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240 THE AMERICAN NATURALIST. [VOL XXXII. 
cubital trachea is typical in both wings, and the anal veins are 
quite similar to those of Nemoura. 
There are two points of especial interest in the fore wing of 
this insect, both showing the importance of ontogenetic study 
in determining the homologies of wing-veins. 
First, it is evident that, correlated with the great reduction 
of the radial sector, vein Mı of the fore wing, which remains 
distinct from vein M2 in this genus, has come to perform the 
function that is performed by vein R445 in Nemoura; and, as a 
result, it has assumed a similar position. So great is the simi- 
larity that one who studied only the wings of the adult Tzeni- 
opteryx would be certain to mistake vein Mı for a branch of 
the radial sector. A glance at Fig. 10, which represents the 
fore wing of the adult of another species of this genus, will make 
this more evident. If the object in view were merely to num- 
ber the wing-veins, it may be that a mistake of this kind would 
not be serious; but when the object is to determine the relation- 
ships of allied forms, such a mistake would surely lead one astray. 
The second point illustrates specialization by addition, and 
it is anticipating somewhat to allude to it here. It will be 
observed that in Fig. 10 a vein which ends in the margin of 
the wing midway between veins Cz: and Cu2 is labeled 1. This 
is what we shall define later as the first accessory cubital vein. 
A reference to Fig. 9 will show that, although this vein has the 
same appearance as other longitudinal veins in the adult, it is 
not preceded by a trachea in the nymph, but, like the cross-veins, 
is formed secondarily. This is an illustration of the beginning 
of a process which is carried to a great extent in those insects 
that have wings with many wing-veins, and which will -be 
described in more detail later. 
It will be seen from these two illustrations that a study of 
the ontogeny of the wings opens a fruitful field to one engaged 
in a study of the genetic relationships of winged insects. 
III. THE VENATION OF THE WINGS OF PSOCUS. 
The determining of the homologies of the wing-veins in 
Psocus and allied genera is a problem that has sorely puzzled 
all who have worked upon it; and it has remained till now 
