AQUATIC INSECTS 931 



54 (65) Lateral appendages of the abdomen more than half as long as the 



inferiors; hind femora longer than the head is wide; when 

 the lateral spines are long, then there is a full series of big, 

 cultriform dorsal hooks on the abdomen 55 



55 (56) Lateral setae four or five; mentum about as long as wide. 



Epicordulia. 



56(55) Lateral setae seven; mentum of labium longer than wide. . . 57 



57 (62) Abdomen with large, laterally flattened, generally cultriform dorsal 



hooks . . 58 



58 (59) Lateral spines of the ninth segment longer than half the length of 



that segment; dorsal hooks on segments 3-9, highest on 6, 

 cultriform, and sharp Tetragoneuria. 



59 (58) Lateral spines of the ninth segment shorter than half of that segment; 



dorsal hooks less developed 60 



60 (61) Dorsal hooks on segments 4-9 laterally flattened, but not cultriform. 



Somatochlora. 



6 1 (60) Dorsal hooks on segments 6-9, longest on 8 and cultriform. 



Helocordulia. 



62 (57) Abdomen with no dorsal hooks, or with these rudimentary, not 



flattened laterally or cultriform, but small obtuse or pointed 

 prominences 63 



63 (64) Hind angles of the head rounded; lateral spines of the ninth abdominal 



segment one-fifth as long as that segment. . . . Cordulia. 



64 (63) Hind angles of the head angulate superiorly; spines of the ninth 



abdominal segment one-third as long as that segment. 



Dorocordulia. 



65 (54) Lateral abdominal appendages generally less than half the length of 



the inferiors; hind femora generally as long as the head is 

 wide; often when the lateral spines of the abdomen are long 

 the dorsal hooks are wanting or reduced. ...... 66 



66 (67) With large, cultriform dorsal hooks on abdominal segments 3-9; eyes 



small and situated on the mid-lateral margin of the head and 

 directed laterally Perithemis. 



67 (66) With no dorsal hook on the ninth abdominal segment; eyes over- 



spreading more or less the anterolateral margins of the 

 head 68 



68 (85) Basal segment of the hind tarsus more than half as long as the second 



segment; lateral appendages of the abdomen not more than 

 half as long as the inferiors (except in Libettula, quadri- 

 maculata) ; superior abdominal appendage regularly tapering 

 to a point 69 



69 (70) Abdominal appendages strongly decurved; lateral spines wanting 



or extremely rudimentary Mesothemis. 



70 (69) Abdominal appendages straight or very slightly declined; lateral 



spines evident on abdominal segments 8 and 9 71 



71 (.74) With no dorsal hooks at all 72 



