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 R 3 and Rj+s- 

 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 



jdA 



2 dA 



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 occur; these are the following: 



The humeral cross-vein (h) 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 R s . 



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

 of the radial sector i. e., from vein Ra+s to vein R4+s or from vein 

 Ra to vein R 4 . 



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 Rj+s to vein MI +2- 



The medial cross-vein (m) extends from vein Mo to vein M 3 . This 

 cross- vein divides cell M2 into cells, ist M2 and 2d M2j see Figure 87 

 where the cells are lettered. 



The medio-cubital cross-vein (m cu) extends from media to 

 cubitus. 



