These caaterpillars get quite a bit bigger than this. I'm thinking this may be 3rd or 4th instar.

Bug Guide has this to say about the caterpillars: "Strange! Eversible tentacles extend from the dorsal surface of A2 and A3. ...when the caterpillar is alarmed these tentacles can be extended to 2x their resting length. These same structures probably occur in other species in the genus; but otherwise I believe they may be unique."