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, /ongiiudina/ 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 norma//y 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 isf 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- 



