Urticating hair

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Young Grammostola rosea showing patch of urticating bristles, reflecting a camera flash

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" (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.

Urticating bristles in plants[edit]

Urticating hairs of a stinging nettle

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 true nettles (Urtica species) possess similar defensive bristles, and the common names often reflect this (e.g., "bull nettle").

Urticaceous bristles in Lepidoptera[edit]

The larva of Eutricha capensis in the family Lasiocampidae is practically covered with urticaceous bristles, but the worst of them are the stiff, shortish ones in the orange and dark-maroon bands across the thorax.
Larva of Lonomia obliqua, the most toxicologically significant species of the genus; severe cases of its sting are life-threatening and require treatment with antivenin

Several lepidopteran families include species whose larvae bear urticaceous bristles. 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 bristles as defence 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. Most such 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.[1]

The stings are not part of a deliberate attack, but mainly the result of brushing against the spines. Many species whose larvae are armed with such hairs however, have behavioural 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 normally are retracted into a shallow fold in the skin, but if the caterpillar is disturbed, it everts the folds and displays the hairs, that usually are of a contrasting colour. If more seriously stimulated for example if gripped, it is likely to lash about, forcing the stinging hairs into any vulnerable organ within reach. Many other species of larvae that lack any such localised concentrations of bristles, but simply are armed more generally with urticaceous hairs will nonetheless lash about frantically if disturbed, making it difficult to handle them without suitable equipment. Toxins from the broken hairs may spill out, causing dermatitis on the surface of the skin. [2] In spite of such defences, 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 bristles 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.[3][4]

Urticating bristles in tarantulas[edit]


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.

  • 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 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.[5] 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.

Types particular to species[edit]

An adult female of Brachypelma smithi, showing a bald patch after kicking bristles off of her abdomen. After molting, the bristles will grow again.

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 bristles 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 bristle 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 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.

Defensive behavior utilizing urticating bristles[edit]

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[which?] 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 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 bristles, the Tarantula will have a bald spot on its abdominal region.

Urticating bristles as territorial markings[edit]

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.

Human reactions to urticating hair contact[edit]

Allergic reaction

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)[citation needed].

It seems likely the hairs cause an accumulative reaction in people.[citation needed]

A solution of 2 to 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.[6] 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.[6]


  1. ^ Pinto, Antônio F.M.; Berger, Markus; Reck Jr., José; Terra, Renata M.S.; Guimarães, Jorge A. Lonomia obliqua venom: In vivo effects and molecular aspects associated with the hemorrhagic syndrome Toxicon Volume 56, Issue 7, 15 December 2010, Pages 1103–1112
  2. ^ https://extension.umd.edu/sites/default/files/_docs/programs/ipmnet/StingingCaterpillarsInMaryland-UMD.pdf
  3. ^ Ian Wyllie (1981). The Cuckoo. Universe Books. ISBN 978-0-87663-371-7. 
  4. ^ McCullough, Deborah G., Raffa Kenneth A., Chris Williamson R. Natural Enemies of Gypsy Moth: The Good Guys. Michigan State University Extension Bulletin E-2700, April 1999
  5. ^ Bertani, R.; Boston, T.; Evenou, Y.; Guadanucci, J.P.L. (2003). "Release of urticating hairs by Avicularia versicolor (Walckenaer, 1837) (Araneae, Theraphosidae)". Bulletin of the British Arachnological Society 12 (9): 395–398. 
  6. ^ a b Norris, J.H.; Carim, Z.I.; Morrell, A.J. (2010). "Spiderman's eye" (PDF). The Lancet 375 (9708): 92. doi:10.1016/s0140-6736(09)61672-x. Retrieved 10 April 2013. 


  • Matheson, R. (1950). Medical Entomology (2nd ed.). Ithaca, New York: Comstock Publications. 
  • Riley, W.A.; Johannsen, O.A. (1938). Medical Entomology: a Survey of Insects and Allied Forms Which Affect the Health of Man and Animals. New York: McGraw-Hill. 
  • Roth, L.M.; Eisner, T. (1 January 1962). "Chemical Defenses of Arthropods" (PDF). Annual Review of Entomology 7 (1): 107–136. doi:10.1146/annurev.en.07.010162.000543. Retrieved 10 April 2013. 
  • Wirtz, R.A. (1 January 1984). "Allergic and Toxic Reactions to Non-Stinging Arthropods". Annual Review of Entomology 29 (1): 47–69. doi:10.1146/annurev.en.29.010184.000403. 

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