water spider

from The Century Dictionary.

• noun A spider of the family Drassidæ, Argyroneta aquatica, which makes a bag of silk on water-plants, and lives in it under water as in a diving-bell, the opening being below, so that the air cannot escape. It is tilled by the spider, which brings down bubbles of air one at a time. See diving-spider, and cut under Argyroneta.
• noun Any one of certain spiders of the lycosid genus Dolomedes, as D. tenebrosus, D. urinator, or D. sexpunctatus, which build nests of leaves and twigs on overhanging rushes, just at the surface of the water in shallow streams; a raftspider.
• noun A water-mite or water-tick.
• noun A bug of the genus Hydrometra; a water-measurer.

from the GNU version of the Collaborative International Dictionary of English.

• 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.
• A water mite.
• 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun diving bell spider