a group of approx 20 of these larvae was found feeding
in a small area of a very large stand of Staghorn Sumac
at Mill Pond Conservation Area. Behaviour was typical of
sawfly larvae (coordinated defensive writhing when disturbed).
The larger larvae were quite red, but a few smaller ones
were somewhat green. I believe this is the larva of
Arge coccinea (Fabricius) -- see next photo in this gallery.