Sawfly larvae

I keep one small willow bush going in my garden just so I can be entertained each year by the amazing antics of these sawfly larvae. They've just about stripped one branch and are eating voraciously, whilst waving in unison if I approach too closely. If the birds don't take quite a few over the next couple of weeks, I may have to intervene to ensure the bush is not a complete skeleton - not that the willow wouldn't be able to withstand the onslaught in the medium term, I think.