Caterpillar to cocoon

This lovely little transparent caterpillar turned up on my thyme bush the other day. Lucky I spotted him: he nearly got picked and put in an omelette! I left him to munch on the herbs, and the next day he had spun himself this little cocoon, right there in the thyme pot on the kitchen windowsill. So far the cocoon is very transparent and I can see right inside. It looks like there are some brown bits starting to develop (not in this photo). I wonder what kind of butterfly he will become?

(A week later)Now the cocoon is really brown inside and looks sort of shrivelled. I hope he is OK in there.