I found several of these small(approx 1 inch) spiky caterpillars on a common buckthorn shrub. I could not figure out what they were, but thanks to the good folk at Bug Guide http://bugguide.net/node/view/15740), I learned that they are 2nd instar cecropia moth caterpillars. I would never, in a million years, have guessed that. I've seen the huge spectacular final instar caterpillars, green with orange and blue "knobs" on the body, so very different from this little one!