forgot about this one for a couple of days. Unfortunately the leaves it was eating dried out. Clearly, though, by this time food is not going to help - the cocoon of a parasitoid has appeared underneath the caterpillar. I was interested also to see its size had shrunk back from 20mm to 12mm.
I found a very similar image on the web here
, which identifies the parasitoid in question as probably "a wasp, Apanteles melanoscelus
, which attacks young larvae."