THE WINGS OF DIPT ERA 



351 



been cited as an illustration of this; and in the Psychodidas (Fig. 362) all of 

 the branches of radius remain distinct. 



But usually the number of the branches of this vein is reduced by a 

 coalescence of some of the branches of the radial sector. Thus in many 

 families the radial sector is three-branched, in others it is only two-branched 



Fig. 362. — Wing of a moth-like fly. 



and in the gall gnats (Cecidomyiidas) it is reduced to a simple, unbranched 

 condition (Fig. 363). 



As this variation in the number of the branches of this vein is due to a 

 greater or less degree of coalescence among them, it is evident that here is a 

 character of considerable taxonomic importance ser\'ing as it does to 

 indicate degrees of divergence from the primitive type. 



Not only do we find differences in degree of reduction of this vein, but 

 differences in the method of reduction are also shown. If the wings of 



Fig. 363. — Wing of a gall-gnat. 



Leptis (Fig. 364) and of Dixa (Fig. 365) be compared it will be seen that 

 although in each the radial sector is only three-branched, the reduction has 

 been brought about in a different way in the two genera. In Leptis veins 

 R2 and R3 have coalesced; while in Dixa it is veins R4 and R5 that have 

 grown together. This is a difference in kind of specialization, which 



