Why do spiders have 8 legs? (2024)

Why do spiders have 8 legs? (1)

There seems to be no ideal number of legs. Humans have two, dogs have four, insects have six and millipedes can have over 1,000. So what made spiders settle for eight legs?

"I think the best answer and the simplest answer is that spiders have eight legs because their parents did," Thomas Hegna, an assistant professor of invertebrate paleontology at the State University of New York at Fredonia, told Live Science. "But then that gets into sort of a regress, and somewhere this all had to start."

If we follow the succession of eight-legged spider parents back to about 500 million years ago, during the middle Cambrian Period, we arrive at the root of the chelicerate lineage, the group of arthropods that contains spiders. If we go even further back, to 541 million years ago, we find the ocean-dwelling lobopods, the ancestors of all arthropods.

The name "lobopod" doesn't refer to a single species but rather a large variety of species with rather simple bodies. Basically, they were wormlike creatures with segmented bodies. Each segment featured roughly identical pairs of short, stubby legs, and this pattern continued along the lengths of their bodies.

Related: What is the deadliest spider in the world?

Why do spiders have 8 legs? (2)

As the lobopods evolved, they began specializing their legs and fusing body segments. The early chelicerates seem to have fused their small body segments into two big ones: the head and the abdomen. Scientists aren't sure why, but the head kept the legs, and the abdomen lost them. By the time spiders appeared 315 million years ago, they inherited a body plan that was likely already 150 million years old.

It's unclear which environmental pressures, if any, caused chelicerates to settle on their eight-legged arrangement. However, we know a great deal about where their legs came from — and it's weird.

Sign up for the Live Science daily newsletter now

Get the world’s most fascinating discoveries delivered straight to your inbox.

"Those legs are actually part of their mouth," Nipam Patel, a developmental biologist and director of the Marine Biological Laboratory, which is affiliated with the University of Chicago, told Live Science.

Because spiders, insects, crustaceans and millipedes all evolved from an ancestor that likely had a segmented body with a set of appendages on each segment, these species are just highly modified riffs on that basic plan. According to Patel, all arthropod appendages — including legs, antennae and even mandibles (the jaws) — can be traced back to a stubby lobopod appendage.

Take a mantis shrimp. It swims with a bunch of little legs on a segmented abdomen. On the cephalothorax (a fused head and thorax) are its walking legs, and then near its mouth are little appendages that not only make up its jaws but also sweep food into its mouth to help it eat.

Compare that to an insect, whose abdomen doesn't have appendages. But it has six legs on its thorax, while its head and mouth are basically set up like the mantis shrimp's.

Then, there are spiders.

"If you look at a spider embryo, it looks exactly like an insect embryo," Patel said. "Except it only grows the legs on its head. But instead of using those as mouthparts, it uses them to walk."

The reason spiders walk with appendages from their faces goes back to lobopods and the original chelicerate body plan. While modern arthropods are spoiled for specialized appendages, the lobopods were wormlike creatures with many sets of roughly similar appendages.

Related: What is the largest arachnid to ever live?

"Initially, all of the legs were the same," Heather Bruce, a research associate at the Marine Biological Laboratory, told Live Science. "But then the first appendages became differentiated for being a sensory appendage, like for sensing and grabbing food."

Related mysteries

Are daddy longlegs really the most venomous spiders in the world?

Is every spiderweb unique?

Why did trilobites go extinct?

From that point, the spider's chelicerate ancestors began to diverge from the other groups. In the ancestors of insects and crustaceans, the lobopod's multitasking front appendages lost their grabbing and feeding ability and became specialized sensory structures called antennae. But for chelicerates, those same appendages lost their sensory capabilities and became fangs.

Meanwhile, chelicerates' second leg pair evolved into a set of grabby appendages called pedipalps; the following four sets of legs remained in their role as walking legs, and all appendages after that were lost.

Well, not all of them. "Spinnerets evolved from spider legs," Bruce said. "There are really cool fossils in amber of a species that looks to be an ancestor of both spiders and scorpions, so it has some intermediate traits between the two. And on that fossil, there are very clear legs hanging off of the abdomen."

Cameron Duke

Live Science Contributor

Cameron Duke is a contributing writer for Live Science who mainly covers life sciences. He also writes for New Scientist as well as MinuteEarth and Discovery's Curiosity Daily Podcast. He holds a master's degree in animal behavior from Western Carolina University and is an adjunct instructor at the University of Northern Colorado, teaching biology.

More about spiders

Giant, invasive Joro spiders with 6-foot webs could be poised to take over US cities, scientists warnDiving bell spider: The only aquatic arachnid that creates a web underwater to live in


Defying the odds, Japan's SLIM lander survives 2nd night on the moon
See more latest►

No comments yetComment from the forums

    Most Popular
    Orca calf refuses to leave a lagoon where its mother stranded and died off Vancouver Island

    By Sascha Pare

    Final Delta IV Heavy rocket launch scrubbed just before liftoff

    By Sascha Pare

    9,000-year-old rock art discovered among dinosaur footprints in Brazil

    By Lobato Felizola

    Watch rare endangered pink handfish walking in 19th-century shipwreck off Tasmania

    By Elise Poore

    VR headsets vulnerable to 'Inception attacks' — where hackers can mess with your sense of reality and steal your data

    By Drew Turney

    'Richly decorated' Roman villa with 'curse tablets' and tiny axes unearthed in England

    By Jennifer Nalewicki

    Active ingredient in Viagra tied to lower Alzheimer's risk — but don't get too excited

    By Emily Cooke

    Milky Way's monster black hole may be shooting superheated jets into our galaxy, groundbreaking images reveal

    By Harry Baker

    1st detection of 'hiccupping' black hole leads to surprising discovery of 2nd black hole orbiting around it

    By Sharmila Kuthunur

    Baltimore bridge collapse: an engineer explains what happened, and what needs to change

    By Colin Caprani

    Hurry! Get these solar glasses delivered before the eclipse

    By Orla Loughran Hayes

    Why do spiders have 8 legs? (2024)


    Why do spiders have 8 legs? ›

    So what made spiders settle for eight legs? "I think the best answer and the simplest answer is that spiders have eight legs because their parents did," Thomas Hegna, an assistant professor of invertebrate paleontology at the State University of New York at Fredonia, told Live Science.

    Do spiders always have 8 legs? ›

    All spiders (and arachnids) have eight legs, and almost all of them have eight eyes, but beyond these similarities there's a lot of variation among the approximately 50,000 spider species inhabiting Earth.

    Does a spider have to have 8 legs to be a spider? ›

    Myth: You can always tell a spider because it has eight legs. Fact: Not exactly. Scorpions, harvestmen, ticks, and in fact all arachnids—not just spiders—have four pairs of legs (see illustrations).

    Can spiders survive without 8 legs? ›

    Most spiders will be fine without a leg, many can even survive with relatively few legs.

    Why do some spiders have 10 legs? ›

    All spiders have the same number of legs. A few spiders have extra bits that look like legs but are not legs, such as pedipalps. Pedipalps are male spider sex organs that transfer sperm to female spiders. In some species, they look like an extra pair of legs.

    Can spiders see out of all 8 eyes? ›

    For the most part, when spiders have more than two eyes, all their eyes are used for vision, but not all eyes see in the same way. Typically, the eyes most central to the spider's face are used to detect the size, shape and color of nearby objects, ScienceLine reports.

    Can spiders live after losing a leg? ›

    Leg loss is a common phenomenon in spiders, and according to the species 5% to 40% of the adults can present at least one missing leg. There is no possibility of regeneration after adult moult and the animal must manage with its missing appendages until its death.

    Do spiders have a heart? ›

    In all spiders the abdomen contains a tube-shaped heart, which usually has a variable number of openings (ostia) along its sides and one artery to carry blood (hemolymph) forward and one to carry it backward when the heart contracts.

    Do spiders have stomachs? ›

    Cephalothorax - the fused head and thorax, also called the prosoma. It contains the brain, jaws, eyes, stomach, and leg attachments. Abdomen - the belly, also called the opisthosoma. It contains the guts, heart, reproductive organs, and silk glands.

    Do spiders have blood? ›

    Circulation. Spiders, like most arthropods, have an open circulatory system, i.e., they do not have true blood, or the veins which transport it. Rather, their bodies are filled with haemolymph, which is pumped through arteries by a heart into spaces called sinuses surrounding their internal organs.

    Do spiders feel pain when squished? ›

    Is there a sensation of pain for spiders when they get squished? Spiders and insects do not have the neurology to feel pain as humans do. Do insects experience pain when squashed? Insects lack the necessary nervous system to feel pain when squished.

    What is the lifespan of a spider? ›

    Some spiders have life spans of less than a year, while others may live for up to twenty years. However, spiders face many dangers that reduce their chances of reaching a ripe old age. Spiders and their eggs and young are food for many animals.

    Can spiders survive a fall? ›

    Due to their low weight compared with body surface area, and the thread acting as a parachute, a true spider should survive a fall. However, if it is a type of house spider, it may not survive outside regardless of any issues with falling.

    Do all spiders have teeth? ›

    In its most common use, spiders bite their prey and inject venom, which immobilizes the prey and starts the process of digestion. Spiders have no teeth and rely on the venom to liquefy their prey in order that their stomachs, known as sucking stomachs, can draw in the meal.

    Do spiders have eyes? ›

    Spiders usually have eight eyes (some have six or fewer), but few have good eyesight. They rely instead on touch, vibration and taste stimuli to navigate and find their prey.

    Where did spiders evolve from? ›

    Spiders probably evolved about 400 million years ago from thick-waisted arachnid ancestors that were not long emerged from life in water. The first definite spiders, thin-waisted arachnids with abdominal segmentation and silk producing spinnerets, are known from fossils like Attercopus fimbriungus.

    Do any spiders have 6 legs? ›

    Spiders have 8 legs. All arachnids have 8 legs. Insects only have 6 legs. Insects have three body parts – a head, thorax and abdomen.

    Do spiders ever have 6 legs? ›

    All arachnids have eight legs. If it has six legs, it's not a spider. However, some spiders have adapted their two front legs for purposes other than walking, and in these cases the front legs may be so diminished in size as to be inconspicuous.

    What spiders do not have 8 legs? ›

    None, there is no species of spiders with six legs. All spiders have 8 legs but may loose some for various reasons during their lives.

    Are there any spiders with 10 legs? ›

    Although the common name given to these spiders may not conjure up anything intimidating, camel spiders are, perhaps, the most intimidating spiders that have ever been described by researchers. These spiders possess ten legs, and the have the largest jaws of any arachnid species.

    Top Articles
    Latest Posts
    Article information

    Author: Velia Krajcik

    Last Updated:

    Views: 6638

    Rating: 4.3 / 5 (74 voted)

    Reviews: 89% of readers found this page helpful

    Author information

    Name: Velia Krajcik

    Birthday: 1996-07-27

    Address: 520 Balistreri Mount, South Armand, OR 60528

    Phone: +466880739437

    Job: Future Retail Associate

    Hobby: Polo, Scouting, Worldbuilding, Cosplaying, Photography, Rowing, Nordic skating

    Introduction: My name is Velia Krajcik, I am a handsome, clean, lucky, gleaming, magnificent, proud, glorious person who loves writing and wants to share my knowledge and understanding with you.