Like all arthropods, the tarantula is an invertebrate that relies on an exoskeleton for muscular support. Like other Arachnida a tarantula’s body comprises two main parts, the prosoma (or cephalothorax) and the opisthosoma (or abdomen). The prosoma and opisthosoma are connected by the pedicel, or 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.
Tarantulas sizes range from as small as a fingernail to as large as a dinner plate when the legs are fully extended. Depending on the species, the body length of tarantulas ranges from 2.5 to 10 centimetres (1 to 4 in), with leg spans of 8–30-centimetre (3–12 in). Leg span is determined by measuring from the tip of the back leg to the tip of the front leg on the opposite side. Some of the largest species of tarantula may weigh over 85 grams (3 oz); the largest of all, the goliath birdeater (Theraphosa blondi) from Venezuela and Brazil, has been reported to attain a weight of 170 grams (6.0 oz) and a leg-span of up to 30 centimetres (12 in), males being longer and females greater in girth. The fang size of this tarantula reaches a maximum of 1.5 inches.
Theraphosa apophysis (the pinkfoot goliath) was described 187 years after the goliath birdeater; therefore its characteristics are not as well attested.Theraphosa 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, rival the size of the two goliath spiders.
Most species of North American tarantulas are brown. Elsewhere species have been found that variously display cobalt blue (Cyriopagopus lividus), black with white stripes (Aphonopelma seemanni), yellow leg markings (Eupalaestrus campestratus), metallic blue legs with vibrant orange abdomen and green prosoma (Chromatopelma cyaneopubescens). Their natural habitats include savanna, grasslands such as the pampas, rainforests, deserts, scrubland, mountains, and cloud forests. They are generally classed among the terrestrial types. They are burrowers that live in the ground.
Tarantulas are becoming increasingly popular as pets and some species are readily available in captivity.
The spider originally bearing the name "tarantula" was Lycosa tarantula, a species of wolf spider native to Mediterranean Europe. The name derived from that of the southern Italian town of Taranto The term "tarantula" subsequently was applied to almost any large, unfamiliar species of ground-dwelling spider, in particular to the Mygalomorphae and especially to the new-world Theraphosidae. Compared to tarantulas, wolf spiders are not particularly large or hairy, so among English speakers in particular, the usage eventually shifted in favour of the Theraphosidae, even though they are barely related to the wolf spiders, being in a different infraorder.
When theraphosids were encountered in the Americas, they were named "tarantulas", causing usage of the term to shift to the tropical spiders. Nevertheless, these spiders belong to the suborder Mygalomorphae, and are not 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-webs (Dipluridae and Hexathelidae), and the "dwarf tarantulas". These spiders are related to tarantulas (all being mygalomorphs), but are classified in different families. Huntsman spiders of the family Sparassidae have also been termed "tarantulas" because of their large size. In fact, they are not related, belonging to the suborder Araneomorphae.
Tarantulas of various species occur throughout the United States, in Central America, and throughout South America. Other species occur variously throughout Africa, much of Asia and all of Australia. In Europe, some species occur in Spain, Portugal, Turkey, Italy, and Cyprus.
Some genera of tarantulas hunt prey primarily in trees; others hunt on or near the ground. All tarantulas can produce silk – while arboreal species will typically reside in a silken "tube tent", terrestrial species will line their burrows with silk to stabilize the burrow wall and facilitate climbing up and down. Tarantulas mainly eat large insects and other arthropods such as centipedes, millipedes, and other spiders, using ambush as their primary method of prey capture. Armed with their massive, powerful chelicerae tipped with long chitinous fangs, tarantulas are well adapted to killing other large arthropods. The biggest tarantulas sometimes kill and consume small vertebrates such as lizards, mice, bats, birds, and small snakes.
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 a tarantula 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. Silk for the sperm web of the tarantula is exuded from these special spinnerets.
A tarantula has four pairs of legs and two additional 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 in the case of 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 haemolymph entering the leg.
Tarantulas, like almost all other spiders, have their primary spinnerets at the end of the opisthosoma. Unlike most spider species in the suborder Araneomorphae, which includes the majority of extant spider species, and most of which have six, tarantula species 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.
In 2006 in the Journal of Experimental Biology a paper described observations suggesting that some tarantulas have silk-producing spigots on their feet. The authors assert that these structures enabled the spiders to cling to smooth surfaces and thus avoid harmful falls. In 2011, Dr. Claire Rind and her colleagues from Newcastle University conducted experiments inferring the likelihood that all tarantulas are able to produce silk from their tarsi (feet). Describing her experiments in a BBC Nature report (16 May 2011), Dr. Rind includes an electron microscope image purportedly revealing microscopic silk producing structures on the spiders' feet, and noting that the three species involved in the research were very distantly related, concludes: "So it's likely that all tarantulas produce silk threads from their feet."
Silk production from organs other than the spinnerets has been documented in other spiders such as from the chelicerae of Scytodidae family spiders, making the extraordinary claim plausible. However, the existence of silk-producing organs on the feet of tarantulas is currently considered controversial. Although the 2006 discovery is supported by Dr. Rind's 2011 study, two 2012 studies refute the claim, one of them proposing that the structures described as spigots are actually chemoreceptors.
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 coating 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. After feeding, the leftovers are formed into a small ball by the tarantula and thrown away. In a terrarium, they often put them into the same corner. As these balls are perfect hosts for molds and parasites, they must be removed regularly.
A tarantula's central nervous system (brain) is located in the bottom of the inner prosoma. A tarantula perceives its surroundings primarily via sensory organs called setae (hairs or spines). Although a tarantula has eyes, touch is its keenest sense, and in hunting it primarily depends on 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.
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 generally have better vision compared with terrestrial tarantulas.
In all types of tarantula 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.
A tarantula’s blood is unique (not only by appearance); an oxygen-transporting protein is present (the copper-based hemocyanin) but not enclosed in blood cells such as the erythrocytes of mammals. A tarantula’s blood is not true blood but rather a liquid called haemolymph, or hemolymph. 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 is breached, loss of hemolymph will kill the tarantula unless the wound is small enough that the hemolymph can dry and close the wound.
Regardless of their fearsome reputation, tarantulas themselves are an object of predation. The most specialized of these predators are large members of the wasp family Pompilidae such as the wasp Hemipepsis ustulata. In the Americas, these insects are termed "tarantula hawks", being parasitoids of tarantulas. The largest tarantula hawks, such as those in the genus Pepsis, will track, attack and kill large tarantulas. They use olfaction to find the lair of a tarantula. The wasp must deliver a sting to the underside of the spider's cephalothorax, exploiting the thin membrane between the basal leg segments. This paralyzes the spider, and the wasp then drags it back into its burrow before depositing an egg on the prey's abdomen. The wasp then seals the spider in its burrow and flies off to search for more hosts. The wasp larva hatches and feeds on the spider's non-essential parts and, as it approaches pupation, it consumes the remainder.
Humans can also be considered predators of tarantulas. In addition to more mundane cuisine, tarantulas are considered a delicacy in certain cultures (e.g. Venezuela and Cambodia). They are usually roasted over an open fire to remove the hairs (described further below) and then eaten.
Some other arthropods, such as giant centipedes are also known to prey on tarantulas.
Tarantulas have evolved specialized hairs to defend themselves against predators. Besides the normal "hairs" covering the body, some tarantulas also have a dense covering of irritating hairs called urticating hairs, on the opisthosoma, that they sometimes use as protection against enemies. These hairs are present on New World species but not on specimens from the Old World.
Urticating hairs are usually kicked off the abdomen by the tarantula, but it is noteworthy that some may simply rub the abdomen against the target, like the Avicularia genera. These fine hairs are barbed and serve 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 respiratory 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 moult. The intensity, amount, and flotation of the hairs depends on the species of tarantula.
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.
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.
Though all tarantulas are venomous and some bites cause serious discomfort that might persist for several days, so far there is no record of a bite causing a human fatality. In general, the effects of the bites of all kinds of tarantula 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 and to produce intense spasms that may recur over a period of several days; the venom of the African tarantula Pelinobius muticus also causes strong hallucinations. For Poecilotheria species, researchers have described more than 20 bites with the delayed onset of severe and diffuse muscle cramps, lasting for several days, that in most cases resolved completely with the use of benzodiazepines and magnesium. In all cases, it is advisable to seek medical aid. 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. Such allergic effects can be life-threatening.
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 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, the tarantulas of the Americas may next turn away and flick urticating bristles toward the pursuing predator. The 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. Some tarantulas are well known to give "dry bites," i.e., they may defensively bite some animal that intrudes on their space and threatens them, but they will not pump venom into the wound.
New-world tarantulas (those found in North and South America) are equipped with urticating hairs (technically bristles) on their abdomen, and will almost always throw these barbed bristles as a first line of defense. These bristles will irritate sensitive areas of the body and especially seem to target curious animals who may sniff these bristles into the mucous membranes of the nose. Some species have more effective urticating bristles than others. The Goliath Birdeater is one species known for its particularly irritating urticating bristles. Urticating bristles can penetrate the cornea so eye protection should be worn when handling such tarantulas.
Old-world tarantulas (from Europe, Africa, Asia, and Australia) have no urticating bristles and are more likely to attack when disturbed. Old-world tarantulas often have more potent, medically significant venom.
There are dangerous spider species which are related to tarantulas and frequently confused with them. A popular urban legend maintains that deadly varieties of tarantula 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, of 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 technically a tarantula but it is fairly large (4–5 inch legspan), somewhat hairy, and is highly venomous to humans. Another dangerous type of spider that has been confused with tarantulas is the Australasian funnel-web spider. 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 antivenom in the 1980s) was responsible for numerous deaths in Australia. These spiders are members of the same suborder as tarantulas. Some Australians use the slang term 'triantelope' (a corruption of the incorrect term 'tarantula', which is also used) for large, hairy and harmless members of the Huntsman spider family which are often found on interior household walls and in automobiles.
Some tarantula species exhibit pronounced sexual dimorphism. Males tend to be smaller (especially their abdomens, which can appear quite narrow) and may be dull in color 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. Males typically have longer legs than the females.
A juvenile male's sex can be determined by looking at a cast exuvia for exiandrous fusillae or spermathecae. Females possess spermathecae except for the species Sickius longibulbi and Encyocratella olivacea. Ventral sexing is difficult, 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 moult. 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 moult as they tend to get their emboli, mature male sexual organs on pedipalps, stuck in the moult. 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.
Like other spiders, tarantulas have to shed their exoskeleton periodically in order to grow, a process called molting. A young tarantula may do this several times a year as a part of the 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. It is clear that molting will soon occur 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 take two to five years to reach adulthood, but some species may take up to ten 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.
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 years. Grammostola rosea spiders are known for only eating once or twice a week and for living up to 20 years.
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 opisthosoma. 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 8 weeks. During this time, the female will stay very close to the egg sac and become more aggressive. Within most species, the female turns the egg sac 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.
Linnaeus placed all spiders in a single genus, Aranea. In 1802, Charles Athanase Walckenaer separated mygalomorph spiders into a separate genus, Mygale, leaving all other spiders in Aranea. However, Mygale had already been used in 1800 by Georges Cuvier for a genus of mammals (in Greek, mygale means "shrew"). Accordingly, in 1869, Tamerlan Thorell used the family name "Theraphosoidae" (modern Theraphosidae) for the mygalomorph spiders known to him, rather than "Mygalidae" (as used, for example, by John Blackwall). Thorell later split the family into a number of genera, including Theraphosa.
As of February 2016, the World Spider Catalog accepted the following genera:
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.