IIO 



be impossible to point out the changes that have taken 

 place in the course of the development of each of the vari- 

 ous types of many-veined wings. We therefore apply the 

 term accessory veins to these secondarily developed longitu- 

 dinal veins, and retain the same nomenclature for the primi- 

 tive veins that we used in describing the few-veined wings. 



The development of accessory veins. Accessory 

 veins may be borne by any of the primitive longitudinal 

 veins ; and they may arise from either of the two sides of 

 such a vein. In most cases it is unnecessary to designate 

 the individual accessory veins, as, usually, it will be sufficient 

 for descriptive purposes to indicate the number of these 

 veins that have been developed upon a particular longitudi- 

 nal vein. In fact, in certain cases more than this could not 

 well be done, owing to the irregularity of the veins. On the 

 other hand, in many cases the accessory veins borne by a 

 single primitive vein present a high degree of regularity, 

 and it is evident that they have been developed in a regular 

 sequence. Under these circumstances it is practicable to 

 designate them individually ; and we have devised the fol- 

 lowing method for this purpose. 



The accessory veins arising from one side of a single 

 primitive vein are considered as a single set, and to each set 

 of veins a distinct set of numbers is applied, beginning with 

 the oldest (i.e., the first-developed) member of the set. 



By this method homologous veins, when a homology ex- 

 ists, will bear the same number. But it should be remem- 

 bered that as accessory veins have arisen independently in 

 many different groups of insects, it often happens that acces- 

 sory veins similar in position, and bearing the same number 

 in our system, are merely analogous and not homologous. 



In order to apply this system it is necessary to know, in 

 the case of each group of insects studied, the sequence in 

 which the members of the particular set of veins under con- 



