Caterpillars are the wormlike, larval forms of butterflies and moths of the insect order Lepidoptera. Next to flies, lepidopterans are the most abundant arthropods with more than 165,000 species worldwide, and with most species posing no human threats. However, caterpillar species from approximately 12 families of moths or butterflies worldwide can inflict serious human injuries ranging from urticarial dermatitis and atopic asthma to osteochondritis, consumption coagulopathy, renal failure, and intracerebral hemorrhage. Unlike bees and wasps, envenoming or stinging caterpillars do not possess stingers or modified ovipositors attached to venom glands, but instead bear highly specialized external nettling or urticating hairs and breakaway spines or setae to defend against attacks by predators and enemies. Since the 1970s, there have been increasing reports of mass dermatolgic, pulmonary, and systemic reactions following caterpillar encounters throughout the world.