from
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]