70 Jolui Henry Cotnstock 



ding to its value. Thus in the Lepidoptera I designate the five 

 branches of radius as IIIi, IIIj, III3, III4, and 1 1 Irrespectively, 

 although all of these branches are convex. 



It should be noted that in numbering the branches of a 

 principal vein, they are numbered in the order in which they 

 reach the margin of the wing, not in the order in which they 

 are given off from the main stem. The system adopted is not 

 only the simpler but insures the same number being applied 

 to homologous veins in different genera, which would not be 

 the case were the other system adopted. 



Having indicated the more general features in the develop- 

 ment of the wings of insects in order to define the nomen- 

 clature of the wing veins that I have adopted, and having 

 explained this nomenclature, I can now pass to the considera- 

 tion of certain details exhibited by the wings of the Lepidop- 

 tera. 



The more important of these are the changes which take 

 place in media ; for this vein in the Lepidoptera is of the high- 

 est value for taxonomic purposes. 



The tendency to abortion of the main trunk of media has 

 already been pointed out. The explanation of this tendency 

 I have not fully determined satisfactorily to myself, I can 

 only suggest the following : In the course of the narrowing 

 of the wing and the strengthening of the main veins which 

 has taken place as a result of a natural selection of the more 

 active flying forms, the veins have become crowded together 

 at the base of the wing. The more important veins, i. e., 

 radius and cubitus, have held their place, while media has 

 been crowded out. This crowding out has probably taken 

 place in this way. The narrowing of the .space occupied by 

 media compressed the large trachea or tracheae which it con- 

 tained. Such a compression tends to shut off the supply of 

 air to that part of the wing supplied by the branches of media. 

 To counteract this evil, communication is established between 

 the branch of media and the veins lying on either side of it. 

 When such a communication is well established there is no 

 longer any need for the basal portion of media and it becomes 

 atrophied. In this connection it should be stated that the mem- 



