This is the caterpillar of the viceroy butterfly, which is a monarch butterfly mimic (as you can see from the inset). This was a whopper of a caterpillar! Normally, the viceroy caterpillars I've seen in the past have been quite small and brown and white. At first we didn't know what this was, but after some searching discovered the ID. Found along the K&P trail in Lanark County.