Tarantula

Tarantulas
Mexican redknee tarantula Brachypelma smithi
Mexican redknee tarantula
Brachypelma smithi
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Arachnida
Order: Araneae
Suborder: Mygalomorphae
Superfamily: Theraphosoidea
Family: Theraphosidae
Thorell, 1870
Diversity
113 genera, 897 species
Distribution.theraphosidae.1.png
Subfamilies

Acanthopelminae
Aviculariinae
Eumenophorinae
Harpactirinae
Ischnocolinae
Ornithoctoninae
Poecilotheriinae
Selenocosmiinae
Selenogyrinae
Spelopelminae
Stromatopelminae
Theraphosinae
Thrigmopoeinae

Tarantula is the common name for a group of hairy and often very large spiders belonging to the family Theraphosidae, of which approximately 900 species have been identified. Tarantulas hunt prey in both trees and on the ground. All tarantulas can emit silk, whether they be arboreal or terrestrial species. Arboreal species will typically reside in a silken "tube web", and terrestrial species will line their burrows or lairs with web to catch wandering prey. They mainly eat insects and other arthropods, using ambush as their primary method. The biggest tarantulas can kill animals as large as lizards, mice, or birds. Most tarantulas are harmless to humans, and some species are popular in the exotic pet trade while others are eaten as food. These spiders are found in tropical and desert regions around the world.

The name tarantula comes from the town of Taranto in Southern Italy and was originally used for an unrelated species of either European wolf spider (see Lycosa tarantula for more information about this spider the appearance of which resembles that of that tarantula family) or the Mediterranean black widow (the effects of whose bite more closely resemble that described in Taranto). In Africa, Theraphosids are frequently referred to as "baboon spiders". Asian forms are known as "earth tigers" or "bird spiders". Australians refer to their species as "barking spiders", "whistling spiders", or "bird spiders". People in other parts of the world also apply the general name "mygales" to Theraphosid spiders.

There are other species also referred to as tarantulas outside this family; the evolution of the name Tarantula is discussed below. This article primarily concerns the Theraphosids.

Contents

Morphology

Like all arthropods, the tarantula is an invertebrate that relies on an exoskeleton for muscular support. A tarantula’s body consists of two main parts, the prosoma or the cephalothorax (the former is most often used) and the abdomen or opisthosoma. The prosoma and opisthosoma are connected by the pedicle or what is often called the pregenital somite. This waist-like connecting piece is actually part of the prosoma and allows the opisthosoma to move in a wide range of motion relative to the prosoma.

Depending on the species, the body length of tarantulas range from 2.5–10 cm (1–4 inches), with 8–30 cm (3 to 12 inch) leg spans (their size when including their legs). Leg span is determined by measuring from the tip of the back leg to the tip of the front leg on the same side, although some people measure from the tip of the first leg to the tip of the fourth leg on the other side. The largest species of tarantulas can weigh over 9.1 grams (0.3 ounces). One candidate for the title of the largest of all species, the Theraphosa blondi (goliath birdeater) from Venezuela and Brazil, has been reported to have a weight of 3 ounces and a leg span of up to 11 inches (28 cm). The males have the longer length, and the females have broader girth.

Theraphosa apophysis (the pinkfoot goliath) was described 187 years after the Goliath birdeater; therefore its characteristics are not as well attested. However, leg spans of up to 33 cm (13 inches) have been reported for that species. T. blondi is generally thought to be the heaviest tarantula, and T. apophysis to have the greatest leg span. Two other species, Lasiodora parahybana (the Brazilian salmon birdeater) and Lasiodora klugi, get very large and rival the size of the two Theraphosa species.

The majority of North American tarantulas are brown. Many species from other parts of the world have more extensive coloration patterns, ranging from cobalt blue (Haplopelma lividum), black with white stripes (Eupalaestrus campestratus or Aphonopelma seemanni), to metallic blue legs with vibrant orange abdomen (Chromatopelma cyaneopubescens, green bottle blue). Their natural habitats include savanna, grasslands such as the pampas, rainforests, deserts, scrubland, mountains, and cloud forests. They are generally divided into terrestrial types that frequently make burrows and arboreal types that build tented shelters well off the ground.

Appendages

Sub-adult Female Poecilotheria regalis

The eight legs, the two chelicerae with their fangs, and the pedipalps are attached to the prosoma. The chelicerae are two double segment appendages that are located just below the eyes and directly forward of the mouth. The chelicerae contain the venom glands that vent through the fangs. The fangs are hollow extensions of the chelicerae that inject venom into prey or animals that the tarantula bites in defense, and they are also used to masticate. These fangs are articulated so that they can extend downward and outward in preparation to bite or can fold back toward the chelicerae as a pocket knife blade folds back into its handle. The chelicerae of tarantulas completely contain the venom glands and the muscles that surround them and can cause the venom to be forcefully injected into prey.

The pedipalpi are two six–segment appendages connected to the thorax near the mouth and protruding on either side of both chelicerae. In most species of tarantula, the pedipalpi contain sharp jagged plates used to cut and crush food often called the coxae or maxillae. As with other spiders, the terminal portion of the pedipalpi of males function as part of its reproductive system. Male spiders spin a silken platform (sperm web) on the ground onto which they release semen from glands in their opistoma. Then they insert their pedipalps into the semen, absorb the semen into the pedipalps, and later insert the pedipalps (one at a time) into the reproductive organ of the female, which is located in her abdomen. The terminal segments of the pedipalps of male tarantulas are moderately larger in circumference than those of a female tarantula. Male tarantulas have special spinnerets surrounding the genital opening. The sperm web of the tarantula is exuded from these special spinnerets.

A tarantula has 4 pairs of legs but 6 pairs of appendages. Each leg has seven segments which from the prosoma out are: coxa, trochanter, femur, patella, tibia, tarsus and pretarsus, and claw. Two or three retractable claws are at the end of each leg. These claws are used to grip surfaces for climbing. Also on the end of each leg, surrounding the claws, is a group of hairs. These hairs, called the scopula, help the tarantula to grip better when climbing surfaces like glass. The fifth pair are the pedipalps which aid in feeling, gripping prey, and mating for a mature male. The sixth pair of appendages are the chelicerae and their attached fangs. When walking, a tarantula's first and third leg on one side move at the same time as the second and fourth legs on the other side of his body. The muscles in a tarantula's legs cause the legs to bend at the joints, but to extend a leg, the tarantula increases the pressure of blood entering the leg.

Tarantulas, like almost all other spiders, have their primary spinnerets at the end of the opisthosoma. Unlike the typical spiders that on average have six, tarantulas have two or four spinnerets. Spinnerets are flexible tubelike structures from which the spider exudes its silk. The tip of each spinneret is called the spinning field. Each spinning field is covered by as many as one hundred spinning tubes through which silk is exuded. This silk hardens on contact with the air to become a threadlike substance.

Digestive system

The tarantula’s mouth is located under its chelicerae on the lower front part of its prosoma. The mouth is a short straw-shaped opening that can only suck, meaning that anything taken into it must be in liquid form. Prey with large amounts of solid parts, such as mice, must be crushed and ground up or predigested, which is accomplished by spraying the prey with digestive juices that are secreted from openings in the chelicerae.

The tarantula’s digestive organ (stomach) is a tube that runs the length of its body. In the prosoma, this tube is wider and forms the sucking stomach. When the sucking stomach's powerful muscles contract, the stomach is increased in cross-section, creating a strong sucking action that permits the tarantula to suck its liquefied prey up through the mouth and into the intestines. Once the liquefied food enters the intestines, it is broken down into particles small enough to pass through the intestine walls into the hemolymph (blood stream) where it is distributed throughout the body.

Nervous system

A tarantula's central nervous system (brain) is located in the bottom of the inner prosoma. A tarantula maintains awareness of its surroundings primarily by using its other sensory organs, setae. Although it has eyes, a tarantula’s sense of touch is its keenest sense and it often uses vibrations given off by the movements of its prey. A tarantula's setae are very sensitive organs and are used to sense chemical signatures, vibrations, wind direction, and possibly even sound. Tarantulas are also very responsive to the presence of certain chemicals such as pheromones.

Closeup of a tarantula's eye

The eyes are located above the chelicerae on the forward part of the prosoma. They are small and usually set in two rows of four. Most tarantulas are not able to see much more than light, darkness, and motion. Arboreal tarantulas see better than terrestrial tarantulas.

Respiratory system

In all types of tarantulas there are two sets of book lungs (breathing organs). The first pair of book lungs is located in a cavity inside the lower front part of the abdomen near where the abdomen connects to the cephalothorax and the second pair slightly farther back on the abdomen. Air enters the cavity through a tiny slit on each side of and near the front of the abdomen. Each lung consists of 15 or more thin sheets of folded tissue arranged like the pages of a book. These sheets of tissue are supplied by blood vessels. As air enters each lung, oxygen is taken into the blood stream through the blood vessels in the lungs. Needed moisture may also be absorbed from humid air by these organs.

Circulatory system

Thermal image of a cold-blooded tarantula on a warm-blooded human hand

A tarantula’s blood is unique; an oxygen-transporting protein is present (the copper-based hemocyanin) but not enclosed in blood cells as are the erythrocytes of mammals. A tarantula’s blood is not true blood but rather a liquid called haemolymph, or hemolymphy. There are at least four types of hemocytes, or hemolymph cells. The tarantula’s heart is a long slender tube that is located along the top of the opisthosoma. The heart is neurogenic as opposed to myogenic, so nerve cells instead of muscle cells initiate and coordinate the heart. The heart pumps hemolymph to all parts of the body through open passages often referred to as sinuses, and not through a circular system of blood vessels. If the exoskeleton were to be breached, loss of hemolymph could kill the tarantula unless the wound were small enough that the hemolymph could dry and close the wound.

Defense

Besides the normal "hairs" covering the body of tarantulas, some also have a dense covering of irritating hairs called urticating hairs, on the opisthosoma, that they sometimes use as protection against enemies.[1] These hairs are present on New World species but not on specimens from the Old World.

These fine hairs are barbed and designed to irritate. They can be lethal to small animals such as rodents. Some people are extremely sensitive to these hairs, and develop serious itching and rashes at the site. Exposure of the eyes and breathing system to urticating hairs should be strictly avoided. Species with urticating hairs can kick these hairs off: they are flicked into the air at a target using their back pairs of legs. Tarantulas also use these hairs for other purposes such as to mark territory or to line their shelters (the latter such practice may discourage flies from feeding on the spiderlings). Urticating hairs do not grow back, but are replaced with each molt. The intensity, amount, and flotation of the hairs depends on the species of Tarantula. Many owners of Goliath Bird Eating Spiders (Theraphosa Blondi) claim that Theraphosae have the worst urticating hairs.

To predators and other kinds of enemies, these hairs can range from being lethal to simply being a deterrent. With humans, they can cause irritation to eyes, nose, and skin, and more dangerously, the lungs and airways, if inhaled. The symptoms range from species to species, from person to person, from a burning itch to a minor rash. In some cases, tarantula hairs have caused permanent damage to human eyes.[2] Tarantula hair has been used as the main ingredient in the novelty item "itching powder".[3] Some tarantula enthusiasts have had to give up their spiders because of allergic reactions to these hairs (skin rashes, problems with breathing, and swelling of the affected area).

Some setae are used to stridulate which makes a hissing sound. These hairs are usually found on the chelicerae. Stridulation seems to be more common in Old World species.

Venomous Bites

Despite their often threatening appearance and reputation, none of the true tarantulas are known to have a bite which is deadly to humans. In general, the effects of the bites of all kinds of tarantulas are not well known. While the bites of many species are known to be no worse than a wasp sting, accounts of bites by some species are reported to be very painful. Because other proteins are included when a toxin is injected, some individuals may suffer severe symptoms due to an allergic reaction rather than to the venom. For both those reasons, and because any deep puncture wound can become infected, care should be taken not to provoke any tarantula into biting. Tarantulas are known to have highly individualistic responses. Some members of species generally regarded as aggressive can be rather easy to get along with, and sometimes a spider of a species generally regarded as docile can be provoked. Anecdotal reports indicate that it is especially important not to surprise a tarantula.

New World tarantulas (those found in North and South America) are equipped with urticating hairs on their abdomen, and will almost always use these as a first line of defense. These hairs will irritate sensitive areas of the body and especially seem to target curious animals who may sniff these hairs into the mucous membranes of the nose. Some species have more effective urticating hairs than others. The goliath birdeater is one species known for its particularly irritating urticating hairs. Old world tarantulas (from Asia) have no urticating hairs, and are more likely to attack when disturbed. Old world tarantulas often have more potent, medically significant venom.

Before biting, tarantulas may signal their intention to attack by rearing up into a "threat posture", which may involve raising their prosoma and lifting their front legs into the air, spreading and extending their fangs, and (in certain species) making a loud hissing noise by stridulating. Their next step, short of biting, may be to slap down on the intruder with their raised front legs. If that response fails to deter the attacker they may next turn away and flick urticating hairs toward the pursuing predator. Their next response may be to leave the scene entirely, but, especially if there is no line of retreat, their final response may also be to whirl suddenly and bite. Tarantulas can be very deceptive in regard to their speed because they may habitually move very slowly, but are able to deliver an alarmingly rapid bite when sufficiently motivated.

There are dangerous spiders which are not true tarantulas but which are related to them and frequently confused with them. A popular urban legend maintains that deadly varieties of tarantulas exist somewhere in South America. This claim is often made without identifying a particular spider although the "banana tarantula" is sometimes named. A likely candidate for the true identity of this spider is the dangerous Brazilian wandering spider (Phoneutria nigriventer), in the family Ctenidae, as it is sometimes found hiding in clusters of bananas and is one of several spiders called the "banana spider." It is not a tarantula but it is fairly large (4–5 inches long), somewhat hairy, highly venomous to humans, and is regarded as aggressive. Another dangerous type of spider that may be confused with tarantulas are the so-called venomous funnel-web tarantulas. The best known of these is the Sydney funnel-web spider (Atrax robustus), a spider that is aggressive, highly venomous, and (prior to the development of antivenin in the 1980s) was responsible for numerous deaths in Australia. These spiders are members of the same suborder as the true tarantulas, but are not found in the family Theraphosidae.

Medical implications

While no fatalities have been attributed to tarantula bites, sometimes spider bites are regarded as the probable source of infections. Medical advice regarding prophylaxis may be helpful in that regard. In addition, there is considerable anecdotal evidence indicating that the venoms of some Old World species can produce symptoms so severe that medical treatment would be appropriate. Medical intervention is also regarded as appropriate when symptoms such as breathing difficulty or chest pain develop, since these conditions may indicate an anaphylactic reaction. As with bee stings, allergic reactions to protein fractions may be many times more dangerous than the direct toxic effects of the venom. Complete airway blockage can occur within 20 minutes of exposure to some allergens.

Sexual dimorphism

Some tarantula species exhibit pronounced sexual dimorphism. Males tend to be smaller (especially their abdomens, which can appear quite narrow) and may be quite dull when compared to their female counterparts, as in the species Haplopelma lividum. Mature male tarantulas also may have tibial hooks on their front legs, which are used to restrain the female's fangs during copulation.

A juvenile male's sex can be determined by looking at a cast exuvium for exiandrous fusillae or spermathecae. Ventral sexing is less reliable, but, if done correctly, it can be relatively reliable. Males have much shorter lifespans than females because they die relatively soon after maturing. Few live long enough for a post-ultimate molt. It is unlikely that it happens much in natural habitats because they are vulnerable to predation, but it has happened in captivity if rarely. Most males do not live through this molt as they tend to get their emboli, mature male sexual organs on pedipalps, stuck in the molt. Most tarantula fanciers regard females as more desirable as pets due to their much longer lifespan. Wild caught tarantulas are often mature males because they wander out in the open and are more likely to be caught, while females remain hidden in their burrows. A stressed tarantula huddles up in the corner with its legs tucked close to it, does not react, or reacts slowly to touch. A dying tarantula will curl its legs like a clutched hand under it. The movements of tarantula legs are produced hydraulically and their legs retract when death lowers the hydraulic pressure. Tarantulas do not die on their backs unless they fail to survive molting.

Excessive dryness can kill tarantulas, especially tropical tarantulas. Although higher humidity helps with molting, it appears that for many tarantulas humidity does not highly affect molting as much as their actual state of hydration prior to molting. Most notably though, Theraphosa species must have conditions of high humidity to molt successfully.

Life cycle

Ecdysis

The molting process

Like other spiders, tarantulas have to shed their exoskeleton periodically in order to grow, a process called molting. Young tarantulas may do this several times a year as a part of their maturation process, while full grown specimens will only molt once a year or less, or sooner in order to replace lost limbs or lost urticating hairs.[4] A tarantula is obviously going to molt (or "shed", as some call it) when the exoskeleton takes on a darker shade. If a tarantula previously used its urticating hairs, the bald patch will turn from a peach color to deep blue.

Tarantulas may live for years—most species taking 2 to 5 years to reach adulthood, but some species may take up to 10 years to reach full maturity. Upon reaching adulthood, males typically have but a 1 to 1.5 year period left to live and will immediately go in search of a female with which to mate. Male tarantulas rarely molt again once they reach adulthood. The oldest spider, according to Guinness World Records, lived to be 49 years old.

Females will continue to molt after reaching maturity. Female specimens have been known to reach 30 to 40 years of age, and have survived on water alone for up to 2.5 years. Grammostola rosea spiders are renowned for going for long periods without eating.

Reproduction

As with other spiders, the mechanics of intercourse are quite different from those of mammals. Once a male spider reaches maturity and becomes motivated to mate, it will weave a web mat on a flat surface. The spider will then rub its abdomen on the surface of this mat and in so doing release a quantity of semen. It may then insert its pedipalps (short leg-like appendages between the chelicerae and front legs) into the pool of semen. The pedipalps absorb the semen and keep it viable until a mate can be found. When a male spider detects the presence of a female, the two exchange signals to establish that they are of the same species. These signals may also lull the female into a receptive state. If the female is receptive then the male approaches her and inserts his pedipalps into an opening in the lower surface of her abdomen, called the Opithosoma. After the semen has been transferred to the receptive female's body, the male will swiftly leave the scene before the female recovers her appetite. Although females may show some aggression after mating, the male rarely becomes a meal.

Females deposit 50 to 2000 eggs, depending on the species, in a silken egg sac and guard it for 6 to 7 weeks. During this time, the female will stay very close to the eggsac and become more aggressive. The female turns the eggsac often, which is called brooding. This keeps the eggs from deforming due to sitting too long. The young spiderlings remain in the nest for some time after hatching where they live off the remains of their yolk sac before dispersing.

Taxonomy

Main article: List of Theraphosidae species

The family Theraphosidae is divided up into 12 subfamilies, containing over 100 genera and around 900 species between them.

A few genera are presently not well-classified, and/or have classifications which are disputed, including Brachionopus, Cardiopelma, and Proshapalopus

Fossil record

Although there are fossils of mygalomorph spiders going back to the Triassic, only two specimens have been found so far which can be convincingly assigned to Theraphosidae. One is from Dominican Republic amber, the other from Chiapas (Mexican) amber. Both these ambers are quite young, being Miocene in age or about 16 million years old.

Consumption

The Goliath birdeater tarantula (Theraphosa Blondi) is considered a delicacy by the indigenous Piaroa of Venezuela.[5] Another appearance of the tarantula as food was made on Anthony Bourdain's A Cook's Tour.[6] Fried tarantulas are also considered a delicacy in Cambodia.[7]

Etymology

The word tarantula applies to several very different kinds of spider. The spider originally bearing that name is one of the wolf spiders, Lycosa tarantula, found in the region surrounding the city of Taranto (or Tarentum in Latin), a town in Southern Italy. Compared to true tarantulas, wolf spiders are not particularly large or hairy.

The bite of L. tarantula was once believed to cause a fatal condition called tarantism, whose cure was believed to involve wild dancing of a kind that has come to be identified with the tarantella. However, modern research has shown that the bite of L. tarantula is not dangerous to human beings. There appears to have existed a different species of spider in the fields around Taranto responsible for fairly severe bites. The likely candidate (and the only spider found in the area which is dangerous to man) is the malmignatte or Mediterranean black widow. This spider, which belongs in the genus Latrodectus, is a close relative of the black widow and redback spiders, and has a bite which is medically significant. However, the so-called tarantulas were fairly large, frequently visible (as is typical of wolf spiders), and thus drew more attention, taking the blame from its relative. These factors, combined with the belief in the fatality of tarantism, assured the other kind of spiders generally called tarantulas a fearsome reputation.

When theraphosidae were encountered by European explorers in the Americas, they were named "tarantulas". Nevertheless, these spiders belong to the suborder Mygalomorphae, and are not at all closely related to wolf spiders.

The name "tarantula" is also applied to other large-bodied spiders, including the purseweb spiders or atypical tarantulas, the funnel-web tarantulas (Dipluridae and Hexathelidae), and the dwarf tarantulas. These spiders are related to true tarantulas (all being mygalomorphs), but are classified in different families. Huntsman spiders of the family Sparassidae are also informally referred to as "tarantulas" because of their large size. They are not related, belonging to the suborder Araneomorphae.

See also

References

  1. ↑ Cooke, J.A.L., Roth, V.D., Miller, F.H. (1972). The urticating hairs of theraphosid spiders. American Museum novitates 2498. PDF (12 Mb) — Abstract
  2. ↑ Blaikie, Andrew J; John Ellis, Roshini Sanders, Caroline J MacEwen (24 May 1997). "Eye disease associated with handling pet tarantulas: three case reports". BMJ 314: 1524. PMID 9183200. http://www.bmj.com/cgi/content/full/314/7093/1524. Retrieved on 2007-03-06. 
  3. ↑ [1]
  4. ↑ [2]
  5. ↑ [3]
  6. ↑ [4]
  7. ↑ Ray, Nick (2002), Lonely Planet Cambodia, Lonely Planet Publications, ISBN 1-74059-111-9. p. 308.

Further reading

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