74 THE GENERAL FEATURES OF WINGS 



Where the veins are added distaUy so as to form a regular series it is 

 practicable to designate them individually. In this case the accessory 

 veins arising from one side of a primitive vein are considered as a single 

 series, and to each series a distinct set of letters is applied beginning with the 



Fig. 6i. — Wing of a May-fly, Epeoms humeralis 

 (After Morgan). 



first developed member of the series of accessory veins. Thus if vein R2 

 bears three accessory veins they are designated as veins R2a, R2b. and R2C 

 respectively (Fig. 60). 



By this method homologous veins, when a homology exists, will bear the 

 same designation. But it should be remembered that as accessory veins 

 have arisen independently in many different groups of insects, it often 

 happens that accessory veins similar in position, and bearing the same 

 designation in our system, are merely analogous and not homologous. 



The discussion of the sequence of development of definitive accessory 

 veins is continued in the Chapter treating of the wings of Neuroptera; as 

 it is in this order that this class of veins reaches its most perfect develop- 

 ment. 



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 line, in a corrugated wing, more 

 or less nearly midway between two preexisting veins, with which primarih^ 

 it was connected only by cross veins. 



In many cases, however, an intercalary vein has become united to one 

 of the two veins between which it was developed in such a way as to appear 

 to be a branch of it. This resemblance to an accessory vein is greatly 

 increased when the intercalary vein is penetrated by a branch of the trachea 

 of the principal vein; this condition is well-illustrated by the intercalary 

 veins of the Odonata. Excellent illustrations of unmodified intercalary 

 veins are common in the Ephemerida, where most of the intercalary veins 

 remain distinct from the veins between which they were developed, being 

 connected with them only by cross-veins, the proximal end of the inter- 

 calary vein being free (Fig. 61). 



