64 AN INTRODUCTION TO ENTOMOLOGY 



wing, except that the trachea? are not crowded together at the base of 

 the wing as are the veins in the wings of adults.* 



Longitudinal veins and cross-veins. The veins of the wing can be 

 grouped under two heads: first, longitudinal veins, those that 

 normally extend lengthwise the wing; and second, cross-veins, those 

 that normally extend in a transverse direction. 



The insertion of the word normally in the above definitions is 

 important ; for it is only in comparatively generalized wings that the 

 direction of a vein can be depended upon for determining to which of 

 these two classes it belongs. 



The principal wing-veins. The longitudinal wing-veins constitute 

 the principal framework of the wings. In the diagram representing 

 the typical venation of an insect wing (Fig. 77), only longitudinal 

 veins are indicated ; this is due to the fact that the diagram was based 

 on a study of the tracheation of wings, and in the more generalized 

 wings the cross-veins are not preceded by tracheae; moreover in the 

 wings of more generalized paleozoic insects there were no definite 

 cross-veins, but merely an irregular network of thickened lines 

 between the longitudinal veins. 



There are eight principal veins; and of these the second, third, 

 fourth, and fifth are branched. The names of these veins and the 

 abbreviations by which they are known are as follows, beginning with 

 the one nearest the costal margin of the wing: 



Names of veins Abbreviations 



Costa C 



Subcosta Sc 



Radius R 



Media . . M 



Cubitus Cu 



First Anal . ist A 



Second Anal 2d A 



Third Anal ... 3<*A 



The chief branches of the wing-veins. The chief branches of the 

 principal veins are numbered, beginning with the branch nearest to 

 the costal margin of the wing. The term used to designate a branch 

 of a vein is formed by compounding the name of the vein with a 



*For"many details regarding the development of the wings of insects, their 

 structure, and the terminology of the wing-veins, that can not be included in 

 this work, see a volume by the writer entitled The Wings of Insects. This is 

 published by The Comstock Publishing Company, Ithaca, N. Y. 



