93 



each the abbreviation of the name of the vein that forms its 

 cephalic (front) margin. It should be borne in mind, how- 

 ever, that by modifications of the typical arrangement of the 

 wing-veins, a vein that normally forms the cephalic margin 

 of a cell may apparently bear a very different relation to it ; 

 and this must be taken into account if we are to apply the 

 same term to homologous cells throughout the insect series. 



The cells of the wing fall naturally into two groups : first, 

 those on the basal part of the wing ; and second, those 

 nearer the distal end of the wing. The former are bounded 

 by the principal veins ; the latter, by the branches of the 

 forked veins ; a corresponding distinction is made in desig- 

 nating the cells. Thus the cell lying behind the main stem 

 of the radius and on the basal part of the wing is designated 

 as cell R ; while the cell lying behind the radius-one is 

 designated as cell R^. 



It should be remembered that the coalescence of two 

 veins results in the obliteration of the cell that was between 

 them. Thus when veins R^ and R^ coalesce, as in Rhypkus 

 (Fig. 2), the cell lying behind R^^ is cell R^ and not cell 

 7? 2+3 , cell R^ having been obliterated. 



When one of these principal cells is divided into two or 

 more parts by one or more cross-veins, the parts may be 

 numbered, beginning with the proximal one. Thus in Rhy- 

 phus (Fig 2) cell M^ is divided by the medial cross-vein into 

 two parts, which are designated as cell ist M^ and cell 2d M^ 

 respectively. 



The application of this system of naming the cells of the 

 wing is an easy matter in those orders where the wings have 

 few veins ; but in those orders where many secondary veins 

 are developed it is more difficult of application. In the lat- 

 ter case we have to do with areas of the wing rather than 

 with separate cells. Thus, for example, it will be seen later 

 that in certain Neuroptera the area R is divided by several 



