FRESH-WATER BIOLOGY 



24 (17) Gills absent from one or more of segments 1-7; one pair more or less 



elytroid, covering those behind it 25 



25 (28) 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 line 26 



26 (27) Head smooth above. . > Ephemerella. 



27 (26) Head armed above with a pair of erect occipital tubercles. Drunella. 



28 (25) Gills absent from the seventh abdominal segment, elytroid on the 



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



29 (30) Elytroid gill cover subquadrate . . .''., . . . Caenis. 



30 (29) Elytroid gill cover subtriangular Tricorythus. 



31 (16) Outer caudal setae fringed only on the inner side 32 



32 (37) Posterolateral angles of the hinder abdominal segments prolonged 



into thin, flat, sharp lateral spines 33 



33 (34) F re I e 8 s conspicuously fringed with long hairs; gill tufts present 



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

 lamellae on abdomen Chirotonetes. 



34 (33) Fore legs without conspicuous fringes; no maxillary or coxal gills; 



no gill tufts at base of lamellae on abdomen 35 



35 (36) Gills double on the basal abdominal segments; end of maxilla fringed 



with simple hairs Siphlurus. 



36 (35) Gill lamellae all single; end of maxilla fringed with pectinated hooks. 



Ameletus. 



37 (32) Posterolateral angles of the hinder abdominal segments hardly more 



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



38 (41) Gill lamellae simple* 39 



39 (40) Lamellae obtuse at apex; maxillary palpus rounded at the apex. 



Baetis. 



40 (39) Lamellae acute at apex; end of maxillary palpus truncated. 



Centroptilum. 



41 (38) Gill lamellae double, at least on some of the anterior abdominal 



segments 42 



42 (43) Antennae shorter than the body; tracheae of gill lamellae pinnately 



branched Callibaetis. 



43 (42) Antennae longer than the body; tracheae of gill lamellae palmately 



branched. . Cloeon. 



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. 



