930 FRESH-WATER BIOLOGY 



32 (31) Third joint of antenna elongate, linear, little flattened. ... 33 



33 (34) Dorsum of the ninth abdominal segment rounded, or with a low, 



obtuse, median longitudinal ridge Gomphus. 



34 (33) Ninth abdominal segment with a sharp mid-dorsal ridge, ending in a 



straight apical spine Dromogomphus. 



35 (24, 46) Labium with a short median cleft; antennae seven-jointed, 



setaceous; tarsi three-jointed; climbing nymphs, with eyes 

 at sides of head 36 



36 (39) Hind angles of the head, viewed from above, sharply angulate. . 37 



37 (38) Lateral lobe of labium squarely truncate on apex Boyeria. 



38 (37) Lateral lobe of labium with taper-pointed apex. . . . Basiaeschna. 



39 (36) Hind angles of the head obtusely rounded 40 



40 (45) With lateral spines on abdominal segments 4-, 5-, or 6-9. . . 41 



41 (44) With lateral spines on segments 4-, or 5-9 42 



42 (43) With dorsal hooks on abdominal segments 7-9. . . Nasiaeschna. 



43 (42) With no dorsal hooks on abdomen Epiaeschna. 



44 (41) With lateral spines on abdominal segments 6-9 Aeschna. 



45 (40) With lateral spines on abdominal segments 7-9 Anax. 



46 (24, 35) Labium with a shallow median cleft; antennae seven-jointed; 



short; squatting nymphs, with face vertical, and eyes on 

 anterolateral angles; depressed; hairy; tarsi three-jointed. 



Tachopteryx. 



47 (23) Labium mask-shaped or spoon-shaped, when closed, covering the 



face up to the bases of the antennae, armed with raptorial 

 setae 48 



48 (49) The prominent median lobe of the labium cleft into two variously 



formed teeth at apex Cordulegaster. 



49 (48) The median lobe of the labium entire 50 



50 (53) Head with a prominent pyramidal frontal horn; abdomen flat and 



almost circular in outline as seen from above; legs long, 

 giving a spiderlike aspect to these big nymphs; the tenth 

 abdominal segment well exposed, not telescoped in the apex 

 of the ninth segment; teeth on the lateral lobes of the labium 

 with deep incisions between them 51 



51 (52) Head hardly as wide across the eyes as across the bulging hind angles; 



lateral spines not incurved, those of the ninth abdominal 

 segment hardly surpassed by the tips of the appendages; 

 dorsum of the tenth abdominal segment with no trace of a 

 dorsal hook Didymops. 



52 (51) Head widest across the eyes; spines of the ninth abdominal segment 



shorter, not nearly reaching the level of the apices of the ap- 

 pendages; dorsum of the tenth segment with a very rudi- 

 mentary dorsal hook Macromia. 



53 (50) Head without pyramidal frontal horn; abdomen less flattened, more 



elongate; teeth on the lateral lobes of the labium much 

 wider than high 54 



