AQUATIC INSECTS 923 



4 (3) Basal space of all wings traversed by cross veins Hetacrina. 



5 (2) Quadrangle without cross veins; antenodal cross veins but two in each 



wing; pterostigma with a brace vein at its proximal end in 

 the space behind vein Ri; wings narrower 6 



6 (9) Vein M3 arising (i.e., separating from vein M1+..) nearer the arculus 



than the nodus 7 



Fig. 1388. Wing venation in the Odonata: (a) a drs.gon?^y, Cordulef^aster. (b) a damselfly, Argia; 

 (c) the region of the stigma, st, with its brace vein, 2, ar, arculus; al, anal loop; br, bridge; n, nodus; 

 o, oblique vein; t, triangle; /', subtriangle; g, quadrangle; sq, subquadrangle; v, basal subcostal cross 

 veins; z, veins as in Fig. 1386. 



7 (8) Vein M2 separating from vein Mi at a distance of several cells beyond 



the subnodal cross vein Lcstes. 



8 (7) Vein M2 separating from vein Mi close to the subnodal cross vein, less 



than the distance of one cell beyond it. Archilestes. 



9 (6) Vein M3 arising nearer the nodus than the arculus 10 



10 (11) Spines on the tibiae very long, twice as long as the intervals between 



them Irgia. 



11 (10) Spines of the tibiae hardly longer than the intervals between them. 1 2 



12 (16) No pale postocular spots on the top of the head; sexes similarly 



colored 13 



13 (14, 15) Colors of dorsum blue and black; yellow beneath the thorax. 



Chromagrion. 



14 (13, 15) Colors of dorsum red and black; stout species. . Amphiagrion. 



15 (13, 14) Dorsum bronzy green; slender species. . . . Nehallennia. 



16(12) With round or ovoid postocular spots on the head 17 



17 (18) Sexes with a general similarity in color, the female often of a lighter 

 shade; the superior abdominal appendages of the male 

 not strongly directed downward and inward. . . Enallagma. 



