Gaping Maws

What's the origin of the cliche "gaping maw?"

Anyway, I was doing one of those random internet searches which is the work equivalent of sitting at home with a beer watching infomercials and came across this site which features pictures, of all things, the wide open mouths of various sharks and whales. Be sure to check out the picture of the orca, they're the Joker of the sea world.