Unidentified caterpillar.
If you've made it this far, you're ready for a little game. Check out the first pic below and test your critter-finding skills. Click on the photo for a larger version—see anything unusual?
If you looked carefully, you might have noticed this fellow (or gal). A great example of cryptic coloration (camouflage). A fine example, also, of how an eye-spot can present a scary-looking, don't-eat-me message to potential predators (that's not the caterpillar's actual eye). As you can see, the caterpillar has begun building its cocoon.
For what it's worth, my preferred field guides for this genre are Caterpillars in the Field and Garden (Allen, et al.) and Butterflies Through Binoculars—The East (Glassberg).