2"inches long. Happily munching out in the open on Hemlock Water Dropwort
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Is there a suggestion in the picture that it is actually on Water Figwort, one of the usual foodplants?
My Flickr photos...
and you're right. This part of the bank was a tangle of weeds. There being lots of Hemlock Water Dropwort, Wild Turnip, Marsh Woundwort and Water Figwort. I did try to separate the plants but didn't want to disturb the caterpillars too much in order to take a picture.
Please see my Flickr photo's www.flickr.com/photos/129804972@N07/
Lat/Lng: 53.732, -1.3557
OS grid ref: SE426263