776 THE AMERICAN NATURALIST.  [Vou. XXXII. 
vein R2, 3. All stages of this switching of vein Ay occur in 
the Myrmeleonidz; but two examples will suffice to illustrate it. 
In Myrmeleon (Fig. 58e) the base of vein R4 appears to be forked; 
one arm of the fork arising from vein Xs, the other from vein 
R243. The former is the true base of vein 24; the latter is 
a cross-vein which is assuming the function of a base of this 
vein. In the hind wing of Ptynx appendiculatus (Fig. 58f) the 
switching has been completed, vein Ay arising from vein R2 + 3. 
In the foregoing illustrations comparisons of allied insects 
have been made in order to determine the ways in which the 
wings are being modified ; frequently a comparative study of 
the fore and hind wings of a single insect is equally suggest- 
Fic. 59. — Wing of a pupa of Hemerobius. 
ive, for it often happens that the two pairs of wings exhibit 
different degrees of the same kind of modification, and thus 
the course of the change is indicated. 
A study of the causes of the changes which we are describ- 
ing is beyond our present purpose, which is merely to determine 
the homologies of the wing-veins. But we can gain a hint of 
the probable reason for the development of the pectinate type 
of veins without entering very deeply into questions of the 
mechanics of flight. 
It is obvious that many styles of flight exist among insects, 
and that for the different styles of flight different kinds of 
wings are required. In Corydalis (Fig. 54) the wing is stiff- 
ened, along a line parallel with the costal margin of the wing, 
by the subcosta, the main stem of the radius, and veins Xr and 
R2. Back of this line there is a broad, flexible area, which 
bends up during the downward stroke of the wing, forming an 
inclined plane, the pressure of which against the air forces the 
