926 FRESH-WATER BIOLOGY 



54 (67) Sectors of the arculus (veins Mi-g and M4) distinctly separate at their 



departure from the arculus; anal loop elongate, but not 

 distinctly foot-shaped, the toe part being Httle or not at all 

 developed; the last antenodal cross vein extending from the 

 costal to the radial veins (except in D. lintneri, in which 

 it generally extends only from the costal to the subcostal); 

 colors often metallic blue or green on thorax and abdomen. 55 



55 (56) Veins M4 and Cui in the fore wing parallel or a little divergent apically, 



the number of rows of cells between them increasing toward 

 the margin of the wing Neurocordulia. 



56 (55) Veins M4 and Cui in the fore wing approximated toward the margin 



of the wing 57 



57 (58) With large brown spots on all wings at nodus and apex. 



Epicordulia. 



58 (57) No brown spots at nodus and apex 59 



59 (60) Four (rarely five) antenodal cross veins in the hind wing. 



Tetragoneuria. 



60 (59) Usually more than five antenodal cross veins in the hind wing. . 61 



61 (62) Stigma very narrowly diamond-shaped, with the ends of it meeting 



the sides by an angle of 30° to 35° Helocordulia. 



62 (61) Stigma broader, less pointed 63 



63 (64) Triangle of fore wings open Dorocordulia. 



64 (63) Triangle of fore wings divided by a cross vein 65 



65 (66) Inferior appendage at end of male abdomen bifurcated. Cordulia. 



66 (65) Inferior appendage simple Somatochlora. 



67 (54) The sectors of the arculus in close apposition or completely fused for 



a little way beyond the arculus; anal loop generally dis- 

 tinctly foot-shaped, with well-developed ''toe"; the last 

 antenodal cross vein often discontinuous at the subcostal 

 vein 68 



68 (69) Triangle of the fore wings four-sided; anal loop poorly developed, not 



foot-shaped Nannothemis. 



69 (68) Triangle of the fore wing fully differentiated, three-sided; anal loop 



well developed and foot-shaped 70 



70 (71) Triangle of the fore wing with its front and inner sides meeting by an 



angle of about 100°; the subtriangle without cross veins; 

 the vein which bisects the anal loop straight. . . Perithemis. 



71 (70) Triangle of the fore wing with its front and inner sides meeting by an 



angle of about 90°; subtriangle divided into three or more 

 cells; bisector of the anal loop sinuous 72 



72 (89) Triangle of the fore wing not placed distinctly beyond the level of the 



apex of the triangle in the hind wing; pterostigma with its 

 ends parallel or not distinctly divergent. 73 



73 (84) The sectors of the arculus (veins Mi-g and M4) in the fore wing more 



or less completely fused for a short distance beyond the 

 arculus; the triangle of the fore wing not greatly produced 

 posteriorly, and (except in Celithemis) normally containing 

 but a single cross vein, and followed by two or three rows 

 of cells 74 



