The Spice of Life
Frequently Missourians send me photos of various caterpillars, and they usually ask me to identify them. One of the more interesting ones is the caterpillar of the spicebush swallowtail butterfly (Papilio troilus). It is a common species in the Ozarks but less so in other Missouri regions. The larva, or caterpillar, feeds on the leaves of the spicebush shrub and on the related sassafras tree. They will roll a leaf around them for protection during the day and mostly feed at night.
The unusual caterpillar attracts interest because its markings look like eyes and a mouth, which allows it to deter some predators by mimicking a snake. A bird that peers into the rolled leaf sees two large "eyes" looking back at it, resembling the eyes of a snake, a natural enemy of small birds. The caterpillar grows and periodically molts (sheds its skin). Stages between molts are called "instars." During the first three instars, the caterpillar impersonates a bird dropping to avoid predators. Then, as it grows and continues to molt, later instars are green snake mimics and then, just prior to forming a chrysalis to pupate, it turns orange or yellow. If formed in the fall, the chrysalis will overwinter and the adult butterfly will emerge the following spring. The butterflies can be found in Missouri from April into October.
It is fascinating to me to see the ways that different species mimic something else to avoid predation. The spicebush swallowtail takes it a step further by mimicing bird poop and snakes at different times within a single life cycle. I guess it's understandable that one would tire of impersonating bird poop after a while, no matter how effective that might be.
All photos courtesy of bugwood.org. Credits from top to bottom are as follows: Sturgis McKeever, Lacy L. Hyche, Gordon Maupin, Sturgis McKeever, Charles T. Bryson