Source : Webster's Revised Unabridged Dictionary (1913)

Water spider \Wa"ter spi"der\ (Zo["o]l.)
   (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}.