Argyroneta aquatica spins a sheet of silk between submerged plant stems and fills it with air ferried down as bubbles trapped by the hairs on its abdomen and legs. The result is a bell of air held under water by the silk.
Measurements of oxygen gradients across the bell showed it does more than hold what was carried in. As the spider consumes oxygen the partial pressure inside falls below that of the surrounding water, and dissolved oxygen diffuses in through the air-water interface while carbon dioxide diffuses out. A spider in well-oxygenated water can stay down for more than a day, surfacing occasionally to top up the nitrogen that slowly leaks away.
One detail runs against the usual pattern. In this species the male is the larger sex, a reversal generally explained by the demands of swimming and searching for females in three dimensions.