Tarantulas comprise a group of often hairy and very large arachnids belonging to the Theraphosidae family of spiders, of which approximately 900 species have been identified. This article only describes members of Theraphosidae, although some other members of the same suborder are commonly referred to as â€œtarantulasâ€. Most species of tarantulas are not dangerous to humans, and some species have become popular in the exotic pet trade
Like all arthropods, the tarantula is an invertebrate that relies on an exoskeleton for muscular support. Like other Arachnida a tarantulaâ€™s body comprises two main parts, the prosoma (or cephalothorax) and the opisthosoma (or abdomen). The prosoma and opisthosoma are connected by the pedicle, 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 8â€“30-centimetre (3â€“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 150 grams (5.3 oz) and a leg-span of up to 30 centimetres (12 in), males being the longer and females greater in girth.
The spider originally bearing the name â€œtarantulaâ€ 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 â€œtarantulaâ€ 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
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 tarantulaâ€™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 tarantulaâ€™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 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