THE WINGS OF CORRODENTIA 



259 



made from photographs of wings, which were so mounted that the develop- 

 ing wing-veins appeared as pale bands, and the wing-tracheae as dark lines. 

 A remarkable feature of the fore wing of Psocus (Fig. 255) is that, 

 although it is braced in every direction, there is not a single cross- vein in it, 



Fig. 256. — Fore wing of a nymph of Psocus (After C. & N.). 



the bracing being accomplished by the zigzag courses of the principal veins. 

 There is an arculus (ar) near the base of the wing, which is formed of the base 

 of media ; and what appear as cross-veins in the central portion of the wing 

 are sections of media and of cubitus. It was these apparent cross- veins 

 that made the determination of the homologies of the wing- veins difficult 

 before the tracheation of the wings was observed. 



Although there are no cross- veins in the wing represented by Figure 255, 

 cross-veins exist in the wings of certain members of this order. In some 

 genera the radial cross-vein is present and in some, instead of an anasto- 



Fig. 257. — Fore wing of a full-grown nj'mph of Psocus (After C. & N.). 



mosis of veins M and Cui, these veins are connected by a medio-cubital 

 cross-vein. Both of these cross-veins exist in the fore wing of Stenopsocus 

 (Fig. 258). 



