224 NEW YORK STATE MUSEUM 



The main phenomena of speciaHzation of wings of craiie flies 

 are three: 



1 The bringing into hne for mutual support of cross veins and 

 adjacent forks. This may be soon understood by comparing figures 

 12 and 13, or by comparing any specialized crane fly wing with the 

 wing of Macrochile [pi. 14, fig. i]. 



2 The reduction of superfluous veins and cross veins. 



3 The differentiation between the veins that remain. 



The last two processes are really one; for both are at bottom 

 and redistribution of the strength-giving material of the wing. 

 When but one cross vein is left where two were before, that one 

 is larger and stronger. This may be perhaps regarded as hypo- 

 thetical, but the converse of it is not, for two cross veins occasion- 

 ally appear adventitiously, where normally there is one (as, for 

 example, cross vein m in Acyphona) and then they are always 

 markedly weaker than the single one would be. The differentia- 

 tion between longitudinal veins consists likewise essentially in 

 putting the strength-giving material where it will do the most good. 

 That differentiation is but little exaggerated in figure 13. Witness 

 the photographs of plates 11 to 13. Strong and weak stems alter- 

 nate.^ Radius is strong and media weak, cubitus is strong and the 

 first anal weak, the second anal is stronger than the first, and the 

 third is usually absent altogether. 



Our knowledge of the mechanics of insects is yet exceedingly 

 meager and unsatisfactory. It is sufficient to be suggestive how- 

 ever of possible reasons underlying the main phenomena of their 

 specialization. 



Elements of the venation individually considered 

 It will be advantageous now to consider the parts of the venation 



severally, looking first at the veins, then at the cross veins, and 



then at their mutual behavior and adjustment. 



Longitudinal veins. The costa is always simple, and forms 



the front border of the wing. 



The subcosta is a weaker vein that lies in the bottom of the 



furrow of the fore border between costa and radius. It is bound 



to radius by a basal fusion and to costa by the humeral cross veins : 



* I have noted a similar differentiation under very different vein arrange- 

 ment in the Odonata [U. S. Nat. Mns. Proc. 1903. 26:737], and have stated 

 the conclusion thus: "The strong vein bounds posteriorly the area m 

 which the weaker one lies." 



