We are not 100% sure, but believe this is a Western Swallowtail Caterpillar. The Two-tailed Swallowtail (Papilio multicaudata) looks very similar in appearance. We’ve read that “The best clue [for the Two-tailed] is the black edge of the pupil of the large eye spot, which is a thin line. In Western it should be a thick border.” (David J. Ferguson, Contributing Editor for BugGuide). Both are among our largest and arguably most spectacular butterflies, sporting bold black and yellow wings.
Size: Wingspan (adult) 90–110 mm
Photo by: Tracy Younger on 9/18/21 near Great Falls, MT