Is there a walnut tree around where you found it? What you found it on can help tell you what it is. The Nature Forum people might know.

Susan will know when she sees the post.

It was just crawling across my driveway. I have an oak in my front yard and my neighbor has a sycamore. No walnuts in the area.

Here are some I took last month. I'm going to piggy-back on this thread. The mass of caterpillars is walnut datana (really bad this year here). I don't know the others, but these were the three most interesting ones that I had never seen before.