THE EXTERNAL ANATOMY OF INSECTS 69 



branches are the secondarily developed accessory veins. Two types 

 of accessory veins are recognized the marginal accessory veins and 

 the definitive accessory veins. 



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 Osmylus (Fig. 81). The number and position of the 

 marginal accessory veins are not constant, differing in the wings of 

 the two sides of the same individual. 



The definitive accessory veins differ from the marginal accessory 



Fig. 82. Base of fore wing shown in Figure 81. 



veins in having attained a position that is comparable in stability to 

 that of the primitive branches of the principal veins. 



In those cases where the accessory veins are believed to have been 

 developed in regular order they are designated by the addition of a 

 letter to the abbreviation of the name of the vein that bears them; 

 thus if vein R 2 bears three accessory veins they are designated as 

 veins R 2a , R 2 b, and R 2c , respectively. 



The intercalary veins. The intercalary veins are secondarily 

 developed longitudinal veins that did not arise as branches of the 

 primitive veins, but were developed in each case as a thickened fold in 

 a corrugated wing, more or less nearly midway between two pre- 

 existing veins, with which primarily it was connected only by cross- 

 veins. Excellent examples of unmodified intercalary veins are com- 



