Violet Sawfly (Ametastegia pallipes, #840)
/Sawfly identification is pretty hard as there are lots of species and not very many good materials on the internet to help in reliable identification, but I believe this white-legged, mostly black sawfly is Violet Sawfly / Ametastegia pallipes. This is a species which as the name suggests is found on members of the Viola family, and apparently is not rare.