Caterpillar

Yesterday evening I spotted this caterpillar on my Silver Birch. It wasn't moving and I wondered if that thing on its back was some other creature that was burrowing in or out. I didn't hold out much hope of finding out what it was - try searching for caterpillars - but there it was and confirmed by other sites. It's the caterpillar of the grey Dagger Moth, a boring looking insect that flies at night but pollinates flowers. As there's only one caterpillar I will be leaving it alone.