No. 382.] THE WINGS OF INSECTS. 771 
circumstances it is practicable to designate them individually ; 
and we have devised the following method for this purpose. 
‚The accessory veins arising from one side of a single primi- 
tive vein are considered as a single set, and to each set of veins 
a distinct set of numbers is applied, beginning with the oldest 
(2.e., the first-developed) member of the set. 
By this method homologous veins, when a homology exists, 
will bear the same number. But it should be remembered that 
as accessory veins have arisen independently in many different 
groups of insects, it often happens that accessory veins similar 
in position, and bearing the same number in our system, are 
merely analogous and ‘not homologous. 
In order to apply this system it is necessary to know, in the 
case of each group of insects studied, the sequence in which 
the members of the particular set of veins under consideration 
have been developed. For additions to such a set of veins may 
be made to the distal end of the series, or to the proximal end, 
or may be interpolated at some distance from either end. 
Frequently an examination of the wing of an adult insect is 
sufficient to determine this sequence. But the determination 
can be made in a much more satisfactory manner by a study of 
the tracheation of the wings of the nymph or pupa. For in 
the many-veined insects the longitudinal veins, both primitive 
and accessory, are developed about trachez; and it is much 
easier to determine the homologies of the tracheæ of an 
immature wing than it is to determine the homologies of the 
wing-veins of the adult. And, too, in this way we are able to 
eliminate the cross-veins which are not preceded by trachez in 
the forms used for illustration here. We will, therefore, use 
for this purpose the wings of immature insects. 
Accessory veins added distally. — If the radial trachez of the 
pupa of Chauliodes (Fig. 53) and of the pupa of Corydalis 
cornuta (Fig. 54) be examined, it will be seen that both differ 
from our hypothetical type in the presence of a greater number 
of branches of the radial sector. And a comparison of the two 
figures shows that the increase in the case of Corydalis has 
been greater than in the case of Chauliodes. Farther, the 
presence of fine twigs at the tip of the trachea R, indicates the 
