THE WINGS OF INSECTS 



403 



Study Figure 418 carefully and letter with a pencil veins Sc, Ri, and the 

 stem of Rg. Submit the lettering to the Instructor for criticism and make 

 changes if necessary. 



There are two types of accessory veins, which arc designated as the 

 marginal and the definitive respectively. 



3^^ A 2d A CU2 Cuia 



Fig. 419. — Wing of a pupa of Corydalus. 



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 Poly- 

 stoechotes piinctatiis. The ntraiber and position of marginal accessory veins 

 are not at all constant ; they differ in the wings of the two sides of the same 

 individual. 



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 



Fig. 420. — Wing of a nymph of a cockroach. 



to designate them individually. In Figure 41 8 one of the definitive acces- 

 sory veins is designated as Ro^- 



Accessory veins are 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 forming a regular series; and second, the number of 



