517 



Length of abdomen, 2 20 mm. 



Length of hind wings, $ 20 mm. 



Length of hind wings, $ 20 mm. 



Width of hind wings, $ 4 mm. 



Width of hind wings, 5 5 mm. 



This species is common at times along the Drainage Ditch north 

 of Urbana, and the nymphs may be taken at almost any season in the 

 black mud on the bottom of the stream. The species has not been col- 

 lected elsewhere in the state and it does not seem to be as common as 

 reported to be in Indiana. The nymphs emerge throughout June and the 

 adults fly as late as the first of September. 



Genus Enallagma Charpentier 



The nymphs of this genus are characterized by the presence of 

 three, rarely four, mental setae, and five lateral setae. The gills are 

 variable but do not possess the long tapering points of Ischnura and 

 Anomalagrion, being relatively blunt at the tip. The lateral keels are 

 well developed and setose and in some cases present characters of diag- 

 nostic value for the species. 



The adults in all cases have vein Mo arising between the fourth 

 and sixth, usually fourth and fifth, postnodal cross-veins in the front 

 wing and between the third and fifth in the hind wing. The number 

 of postnodal cross-veins varies from seven to twelve, and the female 

 ahvays has a long apical seta on the eighth sternum. The eighth 

 sternites at the base of the cephalic pair of gonapophyses of the female 

 are visible and are small and subtriangular. 



The genus is represented in Illinois by more species than any other 

 genus of Zygoptera. 



Key to Species 



NYMPHS 



a. Gills without pigment except in the tracheae. 



b. Dark tracheal branches in algadike patches (Fig. 76) Jiageni. 



bb. Dark tracheal branches not in alga-like patches. 



c. Lateral keel of the first abdominal segment without setae ; axis of 

 the gills clear. 



d. Dorsal marginal setae of the median gill less than twenty in 

 full-grown nymphs ; all of the third antennal segment dark 

 brown; gills rarely more than 4.5 mm. in length (Fig. 72) .... 

 geminatum. 



