No. 377.] THE WINGS OF INSECTS. 337 
gnats (Cecidomyiidz) it is reduced to a simple, unbranched 
condition. 
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 
Fic. 30. — Wing of Leptis. 
importance, serving 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 wing of Leptis (Fig. 30) and of Dixa (Fig. 31) 
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 
mutA Cu, 
Fic. 31. — Wing of Dixa. 
R2 and R, coalesce ; while in Dixa it is veins R4 and R; that 
have grown together. This is a difference in kind of speciali- 
zation, which indicates that the two forms belong to different 
lines of descent. The common progenitor of these two genera 
had a four-branched radial sector; in some of the descendants 
of this primitive form one method of reduction has taken place, 
while in other descendants another method has been followed. 
1 Loc. cit., Fig. 522. 
