Caterpillar Identication

I found this critter in Calais on an old RR track munching on a Balsam poplar. Can’t find it in any of my bug books. Any suggestions? Cheers, FG

Robin’s Journal – From the Observation Stand

Sunday, August 14, 2011 Molunkus (Map 44) The power in the cabin comes from a generator in a nearby shed. It’s loud and disruptive; I don’t like to run it except to run the water and lights at night. The percolator glub-glubbed on the propane stove while we sat on the porch steps in silence. […]