The diving bell spider or water spider, Argyroneta aquatica, is the only species of spider known to live entirely under water.

Argyroneta aquatica is found in northern and central Europe and northern Asia up to latitude 62°N. It is the only spider known to spend its whole life under water. As with other spiders, it breathes air, which it traps in a bubble held by hairs on its abdomen and legs. This gives it a silvery appearance, despite it being velvet-grey.[citation needed] The spider inhabits ponds in Europe and northern Asia, and lives for approximately two years.[citation needed] The appearance of the diving bell gave rise to the genus name Argyroneta, from the Greek "argyros" (ἄργυρος), meaning "silver", and "neta", a neologism (perhaps for *νητής) derived from the verb "neo" (νέω) "spin", intended to mean "spinner of silver".

Argyroneta aquatica paar
Females build underwater "diving bell" webs which they fill with air and use for digesting prey, molting, mating and raising offspring. They live almost entirely within the bells, darting out to catch prey animals that touch the bell or the silk threads that anchor it. However they have to surface occasionally to renew their personal air supplies and those of their webs. Males also build bells, but these are smaller and the males replenish their bells' oxygen supply less often. The males also have a more active hunting style. Although they are better swimmers than females, they prefer to cling to silk threads or underwater vegetation while moving. Prior to mating, the male constructs a diving bell adjacent to the female's, then spins a tunnel from his bell, breaking into to hers to gain entrance. Mating then takes place in the female's bell. The female spider lays between 30 and 70 eggs in her bell.

Males are around 30% larger than females, which is unusual for spiders. This is possibly because their more active hunting style requires greater strength to overcome water resistance and counteract the buoyancy of their mobile air supplies. The size of females may be limited as they put more energy into building and maintaining their larger bells. The spiders prey on aquatic insects and crustaceans. Their bite is quite painful as the fangs can pierce the skin, causing localised inflammation and feverishness. The spiders themselves fall prey to frogs and fish.

The replenishment of air is unnecessary in well-oxygenated water, because the bell permits gas exchange with the surrounding water; 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. 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

Argyroneta aquatica - The amazing water spider

Argyroneta aquatica - The amazing water spider