I spotted what looked to be a young Orgyia antiqua caterpillar on the underside of an oak leaf - it seemed to be clinging to a cocoon. But on closer examination I saw that the caterpillar was dead.
I thnk it has been eaten from the inside by an ichneumon wasp larva. Which then proceeded to construct the cocoon and decorate it with the skin of its victim - stetched over the top of it a bit like a bearskin trophy rug.
The caterpillar skin is covered in protective hairs that would probably do a fine job of deterring anything unwelcome that might come snuffling around the cocoon while the wasp is pupating.
So... as you might have come to expect from ichneumon wasps, things aren't quite as simple as the picture I painted yesterday. It turns out that the wasps alter the caterpillar's behaviour so that when they leave the caterpillar and begin to make their cocoon, the caterpiller stands over the cocoon guarding the wasps while they build. The wasps weave the caterpillar's prolegs into the cocoon so that is is unable to leave, but can still move. Then, while the wasps pupate, the caterpillar guards the pupa, violently thrashing its head at any potential dangers that approach. When the wasps hatch, the caterpillar then dies.
<_>