These are sawfly larvae. It depends on how much you are bothered about them eating the leaves whether or not you view them as a pest.
I had Solomons-seal sawfly for the first time this year, I even saw a female on the plant probably thinking about laying eggs! What did I do? I took a photo!
Later on I was privileged to get photos of lots of fat larvae stripping my plants.
It won't be easy to say what they are, the plant might give a clue if you know what it is as many sawfly are plant specific.
