THE WINGS OF LEPIDOPTERA 



339 



vein Ml coalesces with the radial sector for a considerable distance. It 

 is obvious that in this respect, the extent of the coalescence of veins Mi and 

 Rs, Adoneta is the more highly specialized of the two genera. 



It is an interesting fact that in cases like Anosia (Fig. 346) where the 

 vestiges of the basal part of media are short spurs projecting into the discal' 

 cell, the branches of media are often discontinuous with these spurs. It is 

 obvious that the spurs indicate the positions fonnerly occupied by the 

 branches of vein M and that 

 they have been left stranded 

 upon the discal vein while the 

 functional parts of the branches 

 to which they correspond have 

 moved into new positions. 

 This is especially marked in 

 the case of vein M3 of the fore 

 wing oi Anosia. 



The union of vein Mi with 

 radius and of vein M3 with 

 cubitus after the atrophy of 

 the base of media is what 

 would be expected. But in 

 which direction would one 

 expect the base of vein M2 

 to migrate? Occupying an 

 intermediate position between 

 radius and cubitus it may go 

 either way. It is like a stream 

 in the middle of a level plain, 

 a trifle may change its course. 

 And thus we find that in some 

 families it migrates towards 

 cubitus making this vein appar- 

 ently fotir-branched, while in 

 other families it goes towards 

 radius, leaving cubitus appar- 

 ently three-branched. The 



former condition exists in the Papilionidas (Fig. 350); 

 Pieridas (Fig. 342). 



This difference may be looked upon as a difference in kind of specializa- 

 tion, and is of high value as indicating a dichotomous division of the line 

 of descent. It is obvious that in a family where vein M2 has migrated far 

 towards cubitus and has thus established its chief source of air supply in 

 that direction, it is not probable that genera will arise in which vein M2 is 



Fig. 350. — Wings of Papllio polyxenes. 



the latter, in the 



