I So LEPIDOPTERA. 



nervures. A fashion has arisen of counting them backwards 

 — not from the front, or costal margin, but from the back, or 

 dorsal margin of the wing. On the whole this is the easier 

 method, though it is by no means universally approved or 

 adopted. By it the nervure nearest the dorsal margin is 

 called vein 1. It is very commonly single and undivided, 

 and so forms a convenient starting-point. The next nervure 

 from the base is the median, and it is always divided into 

 three, sometimes into four branches (trifid or quadrifid), and 

 as these divisions become, for practical purposes of expansion 

 of the wings, main ribs, they are called nervures or veins 2, 

 3, 4< and 5, veins 2, 3 and 4 being permanent, and vein 5 the 

 great variable character on which the classification has been 

 made to depend. The remaining main nervure from the 

 base, the subcostal, is also divided into three strong 

 branches, veins 6, 7 and 8, and these nervures, or veins 1 to 

 8, at moderately even distances — following out the com- 

 parison to an umbrella — stretch the wing into shape, the 

 margin being produced a little at the termination of each strong 

 nervure in a faint likeness to the tips of the umbrella, so that 

 the actual margin of the membrane is usually a little crenulated 

 or scalloped, although this formation is not usually com- 

 municated to the cilia. But vein 5 is, in this group, usually 

 not a strong nervure, but in numerous species extremely thin 

 and weak, and in many of such tenuity that it can only be 

 seen, under a good magnifier, by a faint line of reflected 

 light, being otherwise totally lost in the deep fold which 

 takes place down its line. Wherever this vein 5 is strong and 

 stout, the margin takes the usual extension at its tip, and 

 this is diminished or lost in accordance with the tenuity of the 

 nervure. In all such cases it takes its origin from a short, 

 slender, sometimes angulated or oblique, horizontal nervure, 

 which crosses the space from the median to the subcostal 

 nervure, uniting them at about the angle of veins 3 and 4 and 

 of veins 7 and 8. This is known as the discocellularnervule, 

 but may for convenience be called the cross-bar. It forms the 



