88 



student can gain a good idea of the type of the wings of in- 

 sects belonging to the order Diptera, and have a standard 

 with which to compare wings of insects of other orders. 



Longitudinal veins and cross-veins. The veins can 

 be grouped under two heads : first, longitudinal veins, those that 

 normally extend proximo -distad ; and second, cross-veins, 

 those that normally extend more or less nearly cephalo- 

 caudad. In Figure 2, three of the cross-veins are indicated 

 by arrows, near the middle of the wing ; two other cross- 

 veins are represented near the 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 definitions 

 is important ; for it is only in comparatively generalized 

 wings that the direction of a vein can be depended upon for 

 determining to which of these two classes it belongs. A 

 little later the student will study wings in which the direc- 

 tion of some of the longitudinal veins has been so modified 

 in the course of specialization that they extend transversely 

 (i.e., cephalo-caudad), and some cross-veins extend in a longi- 

 tudinal direction (i.e., proximo-distad). 



Simple veins and branched veins. Veins are either 

 simple or branched. The veins lettered Sc and 1st A 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 Rhyphus, counting the thick- 

 ened, cephalic margin of the wing as the first vein, is termed 

 the radius or vein R ; 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 the radius, count- 

 ing from the cephalic margin of the wing, is termed radius- 



