The closely-related species Udea rubigalis
and U. profundalis are very similar (if they are indeed good species!)
and perhaps can be accurately identified only by genitalic dissection. U.
profundalis is supposed to be of more western distribution than rubigalis,
so I have listed this photograph under profundalis. I have often found
the caterpillar here on Stinging Nettle, though the individual shown was reared
on thistle. The caterpillar was colourless and translucent.