At first it was difficult to ID this caterpillar. It was feeding on Meadow Rue (Thalictrum) and I knew that there were a couple of species that fed only on that plant. But this one didn't match the photo in the field guide, which was of the mature caterpillar. Thank goodness for Bug Guide, where I found a match for this, and discovered it is an earlier instar and hence looks rather different from the 'finished product' so to speak.