The caterpillar in the attached photographs was on the boardwalk at Corkscrew Swamp Sanctuary in Naples, Florida, on August 26. There was no plant nearby that might have been a larval host. It was about 4 inches long and close to 1 inch in diameter. Great Batesian mimicry for a Water Moccasin, although considerably smaller. The spot on the top of the tail end (upper left in photo) would pulsate when the caterpillar was agitated — first by some ants, and then when it was moved to the side of the boardwalk so a passerby wouldn’t accidentally step on it. One person suggested an instar of the caterpillar for an Abbot’s Sphinx Moth. Can you identify?
This is a Gaudy Sphinx Caterpillar, Eumorpha labruscae, and it is quite serpentine.