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 hairs do not cover the entire opisthosoma and are distinct from abdominal hairs.
There are six different types of urticating hair known in tarantulas (M. Overton, 2002). They are all different in shape and size.
Each type of urticating hair is believed to target different enemies. Defined targets for some hair types are unknown.
Type II is not kicked off by the tarantula. Direct contact is necessary to penetrate an attacker. Tarantulas from the genera Avicularia, Pachistopelma and Iridopelma possess Type II hairs. (Toni Hoover, 1997)
Type III urticating hairs are most efficient for defence against vertebrates and invertebrates.
Types III and IV are the most irritant to mammalian predators.
Not all urticating hair types are exhibited by each species of tarantula.
Type III urticating hair 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 hair is typical of the species of genus Ephebopus. As mentioned earlier 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 latest studies suggest urticating hairs from tarantulas present not just a mechanical but a chemical influence on the skin and mucous membranes.
The cloud of small hairs will get into the mucous membrane of small mammals and cause edema, which can be fatal.
Reaction and the degree of irritation to a defensive urticating hair barrage can vary tremendously, based on the species in question.
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 a defensive maneuver, an urticating hair lobbing tarantula will have a bald spot on its abdominal region.
There are species with urticating hairs 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.
In humans a defensive cloud of urticating hairs can cause allergic skin reactions which can manifest itself as inflammation, rash and 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. In this case it is necessary to immediately wash the eye thoroughly with copious amounts of cold water and see an ophthalmologist.
Urticating hair is not just thrown at an enemy as a first line defence. It 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 hair 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.