A month ago I blogged a piece about a caterpillar with unusual behavior. It was eventually identified as a Catocala, which pupates into the better-known underwing moth. The one I found had coloration to match the bark of an oak tree; for excellent cryptic coloration, look at this one that matches a lichen-encrusted twig.
The unusual behavior was a quite violent thrashing about, flipping upside-down to reveal what I thought were holes in its abdomen, but were just colored patches. The same behavior is mentioned here, and I've seen it described at other links.
I had thought the behavior was just a way to startle a potential predator, or to wriggle under leaf litter. Recently while reading about aposemitism, I encountered the term "Unken Reflex" and decided that must be what the caterpillar was manifesting. The Unken reflex is a defense mechanism of amphibians ("unken" is German plural for "toad"), which involves exposing a brightly-colored body part that is normally hidden. Bright colors are worldwide cross-species signals of poison and toxicity, used by prey to warn predators against unnecessary attacks. The salamanders in the two photos are displaying their orange and yellow bellies to indicate that they are unpalatable. The amphibian thus has the advantage of crypic coloration for general daily camoflage, but bright coloration for use when attacked or threatened.
I've not seen that term used in association with caterpillars, but I think that's what he was doing when I encountered him - thrashing inverted to show his color spots as a way of saying "don't eat me." It worked. I didn't.