72 



AN INTRODUCTION TO ENTOMOLOGY 



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Fig. 86. — Diagram of an arculus of a dragon-fly. 



The arculus. — 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 is the arculus. The arculus is designated in figures of 

 wings by the abbreviation ar. Usually when the arculus is present 

 the media appears to arise from it; the fact is, the arculus is com- 

 pound, being composed of a section of media and a cross-vein. 



Figure 86 is a dia- 

 gram representing 

 the typical struc- 

 ture of the arculus. 

 That part of the 

 arculus which is a 

 section of media is 

 designated as the 

 anterior arculus (aa) 

 and that part formed by a cross-vein, the posterior arculus (pa). 



The terminology of the cells of the wing. — Each cell of the wing is 

 designated by the name of the vein that normally forms its front 

 margin when the wings are spread. See Figure 87 where both the 

 veins and the cells of the wing are lettered. 



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 stems of the principal 

 veins, the latter, by the branches of these veins; a corresponding 

 distinction is made in designating the cells. Thus a cell lying behind 

 the main stem of radius and in the basal part of the wing is designated 

 as cell R\ while a cell lying behind radius-one is designated as cell i?i. 



Fig. 87. — A wing of Rhyphus. 



It should be remembered that the coalescence of two veins results 

 in the obliteration of the cell that was between them. Thus when 



