chard being eaten from within!

Hi all,

my swiss chard's leaves were wilting and turning white in patches. It took me ages to find the culrit but i've finally discovered it's being done by nasty little caterpilar/grub things inside the leaves.
Can anyone tell me what these things are called, who lays them and what i can do about it, apart from breaking apart each infected leaf and squishing them individually.

thanks
rebecca:-)

thanks mumofstig, i'm off on a squishing spree. (amazing how much less squeemish i'm getting about these things since i started this gardening lark:-)
I've read up a bit about them and i'll be looking for their little white eggs to squish too.

rebecca