THE WATER-MITES (HYDRACARINA) 



869 



46 (53) Genital area usually lying far forwards, at least between epp. IV, 

 and the epimeral groups often separated by a considerable 

 interval, no ancoral process on the maxillary shield. 



Subfamily SPERCHONINAE . 47 



47 (52) Genital acetabula borne on a plate, no flaps present 48 



48 (49) Acetabula numerous Limnesiopsis Piersig 1897. 



A large hydrachnid, about 2 mm. in length, with 

 the surface of the body beset with sharp points. One 

 species, L. anomala (Koenike), described from Canada, 

 and generally distributed in northern lakes but no- 

 where common. 



FIG. 1343. Limnesiopsis a no mala (Koenike). Epimeral 

 field and genital area, male. X 25. Outer side, palpus, 

 male. X 49. (After Koenike.) 



49 (48) Acetabula few, large 50 



50 (51) Leg IV with terminal claws, no swimming-hairs. 



Tyrrellia Koenike 1895. 



Body almost circular, pa pi Hated 

 with one or two dorsal chitinous 

 plates; mouth-opening in the middle 

 of a disk-like surface at the anterior 

 end of the capitulum, resembling the 

 condition seen in the Hydryphantidae; 

 a sluggish, dark-brown mite of medium 

 size averaging 1.2 mm. in length, 

 known from Canada and found 

 abundantly some years since at 

 Reed's lake, near Grand Rapids, 

 Michigan, where it was picked up 

 singly with the pipette in the debris at 

 the margin of the water in close prox- 

 imity to a swampy portion of the lake 

 shore. Very rare in Birge net hauls 

 at the same place. Two species taken, 

 one apparently the same as T. circu- 

 laris Koenike, previously described. 



FIG. 1344. Tyrrellia circularis Koenike. 

 Ventral surface, female. X 26. Inner side, 

 palpus, female. X 4Q- (Modified from 

 Koenike.) 



