THE EXTERNAL ANATOMY OF INSECTS 73 



veins Ri and Rz coalesce, as in the wings of RJtyphus (Fig. 87), the cell 

 lying behind vein i?2+3 is cell Rz, and not cell i?2+3, cell i?2 having been 

 obliterated. 



When one of the principal cells is divided into two or more parts by 

 one or more cross-veins, the parts may be mmibered, beginning v\^ith 

 the proximal one. Thus in Rhyphus (Fig. 87), cell M2 is divided by 

 the medial cross-vein into cell istM-i and cell 2dM2. 



When two or more cells are united by the atrophy of the vein or 

 veins separating them, the compound cell thus formed is designated 

 by a combination of the terms applied to the elements of the com- 

 pound cell. When, for example, the stem of media is atrophied, the 

 cell resulting from the combination of cells R and M is designated as 

 cell R+M. 



The application of this system of naming the cells of the wing is an 

 easy matter in those orders where there are but few cross- veins; but 

 in those orders where there are many cross-veins it is not practicable 

 10 apply it. In the latter case we have to do with areas of the wing 

 rather than with separate cells. These areas are designated as are the 

 cells of the few- veined wings with which they correspond; thus the 

 area immediately behind vein R2 is area R2. 



The corrugations of the wings. — The wings of comparatively few 

 insects present a flat surface ; in most cases the membrane is thrown 

 into a series of folds or corrugations. This corrugating of the wing in 

 some cases adds greatly to its strength, as in the wings of dragon-fiies; 

 in other cases the corrugations are the result of a folding of the wing 

 when not in use, as in the anal area when this part is broadly ex- 

 panded. 



It rarely happens that there is occasion to refer to individual 

 members of either of these classes of folds, except perhaps the one 

 between the costa and the radius, which is the subcostal fold and that 

 which is normally between the cubitus and the first anal vein, the 

 cubito-anal fold. 



Convex and concave veins. — ^When the wings are corrugated, the 

 wing-veins that follow the crests of ridges are termed convex veins; 

 and those that follow the furrows, concave veins. 



The furrows of the wing. — There are found in the wings of many 

 insects one or more suture-like grooves in the membrane of the wing; 

 these are termed the furrows of the wing. The more important of 

 these furrows are the four following: 



The anal furrow when present is usually developed in the cubito 

 anal fold ; but in the Heteroptera it is found in front of the cubitus. 



