6o John Henry Co?nsiock 



were always developed, the marginal [costa] simple and 

 forming the costal border. 



From this brief study of the development of the subcostal 

 area let us pass to the area lying next to the opposite margin 

 of wing, the anal area, omitting for a time any discussion of 

 the three veins (radius, media and cubitus) which occupy the 

 central portion of the wing. 



A striking feature in the structure of the wings of many in- 

 sects is the separation of the anal area from the remainder of 

 the wing by a fold or furrow, along the bottom of which ex- 

 tends a vein. Such a depressed vein has been termed, on ac- 

 count of its position, a concave vein ; and in contradistinction 

 to such veins, those veins which extend along the summit of 

 ridges, or which are more prominent on the upper surface of 

 the wing than on the lower, are termed convex veins. 



This furrow separating the anal area from the preanal por- 

 tion of the wing appeared very early. It is especially promi- 

 nent in all cockroaches both fossil and living ; and can usu- 

 ally be recognized in any insect wing in which the anal area 

 is well developed. I have been unable to determine the sig- 

 nificance of it. But have found it a very useful mark in de- 

 fining the limits of the anal area. It is vein VIII of the 

 nomemclature adopted in this paper. 



The primitive form of the anal area is probabl}' well shown 

 in Mylacris, (Fig. 7), where it closely resembles the primitive 

 form of the subcostal area, as shown in the same genus. But 

 the latter specialization of this area has been very different 

 from that of the subcostal. This specialization has taken 

 place in two opposite directions, i. e., by reduction and by 

 addition. 



In certain lines of development the tendency of natural 

 .selection has been to preserve the narrower winged forms. 

 And the narrowing of the wings has taken place largelj' 

 through a partial or complete reduction of the anal area. 

 The dragon-flies, Odonata, and the ant-lions, Myrmeleon, 

 are examples of the extreme result of this tendenc^^ And in 

 the lyepidoptera there are several instances where a good 

 series illustrating successive stages in this reduction can be 



