392 THE WINGS OF INSECTS 



The medio-cuhital cross-vein. — This extends from media to cubitus (Fig. 

 411, m-cn). 



The arctilus. — In many insects there is what appears to be a cross-vein 

 extending from the radius to the cubitus near the base of the wing. This 

 has been termed the ar cuius by writers on the Odonata, and the use of this 

 term has been extended to all orders in which there is a similar arrangement 

 of the veins in this part of the wing. The arculus is designated by the 

 abbreviation ar. Usually when the arculus is present the media appears to 

 arise from it. The fact is, the arculus is compound, being composed of a 

 section of the media and a cross-vein. The structure of this part can be 

 clearly seen in the Odonata (Fig. 412). In Rhyphus (Fig. 410) the arculus 

 appears as a simple cross-vein extending from the radius to the cubitus, and 

 a part of the base of the media is atrophied. 



R^M 



Fig. 412. — The arculus of a dragon-fly. 



That part of the arculus which is a section of media is designated as the 

 anterior arculus (Fig. 412, a a), and that part formed by a cross-vein, the 

 posterior arculus (Fig. 412, p a). 



Designation of the cells of the wing. — The thin spaces of the wings 

 which are bounded by the veins are called cells. In descriptions of wings it 

 is often desirable to refer to one or more of the cells. It is necessary, there- 

 fore, to have a terminology of the cells of the wing, as well as of the wing- 

 veins. 



Having named the wing-veins, the simplest possible method of desig- 

 nating the cells of the wing is to apply to each the abbreviation of the name 

 of the vein that forms its cephalic (front) margin. It should be borne in 

 mind, however, that by modifications of the typical arrangement of the 

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

 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 designating 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 

 radius-one is designated as cell R\. 



