352 



THE WINGS OF DIPTERA 



indicates that the two fonns belong to different Hnes of descent. The 

 common progenitor of these two genera had a four-branched radial sector ; 

 in some of the descendants of this primitive forai one method of reduction 

 has taken place, while in other descendants another method has been 

 followed. 



That this differentiation took place comparatively early in the history 

 of the order is shown by the fact that in all Nematocera that have a three- 



Fig. 364. — Wing of Leptis. 



branched radial sector veins Ro and R3 remain distinct; while in those 

 Brachycera that have a three-branched radial sector veins R4 and R5 are 

 separate. 



It is evident that as a rule an}- reduction in the number of the branches 

 of radius is the result of a coalescence that has preceded outward, the 



Fig. 365. — Wing of Dixa. 



method of reduction being the same as that which commonly takes place in 

 the Lepidoptera. But this is not always the case. In some of the fungus- 

 gnats, for example, there is shown the result of a coalescence that began at 

 the margin of the wing and preceded inward. This is illustrated by the 

 wings represented in Figure 361. In the wing shown at a in this figure, 

 vein R2+3 occupies the tisvial position of this vein ; but in the wing shown at 

 b it coalesces with vein Ri, and it is evident that the coalescence begati 



