852 FRESH-WATER BIOLOGY 



and Canada. An interesting occurrence was the finding of a spe- 

 cies of Lebertia, a genus usually found in alpine and more northern 

 waters, in a spring at Omaha, Nebraska, the only record of the 

 genus in a state where bodies of water of that character are almost 

 lacking. At present about seventy genera are known, containing 

 several hundred described species, the number of which is fast 

 increasing. 



The water-mites are found at all seasons of the year, even 

 under the ice in winter. Certain ones, especially of the red mites, 

 are abundant in pools in early spring, but the greatest number of 

 species appear as adults during the latter part of the summer or in 

 the fall. They are small forms usually from i to 2 millimeters 

 long, rarely exceeding a length of 5 millimeters, but on the other 

 hand, in the adult condition, rarely measuring less than half a 

 millimeter. 



The color varies greatly, but is most frequently either some shade 

 of red or green; the same species may at the same locality and at 

 the same time be both red and different shades of green or bluish 

 green. The color is partly due to pigment deposited in the epidermal 

 cells, but from above or beneath blackish, brownish or greenish 

 spots are seen, which vary in size and intensity and are due to the 

 stomach and its blind diverticula seen through other more superfi- 

 cial structures. A whitish, yellowish, or reddish Y-shaped dorsal 

 mark, or markings of various form seen on the dorsal, lateral, or 

 posterior surfaces, are due to the presence of excretory matter in 

 the so-called Malpighian vessels, and thus are very variable in 

 number and extent. Hence while color is a clue to identification 

 which may be of service to the experienced observer, it cannot be 

 relied upon, and is of little or no value in the discrimination of 

 species. 



As seen in the water the hydrachnids appear at first glance like 

 small water spiders, possessing, as they do, four pairs of legs and a 

 pair of palpi corresponding to the pedipalps of spiders. But they 

 can at once be referred to the mites when it is noted that there is 

 no trace of segmentation or of division of the body into regions. 



The body is compact and usually more or less globular, ellip- 

 soidal, or ovoidal, though in some cases compressed dorso-ventrally 



