THE EXTERNAL ANATOMY OF INSECTS 



71 



veins will come together at some point more or less remote from their 

 extremities and merge into one for a greater or less distance, while 

 their extremities remain separate. In the fore wing of Prionoxystus 

 (Fig. 84), for example, there is an anastomosis of veins R3 and R4+5. 

 The named cross-veins. — In the wings of certain insects, as the 

 dragon-flies. May -flies, and others, there are many cross-veins; it is 

 impracticable in cases of this kind to name them. But in several of 

 the orders of insects there are only a few cross-veins, and these have 

 been named. Figure 85 represents the hypothetical primitive type 



3dA 



2d A 



Fig. 85. — The hypothetical primitive type of wing- venation with the named 

 cross- veins added. 



of wing-venation with the named cross- veins added in the positions in 

 which they normally occm"; these are the following: 



The humeral cross-vein Qi) extends from the subcosta to costa near 

 the humeral angle of the wing. 



The radial cross-vein (r) extends between the two principal divi- 

 sions of radius, i. e. from vein Ri to vein Rg. 



The sectorial cross-vein (5) extends between the principal divisions 

 of the radial sector — i. e., from vein R2+3 to vein R4+5 or from vein 

 R3 to vein R4. 



The radio-medial cross-vein (r — m) extends from radius to media, 

 usually near the center of the wing. When in its typical position 

 this cross-vein extends from vein R4+5 to vein Mi +2. 



The medial cross-vein (m) extends from vein M2 to vein M3. This 

 cross-vein divides cell M2 into cells, ist M2 and 2d M2; see Figure 87 

 where the cells are lettered. 



The medio-cubital cross-vein {m — cu) extends from media tc 

 cubitus. 



