338 THE AMERICAN NATURALIST. (VoL. XXXII. 
That this differentiation took place comparatively early in 
the history of the order is shown by the fact that in all Nema- 
tocera that have a three-branched radial sector veins Kz and 
Cut rst 4 
Fic. 32. — Wing of Thereva. 
R} remain distinct ; while in those Brachycera that have a 
three-branched radial sector veins R4 and Æ; are separate. 
The coalescence of veins M3 and Cux:.— One of the most 
characteristic methods of specialization exhibited by the Dip- 
Cu, trst A M, + Cu, M, 
Fi. 33. — Wing of Eulonchus. 
tera is the coalescence of veins from the margin of the wing 
‘towards the base. This method of coalescence may occur 
between any two adjacent veins, and sometimes occurs in two 
or three different regions of the same wing. The most strik- 
Fic, 34. — Wing of Pantarbes. 
ing modifications in the courses of the wing-veins have been 
brought about in this way. Let us examine a series illustrat- 
ing different degrees of coalescence of veins M} and Cu. 
In Rhyphus (Fig. 29) these two veins retain their primitive 
position, extending nearly parallel and ending remote from 
