The water spider stores air underwater in a hydrophobic, bell-shaped web.

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References

"Perhaps the most impressive use of gaseous air under water is that of the European water spider, Argyronetes aquatica (Foelix 1996). It is the only spider that lives constantly underwater - Walking, swimming, feeding, mating, and raising young. It makes a bell-shaped web in the submerged vegetation of ponds and then travels repeatedly up to the surface and down again, in each trip carrying down a coating of air on its abdomen (the allusion to silver in the name of its genus reflects this shiny layer). Thus it fills the bell with air; the fine mesh and a water-repellent coating on the strands prevent upward leakage, as will be explained shortly. From time to time the spider adds air to its diving bell to offset use and dissolution." (Vogel 2003:100)

"Argyroneta aquatica is a unique air-breathing spider that lives virtually its entire life under freshwater. It creates a dome-shaped web between aquatic plants and fills the diving bell with air carried from the surface. The bell can take up dissolved O2 [oxygen] from the water, acting as a 'physical gill'. By measuring bell volume and O2 partial pressure (PO2) with tiny O2-sensitive optodes, this study showed that the spiders produce physical gills capable of satisfying at least their resting requirements for O2 under the most extreme conditions of warm stagnant water. Larger spiders produced larger bells of higher O2 conductance (GO2). GO2 depended on surface area only; effective boundary layer thickness was constant. Bells, with and without spiders, were used as respirometers by measuring GO2 and the rate of change in PO2. Metabolic rates were also measured with flow-through respirometry. The water–air PO2 difference was generally less than 10?kPa, and spiders voluntarily tolerated low internal PO2 approximately 1–4?kPa before renewal with air from the surface. The low PO2 in the bell enhanced N2 [nitrogen] loss from the bell, but spiders could remain inside for more than a day without renewal. Spiders appeared to enlarge the bells in response to higher O2 demands and lower aquatic PO2." (Seymour and Hetz 2011:2175)

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Book
Comparative Biomechanics: Life's Physical World, Second EditionJune 17, 2013
Steven Vogel

Web page
Nature's Raincoats

Journal article
The diving bell and the spider: the physical gill of Argyroneta aquaticaJournal of Experimental BiologyJune 8, 2011
R. S. Seymour, S. K. Hetz

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Organism
European Water SpiderArgyroneta aquaticaSpecies


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