928 FRESH-WATER BIOLOGY 



NYMPHS 



1 (22) Three large leaf like respiratory plates at the apex of the slender abdo- 



men, and with the body tapering posteriorly from the head 

 (damselflies) Suborder Zygoptera . . 2 



2 (5) Basal segment of the antenna very large, as long as the other six to- 



gether; median lobe of the labium with a very deep cleft; 

 gills thick, the lateral ones triquetral . 3 



3 (4) Median cleft of labium very deep, extending far beneath the level of the 



base of the lateral lobes Calopteryx. 



4 (3) Median cleft of the labium extending only to the level of the base of 



the lateral lobes Hetaerina. 



5 (2) Basal segment of antenna not longer than succeeding single segments; 



labium with a very shallow closed median cleft or no cleft 

 at all; gills thin, lamelliform. . . . 6 



6 (9) Median lobe of labium with a short, closed, median cleft; lateral lobe 



trifid at end; movable hook bearing raptorial setae; gills 

 showing transverse segmentation 7 



7 (8) Lateral lobe of the labium terminating in three teeth, between the 



middle and external of which is situated a truncated and 

 serrated lobe Lestes. 



8 (7) Three teeth only, terminating the lateral lobe of the labium, no trun- 



cated and serrated lobe between them. . . . Archilestes. 



9 (6) Median lobe of labium entire; lateral lobe bifid at end; hook naked; 



gills various 10 



10 (n) Labium with no raptorial setae on the mentum within; gills broad, 



thick, dark colored, oval or oblong in shape and obtuse at 

 apex Argia. 



11 (10) Labium with mental setae; gills thinner, more pointed and nar- 



rower 12 



12(15) Hind angles of the head strongly angulate 13 



13 (14) Gills widest beyond the middle; body slender; head half as long as 



wide Chromagrion. 



14 (13) Gills widest across the middle; body stouter; head nearly as long as 



wide Amphiagrion. 



15(12) Hind angles of the head rounded 16 



16 (17) Labium with one mental seta (and a rudimentary second one) each 



side; antennae six-jointed; lateral lobe of the labium with 

 the distal end above the end hook hardly denticulated. 



Nehallennia. 



17 (16) Labium with three to five mental setae each side (one may be smaller 



than the others), and end of lateral lobe denticulated dis- 

 tinctly; antennae seven-jointed (with the possible exception 

 of Enallagma antennatum) 18 



1 8 (21) Gills more than half as long as the abdomen, lanceolate; third seg- 



ment of antennae less than a third longer than the second. 1 9 



IQ (20) Labium with four to six lateral setae, generally with five, and with 

 three (rarely four) mental setae each side; gills often with 

 a definite color pattern Enallagma. 



