A western tiger swallowtail caterpillar, on quaking Aspen in the fall. The sun shining through the leaf almost seemed to make this caterpillar glow. In this stage they mimic a snake with an organ resembling a snake's tongue, and eye spots, to hopefully warn off predators. These guys will eat aspen, cottonwood, willows, and the leaves of many other species. It was just in the first stages of building a silk mat to rest, with fine silk strands attaching to the leaf all around itself. This will cause the leaf to curl, although not completely. Eventually, the caterpillar (larvae) will head down into the leaf litter to overwinter and pupate.