522 



with seven or eight setae on the lateral margins of the median lobe ; 

 labium extending just caudad of the first pair of coxae. 



Thorax about half as wide as the head ; the femora all with pre- 

 apical rings on the distal third ; tibiae with the usual apical scales ; tarsi 

 pale ; metathoracic wing-cases extending beyond the cephalic margin of 

 the fourth abdominal segment. 



Abdomen slender, the lateral keels well developed on segments 

 1-8 but almost wholly lacking in setae except the seventh, which some- 

 times possesses a single weak one; gills (Fig. 54) long and slender, 

 somewhat lanceolate, with a gradually tapering tip ; the smaller 

 tracheae are collected in alga-like patches, and the gills are always pro- 

 vided with some dark cuticular pigment ; there is a light spot on the 

 apical third or fourth of the gill on each side of the axis as in exsiilans 

 which is often followed by two dark cross-bands, the extreme tip, how- 

 ever, being light in color; female ovipositor extending to the middle of 

 the tenth sternum. 



Measurements 



Length 13-14 mm. 



Length of abdomen 9 mm. 



Length of gills 7-8 mm. 



Width of gills 1-1.3 mm. 



Length of metathoracic wing-cases ... 4 mm. 



Length of median lobe 1.6 mm. 



Width of median lobe 5-1.2 mm. 



The nymph is very similar to cxsiilans l)ut may be distinguished 

 from that species by means of the more slender gills, the cross-bands 

 at the tip in older individuals, and by the absence of a hinge beyond^ 

 the middle. 



Adult; Male. — Color, blue or greenish yellow and black. 



Head black and orange; mouth-parts buff, the median lobe of the 

 labium subtriangular, the median cleft shallow, acute; proximal seg- 

 ment of the palpus comparatively narrow, the apical half of the distal 

 segment dark ; antennae dark, the apex of the first two segments some- 

 times lighter in color ; a large portion of the postclypeus and a dorso- 

 mesal spot on the labrum, black; anteclypeus, the remainder of the 

 labrum, exposed portions of the mandibles, their trochantins, the genae, 

 and a transverse stripe above the clypeus orange; remainder of the 

 front and vertex dull black ; postocular spots cuneiform, connected with 

 the narrow stripe of the caudo-mesal margin; occiput and postgenac 

 yellow except a black stripe caudo-ventrad of the postocular spots; 

 compound eyes slate-gray. 



