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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 Wa·ter spi·der Zool. (a) An aquatic European spider (Argyoneta aquatica) which constructs its web beneath the surface of the water on water plants. It lives in a bell-shaped structure of silk, open beneath like a diving bell, and filled with air which the spider carries down in the form of small bubbles attached one at a time to the spinnerets and hind feet. Called also diving spider.  (b) A water mite.  (c) Any spider that habitually lives on or about the water, especially the large American species (Dolomedes lanceolatus) which runs rapidly on the surface of water; -- called also raft spider.
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