NORTH AMKRIOAN ODONATA. 225 



Last antecubital of front wings usually not continued to the median vein ; 



third tibiffi with the spines of the antero-inferior row few (5-7) and 



stout; abdomen stout, hardly as long as the hind wings. 



38. Mesotheinis. 

 Last antecubital on front wings continued to the median vein ; vertex 



truncated at tip ; front wings with not more than 7 antecnbitals. 



triangle crossed, 3 rows of posttriangular cells. .39. I'acliydiplax. 



Systematic Characters of Nymphs.* 



L Three caudal tracheal-gills (Zygoptera). 



Legion Calopteryx. — Basal joint of antennae thick, more than twice as long as 



the other six together. Median lobe of labium bifid. 



Front edge of median lobe of labium bifid to form a lozenge-shaped interval 



between, which extends basally beyond the level of the attacliment of 



the lateral lobes. Rear of head with a tooth each side. Median caudal 



gill flat, shorter than the other two...l. t'silopteryx (see fig. 8, pi. II.) 



Front margin of median lobe of labium bifid only as far basally as the level 



of the attachment of the lateral lobes. Abdominal segments with a 



lateral membrane whose margins are denticulated. Median caudal gill 



a little swollen at apex 2. Ketseriua. 



Subfamily Agrioninse. — Basal joint of antennae hardly longer than thick, much 

 shorter than the second or the third. 

 Lateral lobes of labium (excluding the terminal palp) deeply bilobed, median 



lobe barely bifid Legion Lestes. 



Lateral lobes of labium (excluding the terminal palp) not deeply bilobed, 



median lobe entire Legion Agrion. 



The veins on the wing-covers will assist in determining between these 

 two legions as in the imagos. 

 II. No caudal tracheal-gills (Anisoptera). 



Subfamily Gojrajj/iMia?.— Antennae 4-jointed, fourth joint rudimentary ; first and 

 second tarsi 2-jointed ; labium flat, not covering the labrum or frons 

 when closed. 

 First legs less distant from each other at base than are the second legs. Ab- 

 domen much less than twice as long as broad, very flat, almost circular 

 when viewed from above. Third joint of antennae large, flat, circular. 



12. Hageuius. 

 First legs as distant from each other at base as are the second legs. Abdomen 

 at least twice as long as broad. 

 Middle third of front margin of median lobe of labium produced in a very 

 short rounded lobe with pavement teeth and a comb of flat scales. 



13. Oi>liiogoiU|>liu»t. 



Middle third of the same straight, or nearly so 14. Goiiipliiis. 



Subfamily Cordulegasterivse. — Antennae 7-jointed ; all tarsi 3-jointed ; labium 

 spoon-shaped, covering labrum and frons when closed, teeth on the op- 

 posed margin of the lateral lobes long, interlocking when closed so as to 

 form a distinctly zigzag litie of union. Abdomen two to three times as 

 long as broad. 



* Based on Cabot (50), Hagen (52, 53) and Garman "A Preliminary Rejiort on 

 the Animals of the Waters of the Mississippi Bottoms," etc. Springfield, 111., 

 State Printer, 1889. 



TKANS. AM. KNT. SOC. XX. (29) SEPTEMBER. 1893. 



