THE WATER-MITES (HYDRACARINA) 859 



general resemblance between them is close. But the characters 

 also seem to be very constant and few species are subject to marked 

 variation. The accompanying synopsis will aid in placing speci- 

 mens in the proper genus. The statements as to the numbers of 

 species refer to North America only. 



The legs and the corresponding epimera are designated by Roman 

 numerals, beginning with the most anterior, and the palpal and leg 

 segments are referred to by Arabic numerals, numbering from the 

 base outward. Thus, ep. Ill is the third epimeron, leg seg. IV 4, 

 the fourth segment of the fourth leg, and pal. seg. 5 the distal 

 segment of the palpus. In most illustrations are shown the ven- 

 tral surface, only the legs of one side, and the palpus, detached and 

 more highly magnified; these are the characters most important 

 and most readily observed. 



The arrangement of genera and higher groups here used is the 

 same as adopted in a previous paper (Wolcott, 05). It is not in 

 all respects satisfactory, but such a difference of opinion exists 

 among students of the group in this regard that the author is not 

 willing to accept any other system since proposed without himself 

 working the whole matter over again. 



KEY TO NORTH AMERICAN FRESH-WATER HYDRACARINA 



1 (6) Lateral eyes of the two sides close together in the median line and 



borne on a common eye-plate 2 



2 (5) Pal. seg. 5 deeply set into 4, eye-plate long and narrow. 



Family LIMNOCHARIDAE . . 3 

 3(4) Without swimming-hairs . ...... LimnocharesLatieitte 1796. 



A very large clumsy red mite with 

 soft body, variable in form but in 

 general rectangular, found in pools in 

 bogs and swamps. Length 3.5-4 mm. 

 One species, generally distributed and 

 common at times. 



FIG. 1322. Limnochares aquaticus (Lin- 

 naeus). Ventral surface, female. X Q- 

 Inner surface, right palpus. X 95- (Mod- 

 ified from Piersig.) 



4 (3) With swimming-hairs Cyclothrix Wolcott 1905. 



Also red, but oval and more constant in form and recognized at once by the swimming-hairs. 

 One species found also in ponds and lakes with boggy or swampy shores, and known from 

 several northern states. Somewhat smaller than preceding genus. 



