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 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 



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 R 2+3 to vein R4+5 or from vein 

 RS to vein R 4 . 



d 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 M 2 to vein M 3 . This 

 cross-vein divides cell M 2 into cells, ist M 2 and 26. M 2 ; see Figure 87 

 where the cells are lettered. 



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

 cubit us. 



