ANATOMY AND PHYSIOLOGY 59 
In general, the following principal longitudinal veins may 
be distinguished, in the following order: costa, subcosta, 
radius, media, cubitus and anal (Figs. 67-71). 
inGs 67. 
— R1 
Oe a oa 
Hypothetical type of venation. A, anal vein; C, costa; Cu, cubitus; M, media; R, 
radius; Sc, subcosta.—Figs. 67-71 after Comstock and NEEDHAM. 
The costa (C) strengthens the front margin of the wing 
and is essentially unbranched. 
The subcosta (Sc) is close behind the costa and is un- 
branched in the imagines of many orders in which there are 
few wing veins, though it is typically a forked vein. 
The radius (i), though subject to much modification, is 
typically five-branched, as in Fig. 67. The second principal 
‘branch of the radius is termed the radial sector (/s). 
The media (7) is often three-branched and is typically 
four-branched, according to Comstock and Needham. 
The cubitus (Cw) has two branches. 
The anal veins (A) are typically three, of which the first 
is generally simple, while the second and third are many- 
branched in wings that have an expanded anal area. 
The Plecoptera, as a whole, show the least departure from 
the primitive type of venation; which is well preserved; also, 
in the more generalized of the Trichoptera. 
Starting from the primitive type, specialization has occurred 
in two ways: by reduction and by addition. Reduction oc- 
curs either by the atrophy of veins or by the coalescence of 
two or more adjacent veins. Atrophy explains the lack of all 
but one anal vein in Rhyphus (Fig. 68) and other Diptera, 
