There isn’t an optimal quantity of legs. Dogs have two legs, humans have four legs, insects have six legs, and millipedes may have more than a thousand legs. So why did spiders decide to have eight legs?

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

Suppose we follow the lineage of spiders with eight legs back to around 500 million years ago in the mid-Cambrian Period. In that case, we arrive at the beginning of the chelicerate family, the arthropod group, which includes spiders. If we go further back, which is 541 million years, we can encounter the lobopods of the ocean, which are the ancestors of all arthropods.

The term “lobopod” doesn’t refer to one species but a wide range of species with bare bodies. In essence, they were worm-like creatures that had segments of their bodies. Each piece had roughly the same pair of stubby, short legs. This pattern continued throughout the lengths of their bodies.

As lobopods grew into a specialized species, they began to specialize their legs and join parts of their bodies. Early chelicerates appear to have merged their tiny body parts into two large ones: the head and the abdomen. Scientists don’t know why the lead remained in the legs while the core could not keep the legs. When spiders first emerged three hundred million years earlier, they had the body plan probably already more than 150 million years old.

It’s not clear what environmental factors or factors have caused chelicerates to adopt their eight-legged configuration. We do know the majority of the origins of their legs. It’s also a bit bizarre.

“Those legs are actually part of their mouth,” Nipam Patel, the director of the developmental biology lab and the Marine Biological Laboratory, which is part of the University of Chicago, told Live Science.

Since spiders, insects, millipedes, and crustaceans all evolved from an ancestor who likely comprised a segmented body with an array of appendages attached to each segment, They are highly modified variations of that basic concept. According to Patel’s research, all arthropod appendages, including antennae, legs, and mandibles (the jaws), can be linked back to lobopod-like appendages.

You can take a mantis squid. It swims with a lot of legs that are with a segmented stomach. The walking legs are the chest (a combined head and the thorax). Close to its mouth are tiny attachments that don’t just form the jaws but also help to sweep food into its mouth to assist it in getting food.

A peacock mantis ( Odontodactylus scyllarus) is seen walking across the seafloor in West Papua, Indonesia. You can see that it has a segmented abdomen with numerous appendages that aid in swimming. (Image credit: Reinhard Dirscherl/ullstein bild via Getty Images)

Comparing it to an insect whose abdomen does not have appendages. However, it does have six legs along its thorax, and its mouth and head are set up in a similar way to that of mantis shrimps.

And then, there are spiders.

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

The reason spiders wear appendages on their faces goes back to lobopods and the first chelicerate body design. Although modern arthropods are spoilt for special appendages, lobsters are worm-like animals with several sets of remarkably similar appendages.

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

At that point, the spider’s chelicerate ancestral ancestors started to separate from other groups. In the ancestors of insects and crustaceans, the lobopod’s multitasking front appendages lost their gripping and feeding capability and transformed into specific sensory structures known as antennae. However, these same appendages lost their sensory abilities for chelicerates and evolved into fangs.

The second leg pair morphed into an appendage that grabs the user called pedipalps. The next set of four legs continued in their function as walking legs. The appendages that followed were gone.

But not every one of them. “Spinnerets evolved from spider legs,” Bruce explained. “There is amazing fossils that are found within amber of the species that appear like an ancestor to scorpions and spiders as well which means it may have intermediate characteristics between them. The fossil is also interesting because there are clearly visible legs hanging from the abdomen.”


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