AQUATIC INSECTS 925 



36 (25) Eyes approximated on the top of the head 37 



37(42) The radial sector (/?5, Fig. 1388, a) simple 38 



38 (39) But two cubi to-anal cross veins; vein M. undulate; supratriangle 



without cross veins; but one cross vein under the stigma. 



Gomphacschna. 



39 (38) With three or more cubito-anal cross veins; vein Mo not undulate; 



supratriangle divided by cross veins; several cross veins 

 under the stigma 40 



40 (41) Basal space traversed by cross veins Boyeria. 



41 (40) Basal space open Basiaeschm. 



42(37) Radial sector bearing an apical fork. 43 



43 (48) Sectors of the arculus (veins Mi_3 and M4) separating from the 



arculus at or below its middle 4_j. 



44 (47) The radial sector symmetrically forked: between it and the supple- 



mentary vein below it, one or two rows of cells 45 



45 (46) Face strongly produced above, the upper margin of the frons very 



acute; the veins Mi and Mo parallel to the level of the stigma; 

 radial sector and the supplementary vein below it separated 

 by a single row of cells Nasiaeschna. 



46 (45) Face vertical, not sharply angulate at upper edge of frons; veins 



Ml and M2 approximated at the stigma; the radial sector 

 and the supplementary vein below it separated by two rows 

 of cells Epiacschna. 



47 (44) The radial sector strongly deflected toward the stigma at the base of 



its fork, unsymmetric; between it and the supplementary 

 vein below it, three to seven rows of cells. . . . Aeschna. 



48 (43) Sectors of the arculus springing from above the middle of the arcu- 



lus Anax. 



49 (22) Triangle in the hind wing much nearer the arculus than in the fore 



wing; stigma without brace vein 50 



50 (53) The triangle of the hind wing placed considerably beyond the arcu- 



lus; the anal loop well developed and hardly longer than 



broad; more than two cubito-anal cross veins. 



51 (52) Dorsal surface of the head with the occiput larger than the vertex; 



subtriangle of the fore wings usually divided by a cross vein; 

 four to six cross veins in the space above the bridge (Fig. 13 88) . 



Didymops. ■ 



52 (51) Dorsal surface of the head with the occiput much smaller than the 



vertex; subtriangle of the fore wings generally open; two 

 or three cross veins in the space above the bridge. 



Macromia. 



53 (50) The triangle of the hind wing retracted to the level of the arculus, 



or even passing it a little sometimes; the anal loop, greatly 

 elongated (except in Nannothcmis) and becoming foot- 

 shaped; one or two cubito-anal cross veins 54 



