In the ponds of northern Europe lives a tiny brown spider with a bubble on its back. The 10-millimeter-long Argyroneta aquatica is the only spider in the world that spends its entire life underwater. But just like land spiders, it needs oxygen to breathe. So every so often, it leaves its underwater web home to visit the surface and brings back a bubble of air that sticks to its hairy abdomen. It deposits the bubble into a little silk air tank spun for the purpose. This "diving bell," researchers have now found, is not just a repository. It's actually a gill that sucks oxygen from the water, allowing the spider to stay under for up to 24 hours.