236 THE WINGS OF ODONATA 



The postnodal cross-veins. — The two series of cross-veins nearest to the 

 costal margin of the wing and between the nodus and the apex of the wing 

 are termed the postnodal cross-veins (Fig. 230, pn). The first of the two 

 series of postnodal cross-veins extend from the costa to vein Ri ; the second, 

 from vein Ri to vein Mi. The postnodal cross-veins are termed the 

 postcubital cross-veins by some writers. 



The ar cuius. — Near the base of each wing there is what appears to be a 

 cross-vein extending from radius to cubitus and from which media appears 

 to arise; this is the arculus (Fig. 230, ar). The arculus is really composed 

 of a section of media and a cross-vein. 



For a short distance near the base of the wing radius and media coalesce 

 (Fig. 230, R + M). The double nature of this vein is readily seen by an 

 examination of the wings of an adult, and in the grown pupa the radial and 

 medial tracheae are closely parallel in this region of the wing (Fig. 228). 

 At the point where the two veins separate media suddenly bends away from 



R+M< 



Fig. 238. — Diagram of the arculus. 



the radius and extends for a short distance towards the inner margin of the 

 wing; it then makes another sudden bend and extends towards the outer 

 margin of the wing. Frequently, as in Cordulegaster (Fig. 230) the veins 

 Ml +2 +3 and M4 separate before the second bend is made in which case two 

 veins appear to arise from the arculus. That part of the arculus that 

 extends from this second bend in media to cubitus is formed by a cross-vein; 

 this cross-vein is termed the posterior arculus (Fig. 238, pa). 



The triangle of the Anisoptera. — In the suborder Anisoptera or dragon- 

 flies there is near the base of the wing, a well-marked area of the wing, which 

 is usually triangular in outline (Fig. 230, t) ; this area is termed the triangle. 

 This name is applied to this area even when it is not triangular in outline. 

 The various modifications of the triangle and of the adjacent cells are much 

 used in taxonomic work. 



The triangle is formed as follows: a short distance beyond the arculus, 

 the ctibitus makes a sudden bend and extends for a short distance towards 

 the inner margin of the wing, to the point where it divides into veins Cui 

 and Cu2. From each end of the short section of the main stem of cubitus 

 that extends transversely to the length of the wing a cross-vein extends to 

 vein M4, the two cross-veins reaching M4 at the same point or approximately 



