Spiders only bite, although some tarantulas have urticating hairs. Certain caterpillars also have urticating hairs. Several lepidopteran families include species whose larvae bear urticating hairs. It lacks urticating hairs and is arboreal. The urticating hairs of the caterpillar larvae cause harmful reactions in humans and other mammals. After kicking urticating hairs, the Tarantula will have a bald spot on its abdominal region. Most urticating hairs range in effect from mild irritation to dermatitis. Type III urticating hairs are most efficient for defense against vertebrates and invertebrates. The term urticating hairs is a misnomer, as technically only mammals possess true hairs. They are typically docile, though they can be skittish and prone to releasing urticating hairs. Old World tarantulas have no urticating bristles and are more likely to attack when disturbed. Like most New World tarantulas, all species of "Aphonopelma" have urticating hairs. The goliath birdeater is known for its particularly irritating urticating bristles. The urticating hair of stinging nettles contain sodium formate as well as formic acid. Not all urticating hair types are exhibited by each species of tarantula.