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 m clear, shallow, 

 hard waters where plant Ufe 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. 5 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, 

 elHptical 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 T orrenticola Fkisig iSgy . 



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 anomala (Koch). 



Fig. 1332. Torreniicola 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. 



rmm^ 



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. Tanaognathus spinipes Wdicott. Epi- 

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

 side, right palpus, male. X iQS- (After Wolcott.) 



