This is Uraba lugens, a caterpillar that wears a bunch of its old heads on top of its current head like the world's most ridiculously macabre hat. The part of this photo where the otherwise horizontal caterpillar goes vertical? That's a pyramid of exoskeleton head capsules, stacked in descending order from smallest to largest.
The venerable Bug Girl has some better shots of this phenomenon at her blog, along with lots of great information explaining how the heck Uraba lugens ends up making this questionable fashion statement. She also offers this helpful advice:
If you do happen to see one of these, you should not touch it! Apparently these caterpillars are covered with highly itchy and irritating spines–which seems to make their chapeau of old heads a bit redundant.