Western tiger swallowtail caterpillars are familiar to many people in the American West. Their large size, bright green coloration, and distinctive eyespots make them easy to identify. But did you know that when they're getting ready to pupate (form the chrysalis stage), they turn a deep reddish-orange color? Video: Anna Blades/USFWS. Description: A deep reddish-orange caterpillar with large eye spots moves across a rock toward the camera. | U.S. Fish and Wildlife Service