THE WATER-MITES (HYDRACARINA) 857 



companied by the loss of the nymphal appendages and the develop- 

 ment of a new set, and the skin may be cast all at once or in several 

 portions. Instances have been described in which the nymph was 

 produced directly from the egg in the egg-mass. 



These water-mites, like most aquatic animals, spend much of 

 their time in active motion, swimming with comparative rapidity 

 through the open water or more slowly walking over the bottom or 

 climbing about on plants or other objects. At times they stop 

 and remain stationary, clinging to whatever object they may rest 

 upon, but a touch from another animal sends them whirling on 

 again with rapid leg movements. When prey is secured they stop 

 to suck the juices from the body of the victim, casting aside the 

 carcass when it has been drained. Aside from the sense of touch, 

 which seems quite acute, the senses are poorly developed, or at 

 least appear to be little used. They rarely feign death, but almost 

 invariably attempt to escape a threatened danger by rapid flight. 

 The less uniform rate of motion they exhibit is of aid in distin- 

 guishing them from other forms, especially ostracods, with which 

 they may be confused. The leg movement also aids in their dis- 

 crimination. 



Attractive as the hydrachnids are to the student of fresh-water 

 life and to the biologist, they are of economic importance only as 

 they afford an element in the food of fishes. Examinations of the 

 contents of fish stomachs frequently show that they have been 

 eaten, and their abundance at times would seem to indicate that 

 under such circumstances they might make up no inconsiderable 

 portion of the food. But they seem to go to pieces very quickly 

 and so are rarely reported in any numbers in the results of exami- 

 nations of such stomach contents. 



In collecting these little fellows one needs a net, a number of 

 wide-mouthed bottles or jars, a pipette, and, in case he is not to 

 examine his collections within a few hours, a bottle of formalin. 



The most serviceable net is the "cone" or "Birge" net (see 

 page 68). The net may be used from boat or shore and the mate- 

 rial, after being run into a wide-mouthed bottle or jar, be pre- 

 served in toto at once by adding directly a little strong formalin 

 and shaking thoroughly, or it may be carried home in the fresh 



