AQUATIC INSECTS 927 



Vein Cui of the hind wing departing from the triangle at the hind 

 angle. .75 



Sectors of the arculus (veins Mi-a and M4) contiguous, but incom- 

 pletely fused for a distance beyond the arculus; wings 

 generally conspicuously spotted with yellow or reddish brown. 



Celithemis. 



Sectors of the arculus in the hind wing distinctly fused for a distance 

 beyond the arculus. . . . 77 



Stigma short and thick, about twice as long as wide; anal loop with 

 a big heel, there being generally four cells between the bi- 

 sector and the heel point; face pure white. Leucorhinia. 



Stigma more than three times as long as wide; anal loop generally 

 with but two cells between the bisector and the heel point. 



Sympetrum. 



Vein Cui of the hind wing migrated a little way up the outer side of 

 the triangle, separating itself at a distance from the hind 

 angle. . 80 



With a single cross vein under the stigma, and a long vacant space 

 before that cross vein. Pachydiplax. 



With two cross veins under the stigma and the adjacent spaces more 

 normal. . . . .82 



With a single row of cells between veins Mj and Rj. . Mesothemis. 



With two rows of cells for a distance between veins M2 and Rj. 



Micraihyria. 



84 (73) Sectors of the arculus in the fore wing contiguous, but not completely 



fused beyond the point of their departure from the arculus; 

 radial sector distinctly undulate (except in Ladona) ; triangle 

 of the fore wing very much elongated posteriorly and narrow 

 and generally traversed by two or more parallel cross veins, 

 and followed by three to seven rows of cells. ... 85 



85 (86) Vein Mia arising under the proximal fourth of the stigma; fore wings 



with the subtriangle consisting of three cells, and the tri- 

 angle followed by three rows of cells Ladona. 



86 (85) Vein M/a arising under the middle of the stigma; fore wings with the 



subtriangle consisting of four to eleven cells, and the triangle 

 usually followed by four to six rows of cells. 87 



87 (88) Male with no ventral hooks on the first abdominal segment; female 



with the hind tibia a little longer than the hind femur; the 

 sexes alike in wing pattern. Libellula. 



88 (87) Male with a pair of ventral hooks on the first abdominal segment; 



female with the hind femur and tibia of equal length; wings 

 dissimilarly colored in the two sexes. . Plathemis. 



89 (72) Triangle of the fore wing placed beyond the level of the apex of the 



triangle of the hind wing; stigma with its inner end per- 

 pendicular, its outer end very oblique to the bordering veins; 

 wings broad at base and pointed at apex. . . 90 



90 (91) Radial sector regularly curved; hind wings with a broad, basal colored 



band Tramea. 



91 (90) Radial sector distinctly undulate; hind wings not covered at base by 



a broad colored band Pantala. 



