N0.1331. DRAGON-FLY WING VENAriON—NEEDIIAM. 723 



that only the part of the wing which lies back of the median vein has 

 been affected l)v thcni. But thut part has l)eon iiiodiliod j)rofoundly; 

 'in it entirely different ends have l)een wrought out in the two wings, 

 and l)y different means. The fore wing is characterized by greater 

 modification of parts ])resent in the ])i-imitive wing; the hind wing, by 

 the greater development of new parts. 



III. SOME (tENERAI> features OF THE WING. 



VEIN SHIFTING AND VKIN DIKKKKENTI ATION. 



There are two kinds of specialization occurring simultaneously in 

 insect wings, not clearly delimited, but, nevertheless, distinct enough 

 to enable us to understand in a measure the reasons foi- th(> success of 

 both. First, there is the shifting of veins for the mechanical advan- 

 tage of position and the development of strong cross veins to aid in 

 maintaining favorable positions. These developments have occupied 

 our discussion hitherto. We have seen that the course of specializa- 

 tion is traceable in each part, and we shall see further on that the 

 parts, varying independently or being modified together, collectively 

 furnish most excellent characters for interpreting the genealogy of the 

 group. 



Second, there is a specialization which is not confined to any par- 

 ticular part of the wing or to any particular structure, l)ut which con- 

 sists in the progressive differentiation between veins and membrane, 

 in the concentration of strength-giving wing material within the sup- 

 porting veins, and in the reduction of cross veins. Although less tan- 

 gible, perhaps, and certainly less useful for tracing genealogies, this 

 kind of specialization is none the less real. The first furnishes char- 

 acters differing in kind; the second, only those diflering in degree. 

 The first, rath(>r than the second, is characteristic of fossil species, 

 some fossil forms having carried the shifting of veins already described 

 to extremes unparalled at the present time. The second, rather than 

 the first, is characteristic of those modern genera which belong to the 

 cooler regions of the globe. The first is often accompanied by very 

 abundant" venation. The second is indicative of keener competition 

 in recent times and in temperate climates, necessitating greater 

 economy of wing-lniilding substance. 



A comparison of the wings of ^^'urotluuils (fig. JS) and Tetragon,mria 

 (fig. 19), two genera of Libellulida' that nray fairly be taken as expo- 

 nents of the two kinds of specialization, will make the difference 

 ])etw^een them clear. In Nexrothcnilxy^e. have \enatioii •' run wild" — 

 the vein-building material scattered with great i)rodigality throughout 

 the meml)rane, forming a very irregular and inconstant mesh work, 



"One iiiiKlit, almost say luxuriant. Possibly the advantage of the new position 

 attained by the shifting of parts is so great that over development results from it at 



tirst. 



Proc. N. M. vol. xxvi— 02 4i> 



