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Being small is an advantage if you live in small spaces, and many cave critters are. However, some go the other way. Recently researchers in Venezuela reported seeing Scolopendra centipedes nearly a foot long devouring whole roosting bats. Titiotus spiders of Kaweah Cave, blackish and bigger than silver dollars, are more than twice the size of their surface relatives. They do not spin webs but simply run down prey and grab them with their spiny legs. These spiders are found mainly near entrances and still have eyes and pigments, so they are classified as troglophiles—troglobites in training, retaining enough surface characteristics that they might also live under rocks or in soil burrows.

Scientists believe that virtually all terrestrial troglobites evolved from such animals, pre-adapted as they are to cool, moist, confined conditions. It is thought that at some point they moved farther down and stayed, either because they liked it or because they were confined there by climate swings on the surface.

But how long ago particular creatures went underground is rarely clear, for the immediate surface ancestors of most seem to have gone extinct. Scientists think aquatic organisms in the Edwards aquifer are descended from marine creatures stranded there some 60 million years ago when shallow seas receded. Remipedes, Earth’s most primitive living crustaceans, dwell in saltwater coastal caves across the globe. They may have started out over 100 million years ago, when the supercontinent of Pangaea was breaking up. Today related remipedes are scattered from the Caribbean to Australia, possibly brought there by eons of continental drift.

Palmer Cave, the oldest dated cavern in Sequoia-Kings, goes back some 4.7 million years, but the local troglobites could be far older. The Sierra have been uplifting and eroding for tens of millions of years, and mountains long gone may once have held caves; when they wore out, the occupants could just have moved downstairs.

On the other end of the evolutionary timescale are endemic diplura and harvestmen in near-freezing Panorama Cave, in the alpine zone at 10,600 feet (3,230 meters). This area was glaciated only 10,000 years ago, and it is hard to believe anything survived under a mile of ice and meltwater. Like Darwin’s finches, these creatures must have arrived and evolved not over eons, but in human time.

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