500 



cc. Gills with the tracheal branches equally distributed throughout 

 the length of the gill. 



d. Gills with long, tapering points, the cuticular pigment, if 

 present, always in cross-bands; mental setae of the labium 

 usually four. 



e. Gills without cuticular ])igniont (Fig. 60) ; lateral keels with- 

 out setae; nymphs of small size, full-grown individuals 



I'arely more than 14 mm. in length including gills 



Anomalagrion. 



ee. Gills usually wdth cuticular pigment in the form of arcuate 

 cross-bands ; lateral keels with several rows of small setae ; 

 nymphs larger, when full-grown 18-20 mm. in length in- 

 cluding gills Isclinura. 



dd. Gills with blunt points (Figs. 56, 57, 70-72, 75-77a, 80), or 

 if with long points, then with cuticular pigment other than in 

 cross-bands: mental setae usually three (four in E. calverti 

 and E. cyathigerum) Enallagma. 



ADULTS 



a. Cephalic row of setae of all tibiae twice as long as the .spaces between 

 their bases; postnodal cross-veins of the front wings twelve or more 

 in number; JI. arising between postnodal cross-veins five and nine 



in the front wing Argia. 



aa. Cephalic row of setae of all tibiae less than twice as long as the 

 spaces between their bases ; po.stnodal cross- veins of the front ■\vings 

 usually less than twelve in number; ^lo arising between the third and 

 fifth, rarely sixth, postnodal cross-veins in the front wing, 

 b. Dorsum of the thorax metallic green or bronze ; female pronotum 



with the caudal lobe trilobed Xekalennia. 



bb. Dorsum of the thorax not metallic green or bronze; female pro- 

 notum not with the caudal lobe trilobed. 



c. Postocular spots wanting ; mesopleural suture without a distinct 



black stripe, the stripe not wider than the suture itself. 



d. Dorsum of abdominal terga 1-6 reddi.sh or bulif : width of the 



stigma measured between costa and Sc.-1-;R, much greater than 



its length ; female with a heavy spine on the caudal margin of 



the eighth sternum Amphiagrion. 



dd. Dorsum of abdominal terga 1-6. black ; width of the stigma 

 mea.sured Ijetwcen costa and Sc^+R, not greater than its length ; 

 female without a heavy spine on the caudal margin of the 



eighth sternum Chromagrion. 



ee. Postocular spots present ; mesopleural suture usually with a dis- 

 tinct black stripe, the stripe wider than the suture itself. 



