ATARBA. 121 



and I observed the gray variety (A. saxicola, olim) in large 

 numbers, in May, 1859, on mossy stones in a creek, near Wash- 

 ington, D. C, performing a singular, sideways walk along the 

 water's edge, probably for the purpose of oviposition ; some of 

 them were in copulation. I have no doubt now that A. saxicola 

 is only a variety of A. opalizans ; I have received larger speci- 

 mens of it from the north, and I understand that this variety 

 also occurs in Europe. 



Gen. XIII. ATARBA. 



One submarginal cell ; four posterior cells ; a discal cell ; no marginal 

 cross-vein ; tip of the auxiliary vein nearly opposite the origin of the 

 second vein ; the subcostal cross-vein at a distance from this tip which is 

 a little shorter than the great cross-vein (Tab. I, fig. 13). Rostrum short. 

 Antennae 16-jointed, rather long. Tibire without spnrs at the tip (?) ; 

 empodia distinct ; ungues smooth. The large forceps of the male consists 

 of two elongated subcylindrical basal pieces, each bearing a double horny, 

 claw-shaped appendage. 



Eyes glabrous ; front rather narrow ; rostrum but little pro- 

 jecting ; palpi rather long, especially the last joint. Antennae 

 rather long, reaching beyond the basis of the abdomen, when 

 bent backwards ; first joint short, not much longer than the 

 second ; joints of the flagellum elongated, cylindrical, gradually 

 decreasing in length ; they are clothed with a dense pubescence ; 

 a single, somewhat longer hair is perceptible on each segment, 

 above the pubescence ; the antennae of the female are but little 

 shorter than those of the male. Collare short — the head being 

 rather approximated to the mesothorax. Thoracic suture dis- 

 tinct. Feet of moderate length, comparatively stout, finely 

 pubescent ; empodia distinct. The forceps of the male is large 

 and not unlike Tab. IV, fig. 29, in appearance, only more hairy ; 

 the basal pieces leave an open interval between them, even when 

 the forceps is closed ; the ends of the claw-shaped appendages 

 are distinctly bifid, showing that they consist of two closely 

 approximated horny pieces ; there is a short stump in the place 

 of the anal style of the Limnobina (one of my specimens has a 

 long curved aculeus projecting on the under side ; in the other 

 male specimen this organ is apparently concealed internally). As 

 the specimen, which I believe to be a female, has its abdomen 

 broken off, I cannot describe the ovipositor. 



