Millipedes protect themselves by one of two means. They can coil themselves up, like a slimy sort of cinnamon roll. This keeps their tender undersides shielded by their rigid exteriors. Another way they ward off predators is by secreting a toxic liquid, or hydrogen cyanide gas, from their pores. Such a substance is only a mild irritant to humans, but can burn the exoskeleton of ants, or the eyes and skin of larger animals. Some monkeys have even been caught rubbing the substance on themselves as an effective bug spray.