Evolution and Taxonomy 



65 



broadly expanded so as to provide for a sliding flight, there is 

 a necessitj' for the plaiting of these wings when not in use so 

 that they may be carried without impeding locomotion on foot ; 

 second, we find in certain cases where the tendency of spec- 

 ialization has been towards a narrowing of the wings in order 

 to admit of vigorous flight, a corrugation of the wings has 

 taken place in order to strengthen them. The hind wings of 

 a grasshopper illustrate the first ; and the wings of a dragon 

 fly present the extreme of the second form of specialization. 



It is easy to see that a 

 corrugated wing, like that 

 of the dragon fl3', is much 

 stiffer than it would be if 

 the membrane extended in 

 a single plane. If one will 

 examine the cross veins ex- 

 tending between the costa 

 and the radius in a dragon 

 fly, he will find that some 

 of these are in the form of 

 triangular braces which ef- 

 fectually prevent any ten- 

 dency on the part of the 

 wing to become flattened. 

 Evidently the corrugation 

 is of extreme importance. 



The concave veins have 

 arisen in two wa3'S. The 

 first of these is by a change 

 in the position of a convex 

 vein. The subcosta in 



mo.st of the orders of insects is an illustration of this. In 

 the Lepidoptera the subcosta has retained its form as a 

 convex vein, but in most orders of insectst he area be- 

 tween the costa and the radius has been depressed forming a 

 furrow along the bottom of which the subcosta extends. This 

 corrugation has resulted from the need of a stiffening of the 

 costal edge of the wing. The second method of formation of 



