78 University of California Puhlicutions. [Entomology 



produced. In other cases where numerous veins occur, the 

 specialization is one that has fi'xed the many-veined condition. 



Specialization by reduction is a common phenomenon; with- 

 out doubt much of the reduction that has occurred is produced 

 by normal decrease, but in most cases part of the reduction 

 has been brought about through specialization. The speciali- 

 zation of cross veins is always a reduction process. Whenever 

 any cross vein becomes strengthened it decreases the chances 

 for survival of the adjacent cross veins. The cross veins 

 were produced simultaneously over the whole wing, and the 

 natural tendency would be for them to disappear in the same 

 way whenever the conditions ceased to be favorable for their 

 production. Those to survive would be the ones specialized 

 through some special association with the longitudinal veins. 



There are at least four ways in which a cross vein may come 

 into this unusual relationship: It may so lie as to bring the 

 end of an independent into close connection with it, and so 

 become, to some extent, the continuation of the independent 

 vein; it may lie exactly opposite another cross vein as though 

 the cross vein were continuous over the longitudinal vein; it 

 may come to lie in a position parallel with the margin and 

 be favored more on account of the weakness of the membrane 

 in this region; or, finally, it may gain its preeminence over 

 neighboring cross veins by binding the longitudinal veins 

 together at critical points. 



In regard to the first of these methods, it may be remarked 

 that the connection of the independents to the outgrowing veins, 

 making them take on all the appearance of branches, is an 

 extremely common occurrence. An independent vein naturally 

 follows a furrow, and a cross vein has the same peculiarity. 

 When the longitudinal furrow of the membrane begins to dis- 

 appear through the approximation of the adjacent veins, it 

 would not be unnatural to expect that the vein would have a 

 tendency to turn aside, if a transverse furrow were at hand. 

 Thus, the cross vein lying here would be augmented by the tip 

 of the independent and make such a large opening into the 

 adjacent vein that the cross vein and independent vein together 

 would resemble a branch of the stronger vein. 



The development of cross veins exactly opposite one another 

 is a very common occurrence in wings. Their usual place of 

 development is across a vein tliat lies mainly on one side of 



