86 THE AMERICAN NATURALIST. [Vow. XXXII. 
distinction between principal veins and cross-veins. For, al- 
though in certain highly specialized wings, as, for example, those 
of the Odonata, every cross-vein is preceded by a trachea, we 
have found that, as a rule, the secondarily developed cross- 
veins are not preceded by trachez. The figures of Nemoura 
in Chapter I illustrate this. 
In the adult the front, or costal margin, of the wing is usually 
strengthened by a vein or a vein-like structure; this is desig- 
nated as the costa. A study of immature wings shows that, 
although the costa usually extends more or less nearly to the 
apex of the wing, the costal trachea is, as a rule, greatly reduced. 
This reduction of the costal trachea has led to its being over- 
looked by previous writers, and to a denial of its existence by 
Brauer and Redtenbacher.! It is true that Brongniart figures 
what he believed to be the costal trachea in the nymph of a 
dragon fly;? but the structure which he represents is 
evidently the edge of the wing within the wing sheath of the 
nymph. 2 
We have succeeded in finding the costal trachea in nearly all 
of the orders of winged insects, and have found that in widely 
separated forms, as in many Hemiptera and in the more gen- 
eralized Hymenoptera, it extends nearly or quite to the apex 
of the wing. Further details regarding it will be given in the 
treatment of the separate orders. It is only necessary to state 
here that we have abundant evidence to support the view that 
the costa of the primitive insect wing resembled the other wing- 
veins in being preceded by a trachea, and that the origin and 
course of this trachea was probably very nearly as represented 
by Cin Fig. 4. In the photographs of the wings of a nymph 
of Nemoura, reproduced in Chapter I, the costal trachea is not 
evident; but figures will be given of other Plecoptera in which 
this trachea is as distinct as any and extends to the middle of 
the wing. 
The second of the principal veins of the wing is designated 
as the subcosta. This extends more or less nearly parallel with 
the costa and but a short distance from it. In those orders 
1 Zool. Anz., Bd. xi, 1888, pp. 443-447. 
2 Rech. sur les Insectes Fossiles, Pl. viii, Fig. 1, a. 
