I recently shared some photos of likely forth instar azalea moth caterpillars that we found in early August. Steve and I went back to Hawn State Park in early September hoping to find some late/final instars and had some luck! To my great surprise, after returning home from one of these outings on Labor Day weekend, I found a final instar caterpillar on my backyard nannyberry (Viburnum lentago).
Final instar azalea sphinx caterpillar
A Thousand Acres of Silphiums 
Lovely composition and good find on the final instar. Great work, Bill.