Look what I found ...

...when I went to remove a randomly-growing branch from our Kilmarnock Willow! We frequently find that the leaves have been chomped, but so far have only noticed very small white caterpillars; this one is altogether more exotic.

If anyone knows what it will turn into, I'd like to hear - though as it fell off the branch onto the concrete underneath shortly after I took the photo, I don't know what became of it.