Urticating bristles, i.e. irritating bristles, are one of the primary defense mechanisms used by numerous plants, some New World tarantulas, and various lepidopteran caterpillars. Urtica is Latin for "nettle", 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 hairs that cover the dorsal and posterior surface of a tarantula's or caterpillar's abdomen. Many tarantula species will frequently kick bristles off of their abdomens, directing them toward potential attackers. These bristles can embed themselves in the other animal's skin or eyes, thus inducing physical irritation.
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The most common form of urticating bristles in plants are typified by nettles, which possess sharp-pointed hollow bristles seated on a gland which 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 nettles possess similar defensive bristles, and the common names often reflect this (e.g., "spurge nettle").
There are species with urticating bristles in at least eleven Lepidopteran families: the Arctiidae (tiger moths), Anthelidae (lappet moths), Bombycidae, Eupterotidae (bag shelter moths), Lasiocampidae, Limacodidae, Lymantriidae (tussock moths), Megalopygidae, Noctuidae, Notodontidae (processionary caterpillars), Nymphalidae, and Saturniidae (Matheson 1950, Riley and Johannsen 1938, Roth and Eisner 1962, Wirtz 1984). Some adults may also have urticating scales.
Urticating hairs do not appear at birth but form with each consecutive molt, outwardly presenting themselves around areas of more dark hairs on the upper back part of the abdomen of juveniles, widening from molt to molt, but in elder ages merging with the main tone of abdominal coloration. Urticating bristles do not cover the entire opisthosoma and are distinct from abdominal hairs.
There are six different types of urticating bristle known in tarantulas (M. Overton, 2002). They are all different in shape and size.
Each type of urticating bristle is believed to target different enemies. Defined targets for some bristle 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 bristles off of the abdomen, similarly to species from the subfamily Theraphosinae.[1] Tarantulas from the genera Avicularia, Pachistopelma and Iridopelma possess Type II bristles. (Toni Hoover, 1997)
Type III urticating bristles are most efficient for defense against vertebrates and invertebrates.
Types III and IV are the most irritating to mammalian predators.
Not all urticating hair types are exhibited by each species of tarantula.
Type II urticating bristles can be found in the genera Avicularia, Iridopelma and Pachistopelma (subfamily Aviculariinae).
Type I and III urticating bristles are representative on Lasiodora and Acanthoscurria, excluding Grammostola (exhibits types III and IV).
Type III urticating bristle is typically found on the species of Theraphosa spp., Nhandu spp., Megaphoboema spp., Sericopelma spp., Eupalaestrus spp., Proshapalopus spp., Brachypelma spp., Cyrtopholis spp., Iracema spp. and other genera of subfamily Theraphosinae (Rick West, 2002).
Type V urticating bristle 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 bristle. These are easily thrown by the spider into the air (Marshal and Uetz, 1990).
Type VI urticating bristle is found in the genus Hemirrhagus (F. Perez-Miles, 1998).
According to Vellard (1936) and Buecherl (1951), genera with the most urticating bristles 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 bristles in the direction of the enemy. The cloud of small bristles will get into the mucous membrane of small mammals and cause edema, which can be fatal. The latest studies suggest these bristles cause both mechanical and chemical harm to the skin and membranes.
Reaction and the degree of irritation to a defensive urticating bristle barrage can vary tremendously, based on the species in question. Some, such as those of the Chilean rose tarantula (Grammastola rosea) and the pink toe 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 bristles can result in painful rashes, and have been likened to sharp shards of fiberglass.
After kicking urticating bristles, the Tarantula will have a bald spot on its abdominal region.
Urticating bristles 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 bristles 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.
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. A chemical influence upon the skin and mucous membranes explains the different reactions of people to urticating hairs (Rick West, 2002).
It seems likely the hairs cause an accumulative reaction in people.
A solution of 2–2.5% hydrocortisone cream applied to the affected area may help relieve the symptoms.
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.[2] 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.[2]
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