Caterpillars

Took a stroll down to see the cattle earlier on and found a bunch of nettles that had quite a large number of pretty little caterpillars thereon. Anyone know what they will turn into? Thinking they are late developers as they won't have much of the summer left if they don't get on with changing soon... or do they overwinter and pop up as pretty little flying things next year... better investigate if no one can short circuit it and tell me, please...