Hi, please can someone help, my lawn is being eaten by a caterpillar. I dont want to destroy it until I know what it is. If I know its not going to destroy everything then I will leave it, but if it is then.......

There are literally hundreds of them and my lawn is not very big so please could someone help quite quickly!

I have attached a picture

Update - I have found out it is a grass eating sawfly larvae, just in case this helps anyone else.