AQUATIC INSECTS 925 



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



37 (42) The radial sector (Rs, Fig. 1388, a) simple. 3 j 



38 (39) But two cubito-anal cross veins; vein M2 undulate; supratriangle 



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



Gomphaeschna. 



39 (38) With three or more cubito-anal cross veins; vein M2 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. , . Basiaeschna. 



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. . 44 



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 M2 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. . . Epiaeschna. 



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 



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



sub triangle of the fore wings usually divided by a cross vein; 

 four to six cross veins in the space above the bridge (Fig. 1388) . 



Didymops. 



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



vertex; sub triangle of the fore wings generally open; two 

 or three cross veins in the space above the bridge. 



Macromia. 



53 (so) 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 Nannothemis) and becoming foot- 

 shaped; one or two cubito-anal cross veins 54 



