnar, 

 median, and anal; in the fore-wing the ulnar is fused 

 with the radial (fig. 21). In a Fly's wing (fig. 22) 

 there are two 

 sub-costals, a 

 radial, an 

 ulnar, two B^^^g 

 medians, and 

 an anal. In 

 the wings of 

 Moths and 

 other insects 

 there are cubi- 

 tal nervures 

 between the 

 medians and 

 theanals. But 



It is not at alJ p^^ 21.— Mesonotum and metanotum of Male Cock- 

 Certain that roach with wings of left side. Twice natural size. 



, After Miall & Denny. 



the nervures 



which have received the same names in different 



insects really correspond (13, 14). 



Microscopic examination shows that an insect wing 

 consists of a twofold layer of skin, the two layers 

 being in contact except where they are thickened and 

 raised into folds so as to form the firm tubular nervures, 

 which serve as a supporting framework to the wing 

 membrane, enclose air-tubes and serve as blood- 

 channels (15). 



