Tarantulas have a typical spider-like construction, the head and thorax are merged into a cephalothorax or carapace (prosoma) and the abdomen (opisthosoma) is spherical and clearly constricted to the rest of the body. There are eight legs and two pedipalps, the latter are similar to the antennas of all insects, though the attachment is not at the head, as in the insects, but at the chest piece. The pedipalps of tarantulas are often so large that they can easily be confused with their regular legs. The females often have a rounded end of the palp, the males have here - like all spiders - two bald, balloon-like structures that end in a point, called the bulbus pedipalps. This is where the male spider saves his sperm, also see the page Breeding for more information.
Anatomy
A difference between most spiders, but certainly not all spiders, is the body size. Many species can have a body length of 5 to 8 centimeters (2 to 3 inch) measured from the jaws to the tip of the abdomen. The larger species reach a body length of 11 centimeters. If you measure the leg span, most tarantulas will be around 12 to 18 centimeters. They usually got long and dense hair.
One of the biggest tarantulas is the goliath tarantula (Theraphosa blondi). It can reach a body length of 12 centimeters and a leg span of about 30 centimeters.
However, there are tarantula species that are not much larger than a common house spider (in the Netherlands or Belgium).
There are some general differences with many other families of spiders. The eyes of tarantulas relatively small. Like other spiders, there are several pairs of eyes, four in total (ie, eight eyes) which are arranged in two rows, but on the tarantula they are located closer to each other. In other spiders, such as the jumping spider (Salticidae) and the six eyed spiders (Sicarius), the eyes are more widely spaced. The only exception are some species from the subfamily Stromatopelminae, who got enlarged eyes. Another obvious difference are the spinnerets, most spiders got two, but tarantulas got four. Also, the anatomy is different on the tarantulas, though this is not visible on the outside. Spiders got book lungs, which can not be compared with the lungs of other animals. The book lungs are pleated and must remain moist. Most spiders have two book lungs, tarantulas have four.
Digestive System
Tarantulas got large jaws, also called chelicerae. Like all tarantula species the chelicerae are placed in line with the body, unlike other spiders they are not curved. The chelicerae themselves are often hairy and clearly visible, under every cheliceer is a dagger-like, usually dark colored fang present. The tarantula uses the fangs to inject digestive juices into the prey. These, however, are never hairy and in contrast to most spiders, they are movable independently.
Tarantulas can be distinguished by a number of characteristics, especially the combination of these characteristics is typical of many sub-families.
A number of groups got so called scopula on the outside of the venomous fangs, this is dense hair consisting of short, thick hair. With most tarantulas the scopula is missing.
1: fang
2: poison canal
3: venom gland
4: esophagus
5: digestive cecum
6: sucking stomach
7: intestine
8: digestive tubule
9: malphagian tubule
10: stercoral pocket
11: anus
12: silk gland
13: spinneret
A: prosoma
B: opisthosoma
P: petiole
Circulation and reproductive system
1: eye
2: ganglion
3: vein
4: stomach
5: aorta
6: heart
7: ovary
8: book lungs
9: spermatheca
10: oviduct
A: prosoma
B: opisthosoma
