People struggle to identify spiders and this is because they do not know how many legs spiders have. This is a very traditional way to identify a spider. Spiders have 8 legs, attached to their cephalothorax. Their legs are tubular and split into 7 segments, which work together in 3 sections. There are 4 sets of legs having unique functions to create movement. The front 2 sets of legs flex inward, pulling the spider forward, while the third set is used as a pivot to create momentum. The fourth set is used for jumping, after hydraulic pressure, that builds up to 8 times the pressure, enables a spider to jump up to 50 times their length.
Spider Classification
Spiders are classified as arachnida, which are in the family of arthropods, meaning “joint footed”. In lieu of bones and an internal skeleton, arthropods have an outer shell called an exoskeleton that encases their muscles and organs. Along with poor eyesight, spiders lack a nose, ears, and antenna. Antenna are used by other insects to feel, detect wind, sound, smell, and temperature such as hot and cold. Instead, spiders have an extra set of legs with a variety of “hairs” (unlike mammal hair that is made out of keratin, the “hairs” on spiders are made out of the same type of proteins that make up their exoskeletons, called chitin). The eight legs spiders have are not to be confused with the common misconception that spiders always have eight eyes.
Importance Of Spider Hair
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One type of “hair” is trichobothria, which are long vertical “hairs”, hinged to sense vibrations. The frequency at which the “hair” vibrates, both in air and water, allows the spider to detect sounds and differentiate between the wind blowing, water currents and an approaching winged insect such as a fly, or a wasp. The density of these “hairs” trap air which keeps a spider from drowning in water.
Sensors at the base of these “hairs” help detect the direction the movement or sound is coming from. These sensors also seem to help them to remember directions, such as where their burrow is located. Other “hairs” have sensory cells that help them distinguish the difference between tastes and smells, like chemicals emanating from a mate, and to feel the surface of objects.
The Mechanics Of A Spiders Leg
The upper section of the leg, that includes the coxa and trochanter, is connected by large powerful muscles. Flexors decrease the angle of the joint and extensors extend the angle, permitting right and left, in addition to up and down movement. These muscles, along with a hydraulics system, enable jumping in larger spiders which may bounce during the time the hydraulic pressure builds. Spiders then relax their muscles which in turn causes the hydraulic pressure to launch the spider. Large spiders do not appear to use hydraulics for movements other than starting a movement or jumping. Smaller spiders (3g and under) rely on hydraulics and catapult action, from the third set of legs, for jumping and finding prey.
Some spiders, wanting to intimidate or counter-attack predators, will use these muscles to raise and grow their front legs to make themselves look larger. Other spiders, like the male peaco*ck spider, will wave his colorful “hairy” legs to impress other peaco*ck spiders during a showy mating dance.
The body of a spider’s skeleton is filled with hemolymph, which is a fluid like blood. This fluid is stored between the organs in the cephalothorax and allows the spider to use hydraulic pressure to extend their limbs, filling in the gaps around the muscle fibers. A spider can change momentum, timing, and distance by extending or contracting their legs during flight, or stop themselves using their silk as a dragline.
If the cephalothorax, which controls the hydraulics, is ruptured causing the loss of too much fluid, the spider dies, as it has no way to uncurl its legs. Spiders have a mechanism to shut down the flow of fluid if they happen to lose a limb. Spiders rely on hydraulics, because muscles would be too heavy and cumbersome. Being cold blooded, they lack oxygen to maintain large groups of muscles and the energy to keep them warm.
The other 2 sections of the leg, the femur and patella combination, along with the tibia and metatarsus sections, only contain flexor muscles which limits them to up and down movement. The constant inward pull of flexor muscles helps the spider to wrap themselves around their prey and is critical in rappelling and climbing. This is also the reason spiders curl up when they die.
At the end of the legs, is the tarsus, which has a set of claws and a thick strong spine (“hair”) that is used to hold down prey while they inject them with venom from their fangs. In addition, it is used by the male to hold the female during mating. There is a pit located near the end of the tarsus used for sensing temperature and moisture. Based on family classification, spiders may have either an additional claw, or a claw tuft called scopulae.
Web building spiders, such as lace-web spiders, and orb-weavers have an additional claw, modified for handling silk while they build their webs. All spiders produce silk, even though only some build webs. Spiders that are missing legs have webs that are inferior to spiders with 8 legs, but have better agility if they need to maneuver quickly. Hunting spiders, such as ground spiders, wolf spiders, and huntsman spiders have claw tufts on both the tarsus and metatarsus, which are hundreds of “hairs” covered with hundreds of ‘end feet’, called setules. These “hairs” create varying adhesive properties that aid in climbing different types of surfaces.
How Many Legs Do Spiders Have?
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Now that you know the full purpose and anatomy of spider legs, do not forget also the number of legs they have, which is eight. This is a critical attribute of the spider and critical to their webbing, hunting, and ultimate survival.
