THE EXTERNAL ANATOMY OF INSECTS 63 



numeral indicating the number cf the branch ; thus, for example, the 

 first branch of the radius is radius-one or vein Ri. 



In the case of radius and media, each of which has more than two 

 branches, each division of the vein that bears two or more branches 

 has received a special name. Thus after the separation of radius-one 

 from the main stem of radius there remains a division which is typi- 

 cally four-branched; this division is termed the radial sector, or 

 vein Rsi the first division of the radial sector, which later separates 

 into radius-two and radius-three, is designated as radius-two-plus- 

 three or vein R2+3; and the second division is termed radius-four- 

 plus-five or vein R4+5. Media is typically separated into two divi- 

 sions, each of which is two-branched; the first division is media-one- 

 plUs two or vein Mi +2, the second is media-three-plus-four or vein 

 M3+4. 



The veins of the anal area.— The three anal veins exhibit a wide 

 range of variation both as to their persistence and to their form when 



Fig. 78. — A wing of Rhyphns. 



present. In those cases where the anal veins are branched there is 

 no indication that the branching has been derived from a uniform 

 primitive type of branching. For this reason in describing a branched 

 anal vein merely the nimiber of branches is indicated. 



In some cases, as in the Odonata, there is a single anal vein the 

 identity of which can not be determined. In such cases this vein is 

 designated merely as the anal vein or vein A, and its branches as Ax, 

 Ai, Az, etc. 



The reduction of the number of wing-veins. — In many wings the 

 nimiber of the veins is less than it is in the hypothetical type. In 

 some cases this is due to the fact that one or more veins have faded 

 out in the course of the evolution of the insects showing this deficiency; 

 frequently in such wings vestiges of the lacking veins remain, either 

 as faint line-s in the positions formerly occupied by the veins or as 



