Nibbler it is
Thanks to your participation and that of the customers of Blue Moon, the caterpillar is named Nibbler.
Since the first post, it has grown first to this size:
and now to this size:
Notice the change from brown to green. Also the eyes on its back are just coloring - the actual head is tucked out of view. I think the notion is to make itself look frightening to scare off predators.
When you disturb Nibbler, it puffs up its thorax making the eyes stand out more. If you keep prodding it, scent glands emerge from behind its head and tumesce. They look like yellowish horns and give off a vaguely musky-sweet smell. I've raised spicebush swallowtails before and never noticed that - even though the scent glands are on other north american species. Other species use them much more freely I guess. I've seen them on anise, tiger, black, and giant swallowtail larvae.
The rate of growth is astonishing. You really have to wonder if Nibbler is in constant growth pain or is constantly feeling hungry.
At any rate, I moved Nibbler to a sealed container since it looks like it will pupate in 3-5 days. When that happens, most larvae get a wanderlust and head off somewhere secluded since they're an open target during pupation.