226 



GENERAL BIOLOGY 



The order in which they are named is that of their arrange- 

 ment from front to rear. Branches of veins are conveniently 

 designated by numerals added in like order to the abbrevia- 

 tion for the vein (as Sc x and Sc 2 for the two branches of the 

 subcostal vein). But there is one large branch so distinc- 

 tively formed that it has received a special name, the radial 

 sector (Rs) . All these veins and their usual mode of branch- 

 ing are shown in solid lines in the accompanying diagram of 

 a typical wing. In dotted lines are shown the cross veins of 

 most frequent occurrence. Two of these toward the base 

 of the wing the humeral cross vein (h) and the arculus (ar) 

 have received special names; the others are named in 



*_ 



11 



FIG. 138. Diagram of the venation of an insect wing. 



accordance with the positions they occupy in relation to 

 the veins. The radial cross vein (r) and the median (m) 

 occupy the principal forks of the radial and median veins 

 respectively, and radio-median (r-m) and medio-cubital 

 (m-cu) connect the veins whose names they bear. 



These, then, are the materials with which we have to 

 deal in the following exercise. While they appear simple 

 and distinct enough in the diagram, a glance at the three 

 series of wing figures shows that it is not at once easy to 

 be certain as to their identity. For : 



