220 THE ORIGIN AND DEVELOPMENT 



as explained more fully on an earlier page, of a 

 thin double membrane supported by tubular rods, 

 which extend between them and diverge some- 

 what like the rods of a fan. In butterflies, the 

 number and distribution of the main rods or veins 

 are essentially the same in the front and hind 

 wings, although the wings themselves differ 

 greatly in shape. Omitting details, there is a 

 single unbranched vein in front and another be- 

 hind ; while between these are two others, both 

 branched, of which the front one (the second vein 

 of the wing) throws out branches on both sides 

 and the other only behind. This distribution of 

 the veins may, for our purpose, be considered as 

 dividing the wing longitudinally into four sepa- 

 rate areas, each occupied by a distinct vein with 

 its branches. The principal difference between 

 the two wings is found in the branches of the 

 second vein ; in the front wing the anterior 

 branches are numerous and most of them run to 

 the front border of the wing ; this arrangement is 

 plainly for greater strength, the front edge of the 

 fore wings having to bear the brunt of flight , 

 while the front edge of the hind pair requires no 

 such special provision, these wings in flight being 

 practically a part of the front pair ; and they 

 therefore have only a single front branch to the 

 second vein terminating on the outer margin. 

 Notwithstanding these great differences, but in 



