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Understanding tarantula anatomy isn’t just an interest; it is a vital part of owning a pet tarantula. The anatomy of tarantulas relates to the care these arachnids should receive, as well as their behavior and life cycle.
Firstly, it is useful to know how to identify an arachnid. To tell an arachnid apart from an insect, there are a few features to look out for. One difference is that an arachnid has two body parts, while an insect would have three. Second is the number of legs; all arachnids have eight legs, and insects have six. Finally, most insects have wings, but no arachnids, tarantulas or otherwise, have wings.
With this introduction to identify arachnids from insects, it is easier to learn more about a tarantula’s body parts.
The cephalothorax (or prosoma) is the tarantula’s anterior body part. Externally, it features the eyes, legs, and mouth, while the brain, digestive system, and retractor muscles are inside.
Tarantulas have the largest brains, or nervous systems, of all arthropods. The brain is located at the bottom of the inner prosoma.
Despite having eight eyes, sight is not a tarantula’s strongest sense. Tree-dwelling species have better eyesight to hunt and jump from branch to branch, but most species are only able to distinguish between light and dark. They are also able to see large objects move.
Instead of sight, tarantulas rely upon tactile sensations to read the world around them. They are covered in specialized hairs, which increase their sensitivity to vibrations, both on the ground they stand on and in the air.
Tarantulas have eight legs, each consisting of eight sections: claws, tarsus, metatarsus, tibia, patella, femur, trochanter, and coxa. The coxa is the section that connects the legs to the cephalothorax. The first two sections of the legs have scopula pads, which are made up of microscopic hairs that provide an effective grip on the smoothest of surfaces, including glass. With claws at the end of all legs, tarantulas are very successful at climbing.
To move, tarantulas have muscles to retract their legs, while they depend on hydraulics to extend their legs. Tarantulas use a sudden increase of pressure in their cephalothorax to send hemolymph fluid to their limbs, which causes their legs to stretch out. This is why tarantulas and other spiders have skittish movements; they increase and decrease body pressure minutely to expand and contract their legs to walk and run. It also explains why a tarantula’s legs will curl when it dies, as there is no pressure to extend them and the retractor muscles go into rigor mortis.
A mature male will also grow tibial spurs on its front legs.
A tarantula’s mouth is made up of two main parts: the chelicerae and pedipalps. Both are essential for effective hunting, but they are also useful in other situations, as explained below:
A tarantula’s mouth is very small, so much so that it cannot absorb solids. It is almost straw-shaped, only allowing the tarantula to take in liquids sucked from its prey. This is done by mastication; the tarantula releases a special type of saliva from the tiny teeth inside the tarantula chelicerae as it eats to predigest the solid parts. Once solid parts have been crushed and covered in digestive fluids, the liquid meal is then filtered through bristles around the tarantula’s mouth to ensure no solids are consumed.
The chelicerae, which are useful tools, contain the tarantula’s fangs, which immobilize prey by injecting venom. Venom is produced by the venom glands inside the chelicerae, and these are connected to the fangs by a duct. Interestingly, how the chelicerae and fangs open and close differentiates between the Araneomorphae, which are known as “true spiders,” and the Mygalomorphae, where tarantulas are classified. Tarantulas open and close their chelicerae and fangs in parallel, while true spiders cross them to open and close.
Tarantulas also use their chelicerae to dig with the help of their pedipalps and to clean themselves by combing through their pedipalps and legs with the tiny teeth in the chelicerae. They can also clean by moistening their legs and using them to groom their entire bodies.
Although the pedipalps resemble small legs, they are not used for movement. In some instances, the pair of pedipalps work more like antennae or arms, holding prey still as the venom is injected. As sight is not the tarantula’s strongest sense, the pedipalps feel around to find a suitable path, locate mates, or hunt prey.
Male tarantulas use their pedipalps when mating. At the tips of the pedipalps, males have organs called emboli. The embolus is used for sperm storage, which is why a mature male’s pedipalps can look swollen. This organ is used to transfer sperm to the female during mating.
Similar to the legs, the first section of the pedipalps has scopula pads to enable a better grip.
The abdomen, also called the opisthosoma, holds most of the internal organs, such as the book lungs, more of the digestive system, the heart, reproductive organs in females, and spinnerets. This section is connected to the cephalothorax by the pedicel.
Tarantulas have two pairs of book lungs, which makes their respiratory system unique compared to other spiders. As the name suggests, these lungs resemble books; they are arranged in layers that look like book pages. The oxygen flows through these “pages” and is diffused through the tarantula’s hemolymph fluid.
To work adequately, a tarantula’s book lungs require the correct humidity levels.
The tarantula’s heart is located primarily in the abdomen. It is a long, slim tube that runs along the top of the abdomen. Although tarantulas do not have blood vessels like mammals, the heart circulates hemolymph fluid throughout the body’s open circulatory system, reaching all cells. Because of this, if a tarantula wounds its exoskeleton, hemolymph can leak from the injury and could prove fatal.
The reproductive organs of both male and female tarantulas are located at the rear of the abdomen. For females, the epigyne is similar to a small indent on the abdomen’s underside, and it is used to lay eggs.
Male tarantulas deposit sperm from their reproductive parts onto a web before using the pedipalps to collect the sperm, ready for when it mates with a female.
All spiders spin silk threads from the rear end of their abdomens, including tarantulas. Spinnerets serve several purposes, being used to build webs, create egg sacs during egg production, and detect prey. They place the threads that are produced by the silk glands, of which there are four inside the tarantula’s abdomen.
Researchers found that some tarantulas, including the Costa Rican Zebra tarantula (Aphonopelma seemanni), can also produce silk from their feet.