Large birch sawfly larva

Thanks to regular correspondent Bob for this great picture of a Large birch sawfly larva, that he found in Devon after felling a Silver birch tree. This isn't a caterpillar, though obviously it could easily be mistaken for one. It turns into a Large birch sawfly (picture of the adult on that page), a type of sawfly - which is a bit like a wasp but with a fatter waist.