505 



third segments frequently yellow above; wings with twelve to four- 

 teen postnodal cross-veins, the stigma surmounting a single cell or 

 less, and M 2 arising between the seventh and eighth, or eighth and 

 ninth, postnodal cross-veins in the front wing and between the sixth 

 and seventh in the hind wing. 



Abdomen blue or buff and black, the black placed as follows — 

 a spot on the first tergum, longitudinal dorsal stripes on 1-7 which 

 extend well onto the lateral surfaces of terga 3-7 at their apices, and 

 the lateral margins of terga 8-10 inclusive; the yellow or blue forms 

 lateral stripes of pale color on terga 1-7, narrow basal rings on 3-7 

 inclusive, and narrow apical ring on one; dorsum of terga 8-10 blue; 

 anal appendages (Figs. 151, 152) consisting of short black superiors 

 and longer bifurcate inferiors, the dorsal emargination of the tenth 

 tergum about one-third the length of the segment; sterna 2-10 black, 

 one, pale buff. 



Female. — Color similar to that of the male. 



Head : cephalic aspect entirely blue or buff with the exception of 

 black spots ventrad and dorsad of the lateral ocelli and black rings 

 around the postocular spots. 



Thorax as in the male, though usually somewhat lighter in color ; 

 mesostigmal plates as shown in Figure 153 with a very small caudal 

 projection near the caudo-mesal angles ; caudal lobe of the pronotum 

 with more or less blue or buff and the rhesopleural dark stripes of the 

 infraepisternum and epimeron paler than those of the male. 



Abdomen with the dorsum of the eighth and ninth terga black 

 with a dorsal yellow stripe and with dark brown or yellow on the 

 lateral surfaces; tenth tergum dark brown above, paler on the sides, 

 the dorsal, median emargination extending nearlv to the base of the 

 segment; anal appendages of the usual form and pale in color; ovi- 

 positor with light brown lateral valves, the prostyles darker. 



Measurements 



Length, $ 38 mm. 



Length, $ 36 mm. 



Length of abdomen, $ 30 mm. 



Length of abdomen, 5 28 mm. 



Length of hind wings, $ 24 mm. 



Length of hind wings, 9 23 mm. 



Width of hind wings, $ 5.5 mm. 



Width of hind wings, 5 5.5 mm. 



A common species along all large-sized streams in Illinois. The 

 nymphs live in the mud at the bottom, but when mature approach the 



