What is this Caterpillar?

[monkeywrench]

I just found this in my Guinea pigs cage, does anyone know what it is? I am going to assume it was in the lettuce this morning which has come from Spain because the piggies are inside. It's about 2 cm long and smooth.

Hate these threads when I get here too late to be the first with the right answer.

Mine seems to have had a massive poo overnight and eaten some lettuce and is much more green now so perhaps it was just full of carrot and very see-through!!! The poo is a dark orange colour. I will take more pictures when I get in at lunchtime, I have to go to work now!