Thanks to you, I know this bug!
I was showing a friend your What’s That Bug site today. When I happened upon your Polyphemus photos on the moths pages, I remembered these photos that I took with my daughter several years ago. One summer afternoon, we found something wriggling frantically in the grass. It was good sized, about the size of a cocktail wiener, though a little shorter and a little fatter. My grass was long at the time, and the bug seemed to be having trouble, so we let it crawl onto a stick and transferred it onto the trunk of a nearby tree. It settled there and proceeded to pump up its wings, ending up as a gorgeous Polyphemus. Unfortunately, I didn’t time stamp the initial photos, but I think it took about 4-5 hours. When it was wriggling in the grass, the wings were little more than nubs on the body, but they ended up about 5 inches tip to tip. Thought you might enjoy the progression.
Thank you for the marvelous example of metamorphosis in action.