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 pigment (Fig. 60) ; lateral keels with- 

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



rarely more than 14 mm. in length including gills 



Anomalagrion. 



ee. Gills usually with 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 Ischnura. 



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. cyatliigerum) 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; M 2 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 ; postnodal cross- veins of the front wings 

 usually less than twelve in number ; M, 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 Nehalennia. 



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 reddish or buff ; width of the 

 stigma measured between costa and Sc,,-)-]^ much greater than 

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

 the eighth sternum Ampliiagrion. 



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

 measured between costa and SCo-l-E^iiot greater than its length ; 

 female without a heavy spine on the caudal margin of the 

 eighth sternum Cliromagrion. 



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

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



