The water spider lives underwater but still breathes air. It carries a bubble of air on its superhydrophobic abdomen, which it uses as an air supply. The spider only surfaces to collect air for its bubble nests. The trapped air exchanges enough oxygen with the water for it to survive underwater indefinitely. Unlike the water boatman, the spider has very long hairlike structures on its abdomen. These provide its air bubble with some springiness, allowing it to ferry air from the surface to its nest. Exchange takes place most efficiently when the excess air has been lost and the air left is at a lower pressure than the water. The water spider carries air down to a web it has built underwater that has a mesh small enough that it cannot escape.
Plastron properties of a super-hydrophobic surface,
Appl. Phys. Lett. 89 (10) (2006) Art 104106.
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