Tarantula


Tarantulas are a group of large and often hairy spiders of the family Theraphosidae. More than a thousand species have been identified, within almost 200 genera. The term "tarantula" is usually used to describe members of the family Theraphosidae, although many other members of the same infraorder are commonly referred to as "tarantulas" or "false tarantulas". Some of the more common species have become popular in the exotic pet trade. Many New World species kept as pets have setae known as urticating hairs that can cause irritation to the skin, and in extreme cases, cause damage to the eyes.

Overview

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 and the opisthosoma. The prosoma and opisthosoma are connected by the pedicel, or pregenital somite. This waist-like connecting piece is actually part of the prosoma and gives the opisthosoma a wide range of motion relative to the prosoma.
Depending on the species, the body length of tarantulas ranges from about with leg spans of. 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 ; the largest of all, the goliath birdeater from Venezuela and Brazil, has been reported to attain a weight of and a leg-span up to, males being longer and females greater in girth. The fang size of this tarantula reaches a maximum of.
Theraphosa apophysis was described 187 years after the goliath birdeater, so its characteristics are not as well attested. T. blondi is generally thought to be the heaviest tarantula, and T. apophysis has the greatest leg span. Two other species, Lasiodora parahybana 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, black with white stripes, yellow leg markings, metallic blue legs with vibrant orange abdomen and green prosoma. Their natural habitats include savanna, grassland such as in the pampas, rainforest, desert, scrubland, mountains, and cloud forest. 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.

Identification

Tarantulas can be confused with other members of the order Mygalomorphae, such as trapdoor spiders, funnel-web spiders and purseweb spiders. They can also be confused with some members of the order Araneomorphae such as the Lycosidae family. There are multiple ways to identify a tarantula. First the hairs: in the Americas most tarantulas have urticating hairs, though some, such as the Hemirrhagus genus, lack these. The hairs are usually more noticeable than with most other spiders. Another is the size, as tarantulas tend to be bigger, but this is again not a failproof way. They also do not use their webs for hunting, instead using them as building material or tripwire.
One of the most decisive ways to tell is by looking at their fangs. Tarantula fangs face downwards, as opposed to those of true spiders, which face each other, allowing them to make pincerlike motions. They also have two book lungs, as opposed to true spiders, which only have one. Their lifespan is also longer than most spiders.

Etymology

The spider originally bearing the name tarantula was Lycosa tarantula, a species of wolf spider native to Mediterranean Europe. The name is derived from the southern Italian town of Taranto. The term tarantula was subsequently applied to almost any large, unfamiliar species of ground-dwelling spider, in particular to the Mygalomorphae and especially the New World Theraphosidae. Compared to tarantulas, wolf spiders are not particularly large or hairy, and so among English speakers in particular, usage eventually shifted in favour of the Theraphosidae, even though they are not closely related to wolf spiders at all, being in a different infraorder.
The name tarantula is also incorrectly applied to other large-bodied spiders, including the purseweb spiders or atypical tarantulas, the funnel-webs, and the dwarf tarantulas. These spiders are related to tarantulas but fall into different families from them. Huntsman spiders of the family Sparassidae have also been termed tarantulas because of their large size, when, in fact, they are not related. Instead, huntsman spiders belong to the infraorder Araneomorphae.

The element ''pelma'' in genus names

Many theraphosid genera have names, either accepted or synonymous, containing the element pelma. This can be traced back to Carl Ludwig Koch in 1850, who in describing his new genus Eurypelma wrote, "Die Sammetbürste der Fussohlen sehr breit". German arachnologists use the word Fuß to refer to the tarsus. Translations of Sammetbürste into Latin use the word scopula. Hence in English arachnological terminology, Koch meant 'the scopula of the base of the tarsus very wide'. Eury- is derived from the Greek , meaning 'wide', while means 'the sole of the foot', paralleling Koch's use of Fußsohle. Thus Eurypelma literally means 'wide footsole'; however, arachnologists have conventionally taken pelma in such names to refer to the scopula, so producing the meaning 'with a wide scopula'.
Other genus names or synonyms that Estrada-Alvarez and Cameron regard as having 'footsole' or 'scopula' meanings include:
  • Acanthopelma – Greek 'thorn, spine'; overall meaning 'spiny footsole'
  • Brachypelma – Greek 'short'; overall meaning 'short scopula'
  • Metriopelma – Greek 'of moderate size'; overall meaning 'medium length scopula'
  • Schizopelma – from the Greek origin combining form schizo- 'split'; overall meaning 'split footsole'
  • Sericopelma – Greek 'silky'; overall meaning 'silken scopula'
Later, particularly following genus names published by R.I. Pocock in 1901, the element pelma appears to have become synonymous with 'theraphosid'. For example, the author of Cardiopelma writes, "Cardiopelma fait réference aux genitalia de la femelle qui évoquent la forme d'un Coeur", with no reference to either 'footsole' or 'scopula'. Names interpreted in this way include:
  • Aphonopelma – Greek 'soundless'; overall meaning 'theraphosid without sound'
  • Cardiopelma – Greek 'heart'; overall meaning 'heart theraphosid'
  • Clavopelma – Latin 'club'; overall meaning 'theraphosid with club-shaped hairs'
  • Delopelma – Greek 'clear, obvious, visible, conspicuous, plain'; overall meaning 'theraphosid without plumose hair'
  • Gosipelma – the element gosi- means 'desert', relating to the Gosiute people; overall meaning 'desert theraphosid'
  • Spelopelma – Greek 'cave'; overall meaning 'cave theraphosid'

    Distribution

Tarantulas of various species occur throughout the United States, Mexico, 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, southern Italy, and Cyprus.

Habits

Some genera of tarantulas hunt prey primarily in trees; others hunt on or near the ground. All tarantulas can produce silk; while arboreal species typically reside in a silken "tube tent", terrestrial species 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.

Appendages

The eight legs, the two chelicerae with their fangs, and the pedipalps are attached to the prosoma. The chelicerae are two double-segmented appendages 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-segmented appendages connected to the prosoma near the mouth and protruding on either side of both chelicerae. In most species of tarantulas, the pedipalpi contain sharp, jagged plates used to cut and crush food often called the coxae or maxillae. As with other spiders, the terminal portions of the pedipalpi of males function as part of their reproductive system. Male spiders spin a silken platform on the ground onto which they release semen from glands in their opisthosoma. Then they insert their pedipalps into the semen, absorb the semen into the pedipalps, and later insert the pedipalps 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 at the end of each leg are used to grip surfaces for climbing. Also on the end of each leg, surrounding the claws, is a group of bristles, called the scopula, which help the tarantula to grip better when climbing surfaces such as glass. The fifth pair is the pedipalps, which aid in feeling, gripping prey, and mating in the case of a mature male. The sixth pair of appendages is the chelicerae and their attached fangs. When walking, a tarantula's first and third legs on one side move at the same time as the second and fourth legs on the other side of its 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 infraorder 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, tube-like 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 100 spinning tubes through which silk is exuded. As the silk is pulled out of the spinnerets, the shear forces cause proteins in the silk to crystallize, transforming it from a liquid to a solid thread.