Many caterpillars have developed defense mechanisms to ward off predators. Some of those mechanisms include camouflage, mimicking other animals in appearance (e.g. snakes), chemical defense, mimicking poisonous caterpillars in appearance, intense and sudden movement, pretending to bite, regurgitating, and hissing; yet another defense mechanism is possessing a "false head" (Wagner 2005). In some caterpillars, like the Spicebush Swallowtail, the false head is located over the thorax. More commonly, the false head is located at the opposite end of the body.
In the Turbulent Phosphila (Phosphila turbulenta) shown in these photographs, the head appears to be at the end facing my camera in the photograph above. However, the shiny, black, true head of this caterpillar is at the end facing my camera in the photograph below, hidden under the black and white prothoracic shield (the shield over the first segment of the thorax).
Predators often attack the head end of a caterpillar (Purser 2003), so possessing a false head is a way to potentially survive an attack. When the false head is attacked, caterpillars that possess them regurgitate, bite, or emit a poisonous substance (Wagner 2005).
Lindsay and I found this caterpillar at Starved Rock State Park near Utica, Illinois a few weeks ago. It was on a handrail along a trail, with a leaf that had fallen off of a Roundleaf Greenbrier (Smilax rotundifolia) plant. As it turns out, the Turbulent Phosphila feeds on... you guessed it, greenbrier; they often feed in groups, and have been known to defoliate plants (Wagner 2005). You can find this boldly patterned caterpillar in open woods throughout much of the eastern United States (Wagner 2005).
Purser, Bruce. Jungle Bugs: Masters of Camouflage and Mimicry. Buffalo, New York: Firefly Books, 2003.
Wagner, David L. Caterpillars of Eastern North America: A Guide to Identification and Natural History. Princeton and Oxford: Princeton University Press, 2005.