Urticating hair
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Urticating hairs are one of the primary defense mechanisms used by tarantulas. This term refers to the hairs that cover the tarantula's body. Many tarantula species will frequently kick hairs off their abdomens, directing them toward potential attackers. These hairs can embed themselves in the other animal's skin or eyes, thus inducing physical irritation.
The degree of irritation can vary tremendously, based on the species in question. Some, such as those of the Chilean rose hair (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.
Urticating hairs do not appear at birth but form with each consecutive molt, outwardly presenting itself round area of more dark hairs on upper back part of abdomen of juvenil, widening from molt to molt, but in elder ages - merging with main tone of abdomen coloration.
There are six different types of urticating hair known (M. Overton, 2002). As seen in the illustration they are all different in shape and size. It is interesting to note that urticating hairs are absolutely absent in Old World (Asian & African) tarantulas. Only tarantulas from genera Avicularia, Pachistopelma и Iridopelma have urticating hair of type II. Type II urticating hair is not kicked off by the tarantula. Instead direct contact is necessary much like with thorns or cactus, Toni Hoover, 1997). Type V urticating hair is typical of the species of genera Ephebopus. As mentioned earlier is 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 genera Hemirrhagus (Hemirrhagus (F. Perez-Miles, 1998). Types I, II, III and IV urticating hairs can be found in the subfamilies Avicularinae and Theraphosinae. According to Vellard (1936) и Buecherl (1951), generas with the most urticating hairs are Lasiodora, Grammostola and Acanthoscurria. Excluded Grammostola (has types III and IV), representitives of Lasiodora and Acanthoscurria have type I и III urticating hairs. 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 III urticating hair are most efficient for defence against vertebrates and invertebrates. These hairs present dangerous implications to humans as well. The latest studies suggest urticating hairs from tarantulas present not just a mechanical but a chemical influence on the skin and mucous membranes. This would explain the different reactions of people to urticating hairs (Rick West, 2002). It seems likely the hairs cause an accumulative reaction in people.
Tarantulas without urticating hairs have other means of defence. Their first line defence is an aggressive stance with the front legs in the air, body weight on the hind legs. The cheliceras (fangs) bared. This stance is followed by an attack. Tarantulas Stromatopelma griseipes, Citharischius crawshayi, Pterinochilus murinus and Ornithoctoninae spp. are examples of Old World tarantulas that use this defence.
Such behaviour is not typical of New World tarantulas. Old World tarantulas are more aggressive and their venom is more toxic than New World tarantulas.
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 oedema, difficulty in breathing which can be fatal. In humans this 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 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).