930 FRESH-WATER BIOLOGY 



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



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



obtuse, median longitudinal ridge Gomphus. 



34 {s3) 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 



fa-ce up to the bases of the antennae, armed with raptorial 

 setae 4^ 



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 outhne as seen from above; legs long, 

 giving a spiderhke 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 \\dde 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 



