72 



THE GENERAL FEATURES OF WINGS 



Accessory veins are found only in the Orthoptera, Isoptera, Neuroptera, 

 Plecoptera, Mecoptera, and, rarely in the Trichoptera and Embiidina. 



Two types of accessory veins can be recognized, although in many cases 

 it is difficult to detennine with which of the two types certain accessory 

 veins should be classed. These two types may be designated as marginal 

 accessory veins and definitive accessory veins respectively. 



The marginal accessory veins are twig-like branches that are the result 

 of bifurcations of veins that have not extended far back from the margin 

 of the wing. Many such short branches of veins exist in the wings of 

 Osmylns (Fig. 57). Note especially the outer margin of the wing where the 

 forking of the veins is quite regular and where some of the marginal acces- 

 sory veins are themselves forked. 



The most important characteristic of the marginal accessory veins, for 

 the purposes of this discussion, is the fact that their number and position 

 are not at all constant. Not only do they vary in these respects in different 



Fig. 58.- — A wing of a pupa of Corydalus (After C. & N.). 



individuals of the same species but also in the wings of the two sides of the 

 same individual. There is on this account no occasion for providing a 

 terminology for them. 



In the paleozoic insects the marginal accessory veins were very irregular 

 in length, this is shown by the figures illustrating Chapter IV. Strongly 

 contrasting with this condition is that seen in many recent Neuroptera, 

 where there are one or more ranks of these veins, the members of each rank 

 being of comparatively uniform length. 



The definitive accessory veins differ from the marginal accessory veins in 

 having attained a position that is comparable in stability to that of the 

 primitive branches of the principal veins; for this reason it is practicable 

 to designate them individually, either by names or by numbers, depending 

 on the manner in which they have been added. 



Comstock and Needham pointed out that accessory veins arc added to 

 the principal veins in two ways: first, in some insects they are added 

 distally by successive splittings of the tip of a principal vein, thus fomiing a 

 regular series; and second, the number of accessory veins may be increased 



