Many caterpillars are very hairy in appearance. Caterpillar hairs are setae, which, although similar in appearance, are not like the hair of animals. Caterpillars probably evolved their hairy feature because it offers some protection against predators. A few caterpillars have embellished their defensive behavior by evolving sharpened and poisoned hairs. The special venomous setae are called urticating hairs, a name from the plant family that includes nettles.
Caterpillars are usually conspicuously spinose so they can be more readily identified and avoided.
The spines are usually like hypodermic needles. Some caterpillars may have many spines or a smaller number, often very distinctive spine from tubercles. The adults of these stinging caterpillars are moths, each different in size, color, and shape. Moths cannot sting nor do they have the urticating hairs. The severity of the nettling effect depends on the susceptibility of the individual, the tenderness of the skin, and the place of contact.