This unfortunate caterpillar has been parasitized by a wasp, probably a Braconid species. The female wasps lays her eggs in the larva, and when they hatch, the wasp larvae exit the body (you can see the brown exit holes) and spin their cocoons on the poor caterpillar. These cocoons were empty, the wasps already hatched. The caterpillar, amazingly, was still alive. It is a rather gruesome event which is very common in nature, with many moth caterpillar species affected.