64 AN INTRODUCTION TO ENTOMOLOGY 



wing, except that the tracheae 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 

 [)etween 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 

 t bbreviations by which they are known are as follows, beginning with 

 ' he on3 nearest the costal margin of the wing: 



Nam?s of vsins Abbreditions 



Costa C 



Subcosta So 



Radius R 



Media M 



Cubitus Cu 



First Anal ist A 



Second Anal 2dA 



Third Anal 3dA 



The chief branches of the voing-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. 



