Urticating hairs

Urticating hairs or urticating bristles, i.e. irritating hairs, 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 hairs 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 hairs that cover the dorsal and posterior surface of a tarantula's or caterpillar's abdomen. Many tarantula species eject hairs from their abdomens, directing them toward potential attackers. These hairs can embed themselves in the other animal's skin or eyes, causing physical irritation, usually to great discomfort.

The most common form of urticating hairs in plants are typified by nettles, which possess sharp-pointed hollow hairs seated on a gland which secretes an acrid fluid. The points of these hairs 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 hairs, and the common names often reflect this (e.g., "bull nettle").

Several lepidopteran families include species whose larvae bear urticating hairs. Families prominent in this respect include the following:

(Matheson 1950, Riley and Johannsen 1938, Roth and Eisner 1962, Wirtz 1984).

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 poison-producing cells, functioning like a hypodermic needle. Generally most hairs 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 can inject venom that is life-threatening to humans.

The stings are not part of a deliberate attack, but are mainly the result of brushing against the spines. However, many species whose larvae are armed with such hairs have behavioral patterns adapted to present the urticating hairs as a defensive threat and to inflict them on any perceived attackers. For example, many larvae in the family Lasiocampidae bear dense bands of short stinging hairs across their thoracic segments. The hairs are normally retracted into a shallow fold in the skin, but if the caterpillar is disturbed, it everts the folds and displays the hairs, which are usually of a contrasting color.

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