A later instar of this thing http://www.furaffinity.net/view/1753763/

As you can see, he's graduated from his nasty brown ways and become a brilliant green. They will often hide in a leaf like so, and scare away potential predators with those mean looking eyespots.

Although we don't have too many bright green snakes around here, the migratory birds will have seen plenty of snakes during their south-for-the-winter travels. It's pretty odd that these caterpillars mimic animals that aren't even really found here.