Western tent caterpillars (Malacosoma californicum) basking on creosote bush (Larrea tridentata) in Nevada desert. Some can be seen flicking their heads back and forth as a deterrent against parasitic flies. These caterpillars are the larvae of the western tent caterpillar moth and are common in the western U.S., northern Mexico, and southern Canada. They measure 3 cm in length and are responsible for defoliating many species of broadleaf tree and shrubs.