AQUATIC INSECTS 



923 



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



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+2) nearer the arculus 



than the nodus. . ... 7 







Fig. 1388. Wing venation in the Odonata: (o) a dragonfly, Cordulegasler. (6) a damselfly, Argia.; 

 ifi) the region of the stigma, st, with its brace vein, z, ar, arculus; al, anal loop; hr, bridge; «, nodus; 

 0, obUque vein; t, triangle; t' , subtriangle; 9, quadrangle; sq, subquadrangle; ti, basal subcostal cross 

 veins; a, veins as in Fig. 1386. 



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



the subnodal cross vein. Lesles. 



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 . . Argia. 



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 (i4) 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. . . Enallagnm. 



