80 



THE GENERAL FEATURES OF WINGS 



It should be remembered that the coalescence of two veins results in the 

 obliteration of the cell that was between them. Thus when veins R2 and R3 

 coalesce, as in RhypJms (Fig. 67), the cell lying behind vein R2+3 is cell R3 

 and not cell R2+3, ceh Ro having been obliterated. 



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

 one or more cross-veins, the parts are numbered, beginning with the 

 proximal one. Thus in Rhyphus (Fig. 67) cell Mo is divided by the medial 

 cross-vein into two parts, which are designated as cell ist Mo and cell 2d Mo 

 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 



Fig. 68. — Wings of Chalcoplcryx ml Hans (Aftt-r Needham). 



orders where many secondary veins are developed it is difficult to apply it. 

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

 separate cells. Thus, for example, in the Odonata (Fig. 68) that portion 

 of the wing lying between veins Mi and Mo, and which is traversed by one 

 or more intercalary veins and many cross-veins, is designated as area Mi. 

 This area is homologous with cell Mi of the Diptera (Fig. 67). 



In applying this sytem to the Odonata, where the radial sector crosses 

 veins Mi and M2 (Fig. 68) and traverses that part of the wing that really 

 corresponds to cell Mo of the Diptera, the term area M2 is applied to that 

 part of the wing between veins M2 and Rg, and that part of the wing 

 between veins Rg and M3 is designated as area Rg. 



