548 



Thorax nearly uniform in diameter throughout, the prothorax 

 somewhat smaller; legs with faint preapical femoral rings of brown, 

 and the usual apical scales present on the tibiae ; tarsi and claws pale ; 

 metathoracic wing-cases extending to the middle of the fourth ab- 

 dominal segment. 



Abdomen slender, and with a row of spots on the cephalo-lateral 

 angles of the terga and sterna of segments 2-9; lateral keels strongly 

 developed, setose, the first keel usually without setae, the second, 

 third, and fourth with a bunch of four or five at the apices, the fifth 

 with about five apical setae and a weaker row extending to the base 

 of the segment; sixth, seventh, and eighth keels with six setae at the 

 apices and a proximal row of about nine setae on each ; segment nine 

 without lateral keel, but with a row of setae along the line of the lateral 

 keels, composed of two heavy setae near the caudal margin and a 

 straight row of six smaller setae extending cephalad to the margin 

 of the segment; cuticle of the abdomen without dark spots at the 

 bases of the minute setae which cover the surface; dorsum of seg- 

 ments two and three and the base of four with long hairs; gills (Fig. 

 76) lanceolate, almost colorless, widest beyond the middle and rather 

 obtusely pointed ; tracheae pigmented in certain areas which form 

 about twelve alga-like patches around the margins of the gills; dorsal 

 marginal setae of the median gill extending half-way from the base 

 to the tip and composed of more than twenty setae ; ventral setae of 

 the lateral gills extending more than half-way from the base to the 

 tips of the gills ; apical margins of all gills with a few scattered hairs. 



Measurements 



Length 14-15 mm. 



Length of abdomen 9 mm. 



Length of gills 5 mm. 



Width of gills 1.8 mm. 



Length of metathoracic wing-cases. .3.6 mm. 



Length of median lobe 2 mm. 



Width of median lobe 7-1 mm. 



Several specimens obtained from Dr. E. M. Walker have been 

 studied. Specimens of this species are also present in the collection of 

 the Illinois State Laboratory of Natural History and have been iden- 

 tified by comparison. 



Adult; Male. — Color, pale blue and black. 



Head black and yellowish, the mouth-parts buff, the median lobe 

 of the labium subtriangular ; antennae black or very dark brown; 



