No. 374.] THE WINGS OF INSECTS. 87 
where there are many wing-veins it gives off numerous small 
branches to the costa; in the orders where there are few wing- 
veins it appears in the adult to be an unbranched vein. But a 
study of the subcostal trachea in nymphs and in pupz shows 
that it is forked in at least several widely separated orders; we 
have, therefore, represented it so in our type (Fig. 4, Sc, and 
Sez). In adult wings the branches of the subcosta are usually 
either wanting or appear as cross-veins, In those orders in 
which the wing is corrugated the subcosta lies at the bottom of 
a furrow, which stiffens the costal edge of the wing. 
The third vein is the radius. This is the most prominent 
vein in the wing; and it is the one which, from the great variety 
of its modifications, offers more often than any other vein 
‘obvious characters of use in taxonomic work. In spite of the 
wide differences of form of this vein in the different orders, it 
is now clear to us that these various forms have all been 
derived from a type which still exists, but slightly modified, in 
the more generalized Trichoptera, Mecaptera, Diptera, and 
Lepidoptera, and in certain genera of several other orders. In 
its typical form this vein is five-branched (Fig. 4, Ri—Rs). The 
main stem of the vein separates into two divisions; the first of 
these is simple and is more or less nearly a direct continuation 
of the main stem — this is radzus-one (R1); the second of the 
principal divisions of radius is typically four-branched, and on 
account of the frequency of the necessity of making reference 
to it a special name has been applied to it, the radial sector 
(R,). The radial sector separates into two divisions (42+; and 
R4+5); and each of these again separates into two divisions, the 
former into radius-two(Rz) and radius-three (R3), and the latter 
into vadius-four (R4) and radius-five (Rs). 
_ The vein occupying the center of the wing is the media (M). 
In those orders in which it retains most nearly its primitive 
form it is usually three-branched; but the fact that in the more 
generalized members of several widely separated orders it is 
four-branched leads us to believe that it was four-branched in 
the stem form of winged insects. The branches are designated 
as media-one (Mı), media-two (Mz), media-three (M3) and media- 
Sour (M4), respectively. 
