228 



GENERAL BIOLOGY 



Second, there are fewer veins in most of the wings figured 

 than in the diagram. Veins may disappear through fusion 

 of two or more branches into one, or, more rarely, by atro- 

 phy. Fusions may occur between branches: a) from the 

 tips approximated on the wing margin, proximally to the 

 forks ; b) from the forks distally to the wing margin, or c) by 



FIG. 140. Venation of the wings of various flies (order Diptera). a, Rhyphus; b, 

 Conops; c, Erax; d, Dixa; e, Xylophagus ; /, Thereva; g, Eristalis; h, Stratiomyia. 

 All from Comstock. 



the elimination of a cross vein through confluence of branches 

 of adjacent veins, and subsequent fusion distally to the wing 

 margin. Various stages of progress in all the methods of 

 disappearance of branches will be found in the wings figured 

 herewith. 



