THE GENERAL FEATURES OF WINGS 



71 



the veins has taken place in one part of the wing while there is an increase 

 in another part. 



The secondary longitudinal veins. — Omitting certain adventitious veins, 

 which will be discussed later, and branches of the anal veins, the secondary 

 longitudinal veins are of two kinds, which may be designated as the 

 accessory veins and the intercalary veins, respectively. These two kinds of 

 veins differ fundamentally in their mode of origin. Accessory veins arise 

 as secondary branches of the principal veins or of their branches; and their 



Fig. 57. — Wings of Osmyius. 



presence is not correlated in any way with a corrugating of the wing. 

 Intercalary veins, on the other hand, are thickened folds; each of which 

 arises more or less nearly midway between two preexisting veins, with 

 which, as a rule, it has no connection except by cross-veins, and can not, 

 therefore, be considered accessory to either. Frequently, however, an in- 

 tercalary vein becomes joined to one of the two veins between which it 

 was developed in such a way as to appear to be a branch of it. 



The accessory veins. — The secondary longitudinal veins that arise as 

 branches of the principal veins or of their branches are teiTned accessory 

 veins. Accessory veins may be borne by any of the primitive longitudinal 

 ycins; and they may arise from either of the two sides of such a vein. 



