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Spiders are unique in the way they move, scuttling and scurrying in a creepy way. This is due to their skeleton and muscles. Spiders are arthropods, meaning joint-footed, and have a hard outer skeleton called an exoskeleton, made of a substance similar to our fingernails and hair. Unlike vertebrates, spiders do not have internal bones and therefore do not have the same muscles and attachments. They have seven joints for each leg, and while they have flexor muscles, they lack extensor muscles in their major leg joints. Instead, they use hydraulic pressure to extend their legs outward. So, do tarantulas, which are a type of large spider, have muscles?
| Characteristics | Values |
|---|---|
| Muscles | Spiders have muscles to flex their limbs inward, but they use hydraulic pressure to extend them outward. |
| Movement | Tarantulas can move quickly, especially when threatened, but they usually move slowly to sense the world through vibrations. |
| Vision | Tarantulas do not have good eyesight. |
| Venom | Tarantulas have fangs (pedipalps) that release venom, but it is rare for North American tarantulas to inject it. |
| Legs | Tarantulas have eight legs, each with seven joints. |
| Eyes | Tarantulas have eight eyes grouped together in pairs. |
| Colour | Baby tarantulas are dark in colour, while adults are hairy with colourful stripes and other markings. |
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What You'll Learn
Tarantulas use muscles to flex their limbs inward
Spiders have a unique hybrid propulsive system. Unlike most other arthropods, spiders have no extensor muscles in major leg joints. Instead, spiders have muscles to flex their limbs inward and use hydraulic pressure to extend them outward. This is unlike almost all other limbed animals, which have both flexor and extensor muscles, allowing for smoother and less jarring movements.
The femur-patella and tibia-metatarsus joints have hinge axes that interconnect the respective segments at their dorsal rims. As muscles can only pass these joint axes ventrally, a muscle-driven extension is not possible. Therefore, the leg extension is achieved by hydraulic pressure generated in the prosoma, which induces a hemolymph flow into the legs. This pressure is transmitted from the prosoma to the respective joints via lacunae, which are hemolymph-filled spaces between all soft tissues inside a leg. Consequently, the volume of these joints increases, resulting in leg extension.
Smaller spiders, typically those weighing under 3g, use a hydraulic catapult method to move around and catch prey. In contrast, larger spiders, or those weighing over 3g, rely on a combination of hydraulic catapult and muscle-based contraction. When a spider prepares to jump on its prey or escape danger, it pressurizes its legs for extension while simultaneously flexing its muscles. When the spider relaxes its flexed muscles, the pressurized legs extend, initiating the jump.
The hip joint, located at the body, has both extension muscles to push out the legs and flex muscles to pull them in. The femur-patella and tibia-metatarsus only have flex muscles. With flex-only muscles attached to the inside circumference of the spider's body, the spider can maximize its grip on prey.
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Hydraulic pressure is used to extend their limbs outward
Spiders, including tarantulas, have a unique hybrid propulsive system. They have muscles to flex their limbs inward but use hydraulic pressure to extend them outward. This is because spiders have no extensor muscles in their major leg joints. Therefore, hydraulic pressure generated in the prosoma, or the cephalothorax, provides leg extension. The prosoma/cephalothorax is the round, bulb-like midsection to which all the legs are connected.
The process of hydraulics involves the generation, control, and transmission of power through the use of pressurised liquids. To extend their legs, spiders rapidly increase the pressure in their prosoma/cephalothorax, which sends hemolymph (blood) flowing to the extremities, causing the legs to stretch outward. This is achieved by the prosoma/cephalothorax acting as a finely-tuned, fluid-filled bellows that pushes hemolymph around the body of the spider in a fraction of a second.
The use of hydraulics in spiders' legs was first suggested by Ellis in 1944, who observed that spiders die with their legs severely flexed. He posited that if elasticity was responsible for the extension, spiders would more likely die with their legs extended or at least not as flexed as they are when they die. This idea was confirmed by Parry and Brown in 1959, who measured high pressures in spider legs. They found that an isolated leg could lift more weight as the pressure inside it was increased.
The use of hydraulic pressure allows spiders to extend their legs rapidly and powerfully, enabling them to jump and move in a way that can appear unsettling to humans. This unique locomotion system gives spiders their characteristic scuttling and scurrying movement, contributing to the "creep factor" that many people associate with spiders.
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They have seven joints for each leg
Spiders have a unique anatomy that sets them apart from other animals. While humans have one major joint per limb (such as the knee or elbow), tarantulas have seven joints for each leg. This multi-jointed structure is made possible by their exoskeleton, which, unlike the internal skeletons of vertebrates, does not require the support of bones.
The exoskeleton of spiders is composed of a hard, lightweight material called chitin, similar to the composition of human fingernails and hair. This exoskeleton provides the structural framework for the spider's body, including its legs. Each of a tarantula's eight legs is composed of seven segments connected by joints, allowing for a wide range of movement.
The joints of a tarantula's legs function as hinges, enabling the spider to walk, climb, and even jump. However, unlike mammals, tarantulas do not have muscles within their legs to directly control movement. Instead, they rely on a hydraulic system, where a fluid called hemolymph is pumped into the legs to extend them. This system allows tarantulas to move smoothly and efficiently, even in complex environments like trees, rocks, and burrows.
The hydraulic system in tarantulas is controlled through the cephalothorax, which regulates the movement and pressure of hemolymph. The lack of muscles in the legs reduces weight, further enhancing the tarantula's agility. While spiders do have muscles to flex their limbs inward, the outward extension is powered by hydraulic pressure. This unique combination of a multi-jointed exoskeleton and hydraulic locomotion enables tarantulas to move quickly and navigate their environment effectively.
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They have eight eyes grouped together in pairs
Spiders have a unique way of moving that is often described as "creepy". This is due to their skeleton and muscles. Spiders are arthropods, which means "joint-footed". They have a hard outer skeleton called an exoskeleton, which is made of chitin, a material similar to human fingernails and hair. Unlike humans, spiders do not have internal bones, so they do not have the same muscles and attachments as vertebrates.
Tarantulas, like all spiders, have eight eyes grouped together in pairs. Usually, there are two larger eyes in the middle of their head, surrounded by three smaller eyes on each side. Their eyesight is not very good, and they sense the world through vibrations that they pick up via their legs and the hairs on their body. This is why they usually move slowly in a very deliberate, plodding manner.
Spiders have muscles to flex their limbs inward, but they use hydraulic pressure to extend them outward. This is because they lack extensor muscles in the major leg joints. The hydraulic pressure is generated in the prosoma, which provides leg extension. The prosoma acts like a finely-tuned, fluid-filled bellows that push hemolymph (a fluid similar to blood) around the spider's body in a fraction of a second.
The lack of extensor muscles means that almost the entire space inside the exoskeletal tubes can be filled with flexor muscles. These flexor muscles are necessary for gripping prey, climbing, and rappelling. Smaller spiders (usually those weighing under 3g) use a hydraulic catapult method to move and catch prey, while larger spiders (over 3g) rely on a combination of hydraulics and muscle-based contraction.
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They move slowly to sense the world through vibrations
Spiders, including tarantulas, have muscles, but they are not used for extending their legs. Instead, spiders use hydraulic pressure to extend their legs, which is why the legs of a dead spider curl inward. This hydraulic system also gives spiders their characteristic scuttling, scurrying, and jumping movements.
While tarantulas may not possess traditional hearing, they can sense vibrations through their specialised hairs, called trichobothria. These hairs act as sensory receptors that can detect even the slightest vibrations in the environment, allowing tarantulas to perceive a broader range of low-frequency vibrations than humans can. This ability to sense vibrations is crucial for tarantulas to perceive their surroundings and navigate their environment.
Tarantulas use their slit sensillae, small grooves on their legs, to interpret web vibrations when they have caught prey or when a potential mate is approaching. They can also use vibrations to assess each other's size and suitability for mating. For example, males might tap their legs on the ground to attract females.
While tarantulas can distinguish light, shapes, and movement, their vision is not as sharp as humans'. Therefore, they rely on their sense of vibration to detect prey, even in the dark or when their vision is limited. This sense of vibration also allows them to perceive the general presence and direction of vibrations.
Some people have speculated that tarantulas might be able to recognise their names based on the vibrations that are created when their name is spoken. However, this idea is generally met with scepticism, as tarantulas would need to distinguish speech from other vibrations and associate a specific pattern with themselves.
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Frequently asked questions
Yes, tarantulas, like all spiders, have muscles.
Spiders have a unique hybrid propulsive system. They use muscles to flex their limbs inward and hydraulic pressure to extend them outward.
Spiders don't have internal bones, so they don't have the same muscles and attachments as vertebrates. They have a hard outer skeleton called an exoskeleton.
Tarantulas can move quickly, especially when threatened, but they usually move slowly because they don't have very good eyesight and sense the world through vibrations.
