There is net diffusion of oxygen into the bell and net diffusion of carbon dioxide out. This process is driven by differences in partial pressure. The production of carbon dioxide and use of oxygen by the spider maintains the concentration gradient, required for diffusion. However, there is net diffusion of nitrogen out of the bell, resulting in a gradually shrinking air bubble which must be regularly replenished by the spider. Larger spiders are able to produce larger bubbles which have a consequently higher oxygen conductance, but all spiders of this species are able to enlarge their bells in response to increased oxygen demands in low aquatic P(O2) environments. These spiders voluntarily tolerate internal conditions of low oxygen, enlarging their bells with air when the P(O2) drops below 1 kPa; this replenishment process may not need to occur for several days, in some cases. This system has been referred to as "the water spider's aqua-lung of air bubbles", though an aqua-lung lacks gas exchange with the surroundings; this system is more properly regarded as an inorganic form of gill. Bite Their bite is often described as being very painful to humans and as causing localised inflammation, vomiting, and slight feverishness that disappears after 5-10 days. However there is little solid evidence, with most information being based on old and unverified reports due to recent confirmed reports being very rare, leading some sources to refer to its bite as reputedly painful. References External links Water spider pictures on Arkive.org Diving bell spiders use bubble webs 'like gills' Cybaeidae Spiders of Asia Spiders of Europe Palearctic spiders