No. 376.] THE WINGS OF INSECTS. 237 
had been understood by Adolph we would have been spared his 
misleading theory of alternating concave and convex veins.! 
II. THE VENATION OF THE WINGS OF CERTAIN PLECOPTERA. 
If we leave out of consideration the anal area, that portion 
of the wing traversed by the anal veins, we will find that in 
nearly every case each order of insects is characterized by 
either a reduction or a multiplication of the wing-veins; in 
certain orders the tendency is in one direction, while in others 
it is in the opposite. But either of these tendencies may be 
correlated with a similar tendency in the anal area or with the 
opposite one. In this chapter we purpose to point out the ways 
in which the primitive type of wing venation has been modified 
in representatives of several of the orders where a reduction in 
the number of wing-veins in the preanal area has taken place. 
In the order Plecoptera, or stone flies, we find that, although 
in most genera the anal area of the hind wings has been 
expanded and the number of anal veins increased, in the preanal 
areas of both wings the number of wing-veins has been increased 
in certain genera and reduced in others; and we cannot say that 
either of these tendencies has yet attained the ascendency within 
this order. | 
This fact, taken in connection with the generalized condition 
of the basal attachments of the trachez of the wings, already 
pointed out, leads us to believe that the Plecoptera as a whole 
depart less widely from the primitive winged insect than do the 
living representatives of any other order. 
In this place, we have to do only with those Plecoptera in 
which a reduction in the number of wing-veins in the preanal 
area of the wing has taken place. Of these, the genera 
Nemoura and Tzeniopteryx are taken as examples. And we 
use, for the purposes of this study, wings of nymphs taken at 
a stage when the forming wing-veins appear as light-colored 
bands and the trachez, about which they are formed, as dark 
lines. 
1G. Ernst Adolph, Ueber Insectenfliigel. Mova Acta der ksl. Leop.-Carol.- 
Deutschen Akademie der Naturforscher., Ba. xli, pp. 215-251. 1879. 
