THE WINGS OF INSECTS 



391 



two other longitudinal veins (the so-called premedia and postmedia) were present in 

 certain orders of insects, and the numbers IV and VI were applied to these veins. It 

 has since been determined that this conclusion was based on an error. 



Certain writers number the wing veins without omitting the numbers IV and VI, 

 designating the media as vein IV and the cubitus as vein V. 



There are still other writers who do not regard the costa as a true vein, and, there- 

 fore, designate the subcosta as vein I. 



The result is that there are three distinct systems of numbering the wing-veins,^in 

 addition to several old systems which were applied to single orders. It seems better, 

 therefore, to designate the wing-veins by names, and use abbreviations of these names 

 in lettering figures. 



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

 dragon-flies, the 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 411 represents the hypothetical primitive type of wing- 



Fig. 411. — Hypothetical type of wing-venation with the named cross- 

 veins added. 



venation with the named cross-veins added in the positions in which they 

 normally occur; these are the following: 



The humeral cross-vein. — This extends from subcosta to costa near the 

 humeral angle of the wing (Fig. 411, h). 



The radial cross-vein. — This extends between the two principal divisions 

 of radius, i. e. from vein Ri to vein R^ (Fig. 411, r). 



The sectorial cross-vein. — This extends between the principal divisions of 

 the radial sector, i. e. from vein Ro-t-s to vein R4-1-5 or from vein R3 to vein R4 

 (Fig. 411. s). 



The radio-medial cross-vein. — This extends from radius to media, usually 

 near the center of the wing, (Fig. 411, r-m). When in its typical position 

 this cross-vein extends from vein R^-f-o to vein Mi-^2- 



TJie medial cross-vein. — This extends from vein M2 to vein M3 (Fig. 

 411, m). This cross-vein divides cell Mo into cells ist M2 and 2d Mo; see 

 Figure 410, where the cells are lettered. 



