No. 386.] THE WINGS OF INSECTS. IIQ 
cases where a vein has an even number of branches (the radial 
sector and the cubitus) the alternation has been attained by the 
development of an accessory vein. These are indicated in the 
table as chief accessory veins, and are lettered 1 in the figure. 
Many other accessory veins are developed at the margin of the 
wing in a mere or less irregular manner; but whenever a sec- 
ond accessory vein extends far into the disk of the wing it is 
accompanied by a third, one being convex, the other concave. 
The anal area of the wing, where the accessory veins are more 
of the nature of braces, like cross-veins, is not included in this 
statement, nor in that which follows. 
Correlated with the development of a triangular form of 
wing, which involves an expanding of its outer margin, is the 
fact that the accessory longitudinal veins are all added distally 
in the May-flies. But the method of development of these 
veins appears to be radically different from what it is in the 
Neuroptera.! There the accessory longitudinal veins are pre- 
ceded by tracheze, which arise as fine twigs at the tips of older 
trachea, and which in the course of phylogenetic development 
branch off from the parent trachez farther and farther from the 
margin of the wing, thus making room for the development of 
other twigs. Here, in the May-flies, the accessory longitudinal 
veins are evidently thickened folds, which arise more or less 
nearly midway between other veins. A similar thickening of a 
fold occurs in the Diptera, where, in certain Asilidæ, the anal 
furrow is vein-like in structure. 
A fact of prime importance in the study of the homologies 
of the wing-veins of May-flies is that the corrugations of the 
wing are the most persistent features of it. Hence the most 
important criterion for determining the homology of a vein is 
whether it is a concave or a convex one. The basal connec- 
tions of the veins are very inconstant, and are often misleading. 
We have already referred to the separation of the radial sector 
from the main stem of the radius in the adult (its true origin is 
easily seen when the tracheation of the wings of certain nymphs 
is studied) ; and other separations and secondary attachments 
are common. A good illustration is furnished by the wings 
1See American Naturalist, vol. xxxii, pp. 771, 772- 
