68 



AN INTRODUCTION TO ENTOMOLOGY 



abbreviation of the name of the terminal element. Thus a serial 

 vein, the basal element of which is the cubitus and the terminal ele- 

 ment vein MI, is designated as vein Cu MI. A serial vein thus 

 formed exists in the hind wings of certain ichneumon flies. 



The increase of the number of wing-veins. In the wings of many 

 insects the number of veins is greater than it is in the hypothetical 

 type. This multiplication of veins is due either to an increase in the 



Fig. 81. Wings of Osmylus hyalinatus. 



number of the branches of the principal veins by the addition of 

 secondary branches, termed accessory veins, or to the development of 

 secondary longitudinal veins between these branches, termed inter- 

 calary veins. In no case is there an increase in the number of principal 

 veins. 



The accessory veins. The wings of Osmylus (Fig. 81) are an exam- 

 ple of wings in which accessory veins have been developed; here the 

 radial sector bears many more branches than the typical number; 

 those branches that are regarded as the primitive branches are 

 lettered R b R 2 , R 3 , R4, and R 5 respectively (Fig. 82); the other 



