The name tarantula comes from the town of Taranto in Italy
and was originally used for an unrelated species of European
spider. (See Wolf spider for more information about this
spider). In North America the term tarantula commonly refers to
species of theraphosids found in the Americas. 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.
A tarantula is an invertebrate and so has an exoskeleton. A tarantula’s body consists of two main parts, the prosoma or the cephalothorax (the former is most often used because there is no analogous "head") and the abdomen or opisthosoma. The cephalothorax 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 that of the cephalothorax.
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). Legspan 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 90 grams (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 13 inches (33 cm). The males have the long length, and the females have lots of girth.
Theraphosa apophysis (the pinkfoot goliath) was described 187 years after the Goliath bird eater; therefore its characteristics are not as well attested. However, legspans 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 the largest legspan. Two other species, Lasiodora parahybana and Lasiodora klugi, (the Brazilian salmon birdeater) gets very large and rivals the size of both Theraphosa blondi and Theraphosa apophysis, and some have even made claims to same size and even bigger sizes than the two Theraphosa species.
The majority of tarantulas are brown or black, drab, and are adapted to escaping notice, however some species 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 Ccloud forests. They are generally divided into terrestrial types (that frequently make burrows) and arboreal types that build tented shelters well off the ground. The more easily spotted tarantulas tend to be more aggressive and some of these aggressive species have venom that can cause severe pain for extended periods.
The eight legs, the two chelicerae with their hollow fangs, and the pedipalps are attached to the prosoma. The chelicerae are two single 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 (chew). 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 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 larger in circumference than those of a female tarantula.
A tarantula has four pairs of legs but eight pairs of total 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 of legs are the pedipalps which aid in feeling and mating for a mature male. The sixth pair of appendages are the fangs. It is believed that the fangs were at one time legs, but now they are used as weapons, masticating, matings, occasional for mobility. They are mostly used to subdue prey.
The seventh and eighth pairs of appendages are the four spinnerettes, which also are hypothesized by some to have been leglike appendages. 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 spinnerets at the end of the opisthosoma. Unlike 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 produced. This silk hardens on contact with the air to become a thread like substance.
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 which 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 excreted 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 liquids up through the mouth and into the intestines. Once the liquified food enters the intestines, it is broken down into particles small enough to pass through the intestine walls into the haemolymph (blood stream) where it is distributed throughout the body.
A tarantula's central nervous system (brain) is located in the bottom of the inner prosoma. The central nervous system controls all of the body's activities. A tarantula maintains awareness of its surroundings by using its sensory organs, setae. Although it has eyes, and the eyes of arboreal tarantulas appear to be relatively good, a tarantula’s sense of touch is its keenest sense and it often uses vibrations given off by its prey's movements to hunt. Some of a tarantula's hairs are very sensitive organs and are used to sense chemical signatures, vibration, wind direction and possibly even sound. Tarantulas are also very responsive to the presence of certain chemicals. A tarantulas main senses would perhaps be like being able to taste-smell the world.
The eyes, which, unlike those of insects, are simple lenses, 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.
In all types of tarantulas there are two book lungs (breathing organs). The book lungs are located in a cavity inside the lower front part of the abdomen near where the abdomen connects to the cephalothorax. 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 many 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 a pale liquid; an oxygen-transporting protein is present (the copper-based hemocyanin) but not enclosed in blood cells like 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.
Besides the normal hairs covering the body of tarantulas, some also have a dense covering of irritating hairs (about 10,000 per mm²), called urticating hairs, on the opisthosoma, that they sometimes use as a protection against enemies. These hairs are only present on some New World species of the subfamilies of Ischnocolinae, Aviculariinae, Grammostolinae and Theraphosinae, and are absent on specimens of the Old World. They help in phylogenetic studies of Theraphosinae.
These fine hairs are barbed, and designed to urticate, but do not contain venom. Some species can 'kick off' these hairs: they are launched into the air at a target. Tarantulas also use these hairs for other means; using them to mark territory or to line the web or nest (the latter such practice may discourage flies from feeding on the spiderlings).
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. Tarantula hair has been used as the main ingredient in the novelty item "itching powder". 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 or buzzing sound. These hairs are usually found on the chelicerae. Stridulation seems to be more common in Old World species. Many of the New World species, especially those in the Ischnocolinae, Aviculariinae, Grammostolinae, and Theraphosinae subfamilies, have urticating hairs (barbed hairs often kicked at or pushed into predators to discourage their approach) on certain parts of their bodies, but especially the top of the opisthosoma. These urticating hairs can be flicked onto predator animals such as mice to cause itchiness, burning, swelling, redness, and other irritation. These hairs, like the quills of porcupines, are serious defensive weapons. Even large animals like humans can be blinded if urticating hairs are delivered to their eyes and then left untreated. Lung damage is another possible danger. Urticating hairs are usually the main ingredient of itching powder. Some people react especially badly to urticating hairs, notably people with dermatitis, and there seems to be evidence that there is a chemical property to urticating hairs in addition to physical properties.
Tarantulas are nocturnal predators, killing their prey by
injecting venom through their fangs. The hungry tarantula
typically waits partially hidden at the entrance to its retreat
to ambush passing prey. It has sensitive hairs that enable it to
detect the size and location of potential victims from the
vibrations caused by their movements. Some species also use
their silk fiber to detect motion (when prey triggers a line).
Like many other spiders, it cannot see much more than light,
darkness, and movement (see spiders for more about their
eyesight), and uses its sense of touch to perceive the world
around it. That being said, they are anything but sloppy or
imprecise about the way they capture their prey. They generally
seem to choose prey on the basis of how dangerous it is
perceived to be, the general size of the potential prey animal,
etc. Some tarantulas succeed in occasionally capturing small
birds, small lizards, small snakes, small mammals such as mice,
and even small fish, but their ordinary prey consists of insects
such as crickets (for ground dwellers) and moths (for arboreal
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 every year or so, or sooner in order to replace lost limbs or lost urticating hairs.
Tarantulas may live for many 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. It is rare that upon reaching adulthood the male tarantula will molt again.
The habit of male spiders wandering in search of mates makes them especially visible. In late summer and early autumn (September and October in the northern hemisphere), the males will leave their hiding places and walk about, hoping to encounter the hiding place of a female with which to mate. They are willing to cross roads and trails in this quest, and that is when they are most likely to be observed.
When the mature male encounters the burrow of a female, he will draw the female out and signal his intentions to mate by vibrating his body and tapping his front legs. If the female is receptive to mating, she will also vibrate and tap her legs. After mating, the male must get away quickly, or it is possible that he will be eaten. A female tarantula who is unreceptive to mating may also eat the male if he attempts to mate. This result, however, is less common among tarantulas than other spiders. Certain species of tarantulas have been known to mate multiple times over the course of several weeks.
Since females will continue to molt after reaching maturity, they are able to regenerate lost limbs. 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.
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. After the semen has been transferred to the receptive female's body, the male will generally quickly leave the scene before the female recovers her appetite.
Females deposit 50 to 2000 eggs, depending on the species, in a silken egg sac and guard it for 6 to 7 weeks. The young spiderlings remain in the nest for some time after hatching where they live off the remains of their yolk sac before dispersing.
Tarantulas usually live in solitude and, being cannibalistic, will attack and eat others of their own kind. There are however, exceptions such as the pinktoe tarantula (Avicularia avicularia), which can be kept communally, as members of this species are more tolerant of each other. If the vivarium is big enough, has enough hiding spots, and the specimens are about the same size and well fed, there should be little or no cannibalism. However, keeping tarantulas communally is not recommended and should not be attempted except by experienced keepers.
This Tarantula Page is Copyright The Animal Web Guide © 2004 - 2009 Chuck Ayoub