922 FRESH-WATER BIOLOGY 



Gills absent from one or more of segments 1-7 ; one pair more or less 



elytroid, covering those behind it 25 



Gills present on the seventh abdominal segment, elytroid on the third 



or fourth segment; a pair of tubercles on the apical margin 



of each segment beside the middorsal Une. . . 26 



Head smooth above. . Ephemerella. 



Head armed above with a pair of erect occipital tubercles. Drunella. 

 GiUs absent from the seventh abdominal segment, elytroid on the 



second segment; no dorsal abdominal tubercles. ... 29 



Elytroid gill cover subquadrate. . Caenis. 



Elytroid gUl cover subtriangular. Tricorythus. 



Outer caudal setae fringed only on the inner side 32 



Posterolateral angles of the hinder abdominal segments prolonged 



into thin, flat, sharp lateral spines 33 



Fore legs conspicuously fringed with long hairs; gill tufts present 



upon the base of maxillae and front coxae and at bases of 



lamellae on abdomen. . . ... Chirotonetes. 



Fore legs without conspicuous fringes; no maxillary or coxal gills; 



no gill tufts at base of lamellae on abdomen. . . 35 



GUIs double on the basal abdominal segments; end of maxilla fringed 



with simple hairs. . .... Siphlurus. 



Gill lamellae aU single; end of maxilla fringed with pectinated hooks. 



Ameletus. 

 Posterolateral angles of the hinder abdominal segments hardly more 



than acute — not prolonged in thin flat lateral spines. . 38 



GiU lamellae simple . . .39 



Lamellae obtuse at apex; maxfllary palpus rounded at the apex. 



Baetis. 

 Lamellae acute at apex; end of maxillary palpus truncated. 



Centroptilum. 

 GUI lamellae double, at least on some of the anterior abdominal 



segments. . . . 42 



Antennae shorter than the body; tracheae of gill lamellae pinnately 



branched. Callibaelis. 



Antennae longer than the body; tracheae of gUl lamellae palmately 



branched Cloeon. 



33 (34 



34 (33 



35 (36; 



36 (35 



37 (32; 



38(41 



39 (40! 



40 (39' 



41 (38: 



42 (43' 



43 (42^ 



KEY TO NORTH AMERICAN DRAGONFLIES 



Imagos 



1 (21) Fore and hind wings similar, usually held vertically in repose (damsel- 



flies) Suborder Zygoptera . . 2 



2 (5) Quadrangle (Fig. 1388) of the wings divided by a number of cross veins; 



antenodal cross veins numerous; pterostigma lacking a 

 special brace vein; wings rather broad 3 



3 (4) Basal space (space before the arculus) in all wings free from cross veins. 



Calopteryx, 



