864 



FRESH-WATER BIOLOGY 



seg. IV 4 is also usually modified in the male by the possession of a peg-like projection and 

 characteristic hairs. The species vary considerably in size, from about 0.55 mm. to nearly 

 2 mm. One of the most abimdant and widely distributed genera, common in clear, shallow, 

 hard waters where plant life is abundant, with about 200 species, all of various shades of bluish 

 or brownish green, or red. There are about 50 species described from North America. 



22 (19) Pal. seg. s not opposable to 4, and bearing at the distal end small 



more or less distinct teeth or claws 23 



23 (44) Epimera in both male and female united and more or less fused 



into a single epimeral plate .... 24 



24 (39) Body more or less compressed dorso-ventrally and completely en- 



closed in a chitinous covering usually divided into a smaller, 

 elMptical or oval, dorsal plate and a larger ventral plate. 



Subfamily Aturinae. 25 



25 (26) Four smaller plates, variously shaped, anteriorly, between the dorsal 



and ventral Torrenticola Piersig 1897. 



A rather small mite, 0.6 to 0.75 mm. long, of oval form, 

 with the capitulum produced into a sort of snout; no 

 swimming-hairs. One American species, rarely found, 

 and apparently identical with the European Torrenti- 

 cola anomda (Koch). 



Fig. 1332. Torrenticola anomala (Koch). Ventral surface, 

 female. X 27. Outer side, right palpus, female. X no. 

 (Modified from Piersig.) 



26(25) The two plates, dorsal and ventral, covering the whole surface . . 27 



27 (30) Rostrum developed, prolonged and curved upward 28 



28 (29) Genital area without flaps or valves, with numerous acetabula free 



in the body surface ... . Tanaognathus Wolcott 1900. 



A rather small mite, strongly compressed 

 dorso-ventrally, and with few swimming- 

 hairs. One species, T. spinipes Wolcott, 

 about 0.7 mm. long, known only by a few 

 specimens from Michigan. 



Fig. 1333. Tanaognath%isspinipesVlo\coit. Epi- 

 meral field and genital area, male. X 42. Outer 

 side, right palpus, male. X igs. {After Wolcott.) 



