Urticating hairs or urticating bristles are one of the primary defense mechanisms used by numerous plants, almost all New World tarantulas, and various lepidopteran caterpillars. Urtica is Latin for "nettle" (stinging nettles are in the genus Urtica), and bristles that urticate are characteristic of this type of plant, and many other plants in several families. This term also refers to certain types of barbed bristles that cover the dorsal and posterior surface of a tarantula's or caterpillar's abdomen. Many tarantula species eject bristles from their abdomens, directing them toward potential attackers. These bristles can embed themselves in the other animal's skin or eyes, causing physical irritation, usually to great discomfort. The term urticating hairs is a misnomer, as technically only mammals possess true hairs. In plants The most common form of urticating hairs in plants are typified by nettles, which possess sharp-pointed hollow bristles seated on a gland that secretes an acrid fluid. The points of these bristles usually break off in the wound, and the acrid fluid is pressed into it. Various plants unrelated to true nettles (Urtica species) possess similar defensive bristles, and the common names often reflect this (e.g. "bull nettle"). Many cacti of the sub-family Opuntioideae feature fine, loosely attached short spines called glochids. When the plant is disturbed many of these spines fall off and penetrate the skin, causing irritation. Many glochidia are barbed, complicating their removal and enhancing their persistence in the skin. Exposure to glochidia is an occupational hazard to fruit pickers and other outside workers in areas where Opuntioideae thrive, as the spines can persist in clothing and gloves and can become airborne under the right conditions. In Lepidoptera Several lepidopteran families include species whose larvae bear urticating hairs. Families prominent in this respect include the following: Arctiidae Anthelidae Bombycidae Erebidae Eupterotidae Lasiocampidae Limacodidae Lymantriidae Megalopygidae Noctuidae Notodontidae Nymphalidae Saturniidae Thaumetopoeidae Adults of some species also have urticating scales, and some species shed some of their urticating hairs as defense for their pupae and eggs. The urticating setae or spines can cause irritation when they break off and lodge in skin. In some species these structures are hollow and connected to venom-producing cells, functioning like a hypodermic needle. Generally most bristles are only irritating, but some are dangerous to the eyes and respiratory tract, and some can cause severe skin necrosis and shedding. Certain species of Lonomia in the family Saturniidae actually