— 77 — 



indicated by the letters c v ; two other cross- veins are repre- 

 sented near the base of the wing, but are not lettered. All 

 other veins represented in this figure are longitudinal veins. 

 The insertion of the word normally in the above defini- 

 tions is important ; for it is only in comparatively general- 

 ized wings that the direction of a vein can be depended 

 upon for determining to which of these two classes a vein 

 belongs. A little later the student will study wings in 

 which the direction of some of the longitudinal veins has 

 been so modified in the course of specialization that the 

 veins extend transversely (z. e. , cephalo-caudad), and some 

 cross- veins extend in a longitudinal direction (/. e. , proximo- 

 distad). 



Simple veins and branched veins — Veins are either 

 simple or branched. The veins numbered II and IX in 

 Figure 2 are simple veins ; between these there are three 

 branched veins. 



In the case of branched veins the entire vein including all 

 of its branches is often referred to as a single vein. Thus 

 the third vein in the wing of Rhyphtcs, counting the 

 thickened cephalic margin of the wing as the first vein, is 

 termed vein III ; and by this expression we include both 

 the main stem of the vein and its three divisions. On the 

 other hand, each division of a branched vein is often termed 

 a vein. Thus the first division of vein III, counting from 

 the cephalic margin of the wing, is termed vein Illr, and 

 the second division, vein III2, and so on till all are 

 numbered. 



Note. — In the most generalized flies known to us vein III is five- 

 branched. But in most flies some of the branches of this vein 

 coalesce so that the number of apparent branches is less than five. 

 In JRhyphus, veins III 2 and III 3 coalesce so as to appear as a single 

 vein. In order to indicate that this apparently simple vein is com- 

 posed of two veins, and in order that homologous veins in different 

 insects shall bear the same number, this compound vein is numbered 

 vein III2 + 3. In the same way, what appears to be the third branch 



