These caterpillars form groups as a defense mechanism. By clumping like this they look much larger than as individuals. Additionally, some processionary caterpillars are also poisonous. Randol, our night hike guide had no fear of picking up scorpions or handling poisonous frogs but was afraid of these caterpillars. As we descended the mountain that day, we first saw this group of caterpillars and then later saw another group of caterpillars, but this second group had been flattened by a Jeep tire. So much for their defense mechanism.