A species endemic to North America, the asp caterpillar—Megalopyge opercularis—looks quite harmless. However, it should be treated with extreme caution as it has very powerful venom and is one of the most toxic caterpillars in North America. At the end of the fluffy hairs lie sharp, poisonous spines which pierce the skin and can cause extreme allergic reactions such as inflammation, blisters, headaches, nausea and breathing difficulties. These symptoms can last for days.

Many cases of injury have been reported in the United States over the years—the injuries occur mainly in children as they try to pick up these creatures. The asp caterpillar grows to two centimeters in length an the adult is the crinkled flannel moth, which also has a bizarre, yeti-like appearance.