I'm pleased to learn that it's possibly a Pyractomena borealis larva! This specimen was found nestled (head down) in the cease of the painted, north-facing, exterior concrete wall of the men's outhouse at the Rydell NWR, Minnesota (which is very tree-trunk-like!). ;-)

Interesting! a concrete bathroom wall and north facing too? it is confused!! The head down posture is typical right before they glue their tail permanently to the substrate (wedged in a bark furrow upside down) and pupate soon after. Usually this species chooses the south facing aspect on a larger diameter tree that gets direct sun with furrowed bark (tulip poplars and hickories and oaks are often favorite types). Have you released it? it definitely was wanting to pupate.

Thanks for the information about the behavior of this species, LFaust. Happily, I left the specimen right where I found it. I took two photos, and then trudged on my way to other parts of the Rydell NWR look for spring moths and migrant warblers. I visit this Minnesota refuge regularly during the summer to photograph insects, and so I'll make an effort to find an adult Pyractomena firefly in the coming weeks. I don't imagine that I'll find another larvae--they are easy to overlook! ;-)