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). 



M4 

 Cu2 



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 (R), 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 (Rs). 



The media (M) is often three-branched and is typically 

 four-branched, according to Comstock and Needham. 



The cubitus (Cu) 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, 



