Several species of 'trapdoor' spiders are found in the western US.   Relatives of the tarantulas, they live in silk-lined burrows with hinged top doors that are highly camouflaged with dirt, pebbles, or other debris.   The spider lurks inside its burrow and when a small animal (usually an insect) comes close, the trapdoor is popped open and the spider lunges out to grab the prey.   Silk 'trip wires' are usually laid out around the trapdoor, which help detect prey.   Female trapdoor spiders, like this one, usually stay more or less permanently in or near their burrows; the somewhat smaller males may travel about to look for females.   Although large and somewhat scary-looking, they are harmless to humans (as are most spiders).   This spider is in the genus Aptostichus (I don't know the full species name).  It was captured by a colleague in the southern Sierra Nevada mountains of California.   I photographed them in Riverside, California; the image at upper left shows the spider on a lab countertop.   A related species, Aliatypus janus, lives in the same general vicinity.