AQUATIC INSECTS 929 



20 (19) Labium with five or six lateral setae, and with four mental setae each 



side; gills generally with no distinct pattern. . . Ischnura. 



21 (18) Gills less than half as long as the abdomen, narrower and with a long 



tapering point; third segment of antenna more than a third 

 longer than the second Anomalagrion. 



22 (i) Without external respiratory plates, but with a respiratory chamber 



inside the wide abdomen; body less slender, and not widest 

 across the head. (Dragonflies; proper.) 



Suborder Anisoptera . . 23 



23 (47) Labium flat or nearly so (the edges of the lateral lobes slightly up- 



turned in Tachopteryx) , without raptorial setae 24 



24 (35, 46) Labium with its median lobe entire; antennae four-jointed, the 



fourth joint rudimentary; fore tarsi two-jointed: burrow- 

 ing nymphs 25 



J. 1389. Recognition characters of dragonfly nymphs. A, inner aspect of the labium; m, mentum; 

 ubmentum; ml, median lobe; //, lateral lobe; ms, mental setae; .Is, lateral setae; h, end hook. B, end 



FIG. 



sm, submet 



of the abdomen as seen from above: 7, 8, Q, 10, abdominal segments; d, dorsal hooks; /, lateral spines; 

 s, superior appendage; I, paired lateral appendages; c , inferior appendages (cerci). 



25 (26) Middle legs more approximate at the base than are the fore legs; 



fourth segment of the antenna slender, erect, about as long 

 as the third segment is wide; the tenth abdominal segment 

 about as long as the ninth Progomphus. 



26 (25) Middle legs not more (usually less) approximate than the fore legs 



at base; the fourth segment of the antenna a mere rudiment, 

 orbicular or discoid, much shorter than the third segment 

 is wide; the tenth abdominal segment much shorter than 

 the ninth 27 



27 (28) Wing cases strongly divergent on the two sides; lateral lobe of 



labium blunt at apex Ophiogomphus. 



28 (27) Wing cases laid closely parallel along the back; lateral lobe of labium 



ending in a sharp, incurved hook 29 



29 (30) Abdomen very thin and flat, circular in outline as seen from above; 



third segment of antenna flat and subcircular. . Hagenius. 



30 (29) Abdomen less depressed, ovate to lanceolate in outline, at least twice 



as long as wide 31 



31 (32) Third joint of antenna very flat, thin, and in outline circular or 



broadly oval Lanthus. 



