The Collaborative International Dictionary of English v.0.48:

Water mite \Wa"ter mite`\ (Zool.)
   Any of numerous species of aquatic mites belonging to
   Hydrachna and allied genera of the family Hydrachnidae,
   usually having the legs fringed and adapted for swimming.
   They are often red or red and black in color, and while young
   are parasites of fresh-water insects and mussels. Called also
   water tick, and water spider.
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