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.
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.
Several lepidopteran families include species whose larvae bear urticating hairs. Families prominent in this respect include the following:
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 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 bristles 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 bristles across their thoracic segments. The bristles are normally retracted into a shallow fold in the skin, but if the caterpillar is disturbed, it everts the folds and displays the bristles, which are usually of a contrasting color.
If seriously stimulated or gripped, lasiocampid larvae are likely to lash about, forcing the stinging bristles into any vulnerable organ within reach. Many other species of larvae lack any such localized concentrations of bristles and are armed more generally with urticating hairs; even so, they too will lash about frantically if disturbed, making them difficult to handle without suitable equipment. Toxins from the broken bristles may spill out, causing dermatitis on the surface of the skin. For brown-tail moths (Euproctis chrysorrhoea) native to Europe and invasive in other parts of the world, bristles are shed or broken off during molts and can be wind-borne, so that direct contact with live or dead larvae is not required to trigger a rash.
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 until the majority of the bristles have been stripped or damaged, but at least some species of cuckoos apparently collect the bristles 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 (setae) are found in about 90% of the species of tarantula (spiders of the family Theraphosidae) found in the New World. They are not found in tarantulas from other parts of the world.
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 bristles 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 bristles, such as their tendency to cover only a portion instead of the entirety of the opisthosoma.
There are seven different types of urticating hair known in tarantulas, varying in size and shape, particularly the distribution of barbs.
- 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
- Type VII
Each type of urticating hair 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—Caribena 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 II urticating hairs can be found in the genera Avicularia, Iridopelma and Pachistopelma (subfamily Aviculariinae). 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.. 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. Type VI urticating hair is found in the genus Hemirrhagus.
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 bristles will get into the mucous membrane of small mammals and cause edema, which can be fatal. The bristles 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 bristles 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 are also used as an indication of territory. They can be found on and around the burrow entrance and in webbing for protection (for example, some subfamily Theraphosinae species include these bristles in cocoon silk).
Urticating hairs protect tarantula egg sacs (Avicularia spp. and Theraphosa blondi, respectively). This is thought to discourage fly larvae from consuming their eggs and young.
In humans, urticating hairs can cause allergic skin reactions which can manifest as inflammation, rash and itching. The reactions can last from several hours to weeks. Ophthalmia nodosa, an irritation reaction, can result when the barbed bristles lodge in the cornea. Handlers are advised to wear eye protection.
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