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.
   [1913 Webster]
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Water spider \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.
       [1913 Webster]