48 THE AMERICAN NATURALIST. 
two sets of structures is sometimes very wide, and must be 
taken into account in an interpretation of the characters pre- 
sented by a wing. 
While this increases the difficulty of determining the homolo- 
gies of the wing-veins, it is often of great aid in taxonomic 
work, for it may. afford an indication of the degree of diver- 
gence from a primitive type in the structure of a wing; and 
when a series of forms is studied the course of this divergence 
is often clearly indicated. 
The figures also show that in some cases what appears as a 
single vein is formed about two closely parallel trachez. This 
is shown in the case of the bases of the second and third prin- 
cipal trachez, counting from the costal margin of the wing, 
the radial and medial tracheze. This illustrates a fact of 
frequent occurrence, — that what appears to be a single vein 
may be formed by the coalescence of two primitive veins. 
In these figures the tracheze just mentioned, except one of 
them in the fore wing, appear not to extend to the base of the 
wing. This is due to the fact that in the preparations photo- 
graphed the mounting medium had penetrated these tracheæ 
for a distance, rendering the basal portion of them invisible. 
The figure of the hind wing illustrates also another way in 
which specimens may be injured during their preparation, and 
which may lead to a misinterpretation of them. In this wing 
the first branch of the first main trachea, the subcostal tra- 
chea, has been broken and moved out of place within the 
wing-sac. The normal position of this branch is well shown 
in the figure of the fore wing. 
We will not go farther into the discussion of the technique 
of this method of study. Enough has been said to show that 
we have at hand a comparatively simple method of determining 
those questions of homologies of wing-veins that have sorely 
puzzled all investigators that have attempted to deal with them, 
and to indicate the nature of the material upon which we have 
based the conclusions that we purpose to offer in succeeding 
chapters of this paper. 
ENTOMOLOGICAL LABORATORY, 
CORNELL UNIVERSITY, November, 1897. 
