62 



REPOET 108, U. S. DEPARTMEN^T OF AGRICULTURE. 



at tip. The palpi have the basal joints enlarged and often with a spur below. They 

 usually occur in lakes. 



Fig. 87.— Coxal plates of By- Fig. 88.— Palpus of Hygrdbates. 



grobatcs. (Original.) (Original.) 



Of Atractides (fig. 89) one European species has been recorded from Canada. The 

 palpi are close together on a short elevation, the fourth joint rather swollen and bristly 

 above; there are shields on the dorsum of the body. 



Lebertia (fig. 92) (and its subgenera) has many species in 

 Europe, one of which has been recorded from Canada, but 

 the American form has much longer hairs than the European 

 specimens. 



Tyrrellia (fig. 83) was based on a round-bodied, short-legged 

 species from Canada; the palpi are rather short, and the second 

 joint has a spine beneath. 



To Thyopsis (near Thyas, but without median eye) belongs 

 a remarkable species, T. cancellata Koenike. The entire dor- 

 sum is densely reticulate, the central part with a large and 

 heavy reticulation. 



Koenikea is represented by one species ^videly distributed in the northern parts of 

 our country. The species of Hygrobates (figs. 87, 88) have soft bodies, the slender legs 



Fig. 89.— Palpus of Atrac- 

 tides. (Original.) 



Fig. 90.— Coxal plates of Fig. 91.— Palpus of Spcrchoyi. 



Sperchon. (Original.) (Original.) 



destitute of true s-svimming hairs, the palpi have no tooth below, and the epimera form 

 three plates. There are three sucking disks each 

 side of the genital orifice. Four species are known 

 from small rivers in Canada. 



Atax (figs. 95, 96, 97, 98) is one of the largest genera, 

 and Dr. Wolcott has worked out some 12 species, 

 and another has since been added from Texas. The 

 genital opening is situate at the tip of body, and the 

 sucking di.sks each side are 10 or more in number. 

 The legs have swimming hairs, and the first pair is 

 thicker than usual. The palpi (fig. 98) are enlarged 

 at base, and the fourth joint bears three papilUie or 

 spurs below. Most of the species occur, for at least 

 a time, in certain fresh-water mussels, particularly 

 of the genera Unio and Anodontn. They are found 

 in the mantle folds or gills of the mussel, and feed 

 on th(^ minute animals drawn in by the mussel. 



The genus Ncumauia is closely allied to Atxix, l)ut peculiar 

 ringed bristles on legs I and II. 



Fig. 92. 



A'cnter of Lebertia. (.Vutlior's 

 illustration.) 



Ill account of the spirally 



