932 FRESH-WATER BIOLOGY 



72 (73) Abdomen smooth, depressed; head twice as wide as long, with eyes 



very prominent laterally; lateral spines large and straight; 

 superior appendage one-third shorter than the inferiors. 



Pachydiplax. 



73 (72) Abdomen hairy at the apex; lateral spines small and sharply in- 



curved; superior appendage as long as the inferiors. 



Nannothemis. 



74 (71) Dorsal hooks present, at least on the middle abdominal segments. 75 



75 (80) Abdomen ovate in outline, rather abruptly narrowed to the posterior 



end; hind margin of the eyes behind the middle of the 

 head. . 76 



76 (77) Lateral spines long and straight; abdomen not narrowed posteriorly 



before the eighth segment. ... .... CelUhemis. 



77 (76) Lateral spines shorter and more or less incurvate; the abdomen 



more or less narrowed before the eighth segment. ... 78 



78 (79) Dorsal hooks as long as the segments which bear them. Lettcorhinia. 



79 (78) Dorsal hooks shorter than the segments which bear them. 



Sympetrum. 



80 (75) Abdomen lanceolate in outline, slowly narrowed to the pointed 



posterior end; eyes capping the prominent anterolateral 

 angles of the head, their hind margin generally before the 

 middle of the top of the head; body generally hairy. . 81 



81 (82) The tenth abdominal segment with subcarinate lateral margins; 



appendages very long; lateral setae 0-3 Ladona. 



82 (81) The tenth abdominal segment shorter, cylindric; appendages shorter; 



lateral setae 5-10 83 



83 (84) Head a little narrowed behind the eyes; fror^t border of the median 



lobe of the labium entire Libellula. 



84 (83) Head not narrowed behind the eyes to the hind angles; front border 



of the median labial lobe crenulate Plathemis. 



8s (68) Basal segment of the hind tarsus half as long as the second segment; 

 lateral appendages of the abdomen at least three-fourths 

 as long as the inferiors; lateral setae 10 or more; superior 

 appendage of the abdomen suddenly contracted at its basal 

 third, the dorsal two-thirds forming a long slender point. 86 



86 (87) Movable hook of labium long and slender, setiform; teeth much 



broader than high; spines of the eighth segment one-half 

 longer than the ninth segment; superior abdominal append- 

 age shorter than the inferiors Tramea. 



87 (86) Movable hook of the labium short, hardly longer than the teeth; 



teeth higher than broad; spines of the eighth segment as 

 long as the ninth segment; superior appendage equaling 

 the iirferiors Pantala. 



