Urticating hairs or urticating bristles, i.e. irritating hairs, are one of the primary defense mechanisms used by numerous plants, some 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 urticaceous 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 urticaceous 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 urticaceous hairs as a defensive threat and to inflict them on any perceived attackers. For example, many larvae of 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.
If seriously stimulated or gripped, Lasiocampidae larvae are likely to lash about, forcing the stinging hairs into any vulnerable organ within reach. Many other species of larvae lack any such localized concentrations of hairs and are armed more generally with urticaceous hairs; even so, they too will lash about frantically if disturbed, making them difficult to handle without suitable equipment. Toxins from the broken hairs may spill out, causing dermatitis on the surface of the skin. 
In spite of such defenses, some species of birds feed avidly on hairy caterpillars. Usually they grab them in their beaks and scrub them on the ground till the majority of the hairs have been stripped or damaged, but at least some species of cuckoos apparently collect the hairs in the digestive tract until they form pellets that can be regurgitated. Examples of avian predators other than cuckoos that feed on hairy caterpillars include at least tens of species from several continents.
Urticating hairs do not appear at birth but form with each consecutive molt, widening from molt to molt and outwardly presenting themselves around areas of more dark hairs on the upper back part of the abdomen of juveniles. In elder ages their coloration shifts to match the main tone of abdomen. Despite this shift, urticating hairs nonetheless retain unique characteristics that render them visually distinct from abdominal hairs, such as their tendency to cover only a portion instead of the entirety of the opisthosoma.
There are six different types of urticating hair known in tarantulas (M. Overton, 2002). They are all different in shape and size.
- Type I (0.2–0.6 mm)
- Type II (0.5–1.5 mm)
- Type III (0.3–1.2 mm)
- Type IV (0.06–0.2 mm)
- Type V
- Type VI
Each type of urticating hair is believed to target different enemies. Defined targets for some hair types are unknown.
Type II is usually not kicked off by the tarantula, rather delivered by direct contact. However, there is at least one aviculariine species - Avicularia versicolor - which can kick type II urticating hairs off of the abdomen, similarly to species from the subfamily Theraphosinae. Tarantulas from the genera Avicularia, Pachistopelma and Iridopelma possess Type II hairs. (Toni Hoover, 1997)
Type III urticating hairs are most efficient for defense against vertebrates and invertebrates.
Types III and IV are the most irritating to mammalian predators.
Types particular to species
Not all urticating hair types are exhibited by each species of tarantula.
Type I and III urticating hairs are representative on a wide diversity of large bodied genera in the subfamily Theraphosinae Lasiodora and Acanthoscurria Nhandu spp., Megaphobema spp., Sericopelma spp., Eupalaestrus spp., Proshapalopus spp., Brachypelma spp., Cyrtopholis spp., and others, although some only have Type I in mature males. Unusually, Type III urticating hair is found alone on the species of Theraphosa spp., but these otherwise are similar to many species that also have Type I hairs.
Type III alone is found in many 'dwarf' new world genera "Hapalopus spp." etc.
Type IV is found in several South American genera Grammostola', "Euathlus" etc (exhibits types III and IV).
Type V urticating hair is typical of the species of genus Ephebopus. They are located on the pedipalps. They are much shorter and lighter in contrast with other types of urticating hair. These are easily thrown by the spider into the air (Marshal and Uetz, 1990).
Type VI urticating hair is found in the genus Hemirrhagus (F. Perez-Miles, 1998).
According to Vellard (1936) and Buecherl (1951), genera with the most urticating hairs are Lasiodora, Grammostola and Acanthoscurria.
New World tarantulas will at the moment of danger, turn toward the attacker and briskly rub their hind legs against the opisthosoma throwing the urticating hairs in the direction of the enemy. The cloud of small hairs will get into the mucous membrane of small mammals and cause edema, which can be fatal. The latest studies[which?] suggest these hairs cause both mechanical and chemical harm to the skin and membranes.
Reaction and the degree of irritation to a defensive urticating hair barrage can vary tremendously, based on the species in question. Some, such as those of the Chilean rose tarantula (Grammastola rosea) and the Pinktoe tarantula (Avicularia avicularia), are fairly mild and innocuous to humans. Others, such as those of the Brazilian giant white knee tarantula (Acanthoscurria geniculata), are moderately irritating. Still others, such as the Goliath Birdeater (Theraphosa blondi), are far more severe. These hairs can result in painful rashes, and have been likened to sharp shards of fiberglass.
After kicking urticating hairs, the Tarantula will have a bald spot on its abdominal region.
As territorial markings
Urticating hairs are not just thrown at an enemy as a first line defense, but is also used as an indication of territory. It can be found on and around the burrow entrance and in webbing for protection (for example, some of Theraphosinae subfamily species include these hairs in cocoon silk).
Urticating hairs can also be found protecting tarantula egg sacs (Avicularia spp. and Theraphosa blondi respectively). This is thought to discourage fly larvae from consuming their eggs and young.
Human reactions to urticating hair contact
In humans a defensive cloud of urticating hairs can cause allergic skin reactions which can manifest as inflammation, rash and/or itching. The reactions can last for several hours or days. 
It seems likely the hairs cause an accumulative reaction in people.
A more serious consequence is urticating hair in the eyes. Ophthalmia nodosa, an irritation reaction, can result when the barbed hairs lodge in the cornea. In this case it is necessary to immediately wash the eye thoroughly with copious amounts of cold water and see an ophthalmologist. Handlers are advised to wear eye protection.
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