It seems there is no perfect number of legs. Humans have two, dogs have four, and insects have six and Millipedes can contain more than 1,000. So what made spiders settle with eight legs?
“I think the best and simplest answer is that spiders have eight legs because their parents did,” Thomas Hegna, assistant professor of invertebrate paleontology at the State University of New York at Fredonia, told Live Science. “But then it becomes a kind of backtracking, and somewhere this all has to start.”
If we follow the succession of the eight-legged spider back to approx 500 million years agoDuring the middle Cambrian period, we got to the root of the chelicerae linea, the group of arthropods that includes arachnids. If we go further, to 541 million years agowe find ocean-dwelling lobules, the ancestors of all arthropods.
The name “lopopod” does not refer to a single species but to a large variety of species with rather simple bodies. Basically, they were worm-like creatures with segmented bodies. Each segment featured nearly identical pairs of short, stubby legs, and this pattern continued along their bodies.
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As their lobules develop, they begin to customize their legs and fuse body parts. Early chelicerates seem to have combined their small body parts into two large parts: the head and the abdomen. Scientists aren’t sure why, but the head was kept on the legs, and the belly lost it. By the time spiders appeared 315 million years agothey inherited a body plan that was likely 150 million years old.
It is unclear what, if any, environmental pressures caused the claw-footed to settle in their eight-legged arrangement. However, we know a lot more about where their legs came from—and it’s strange.
“These legs are actually part of their mouths,” Nipam PatelThe developmental biologist and director of the University of Chicago’s Marine Biology Laboratory, told Live Science.
Since 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 notches on that basic plan. According to Patel, all arthropod appendages—including legs, antennae, and even the mandible(s)—can be traced back to a short arthropod appendage.
Take the mantis shrimp. It swims with a set of small legs on a segmented abdomen. On the cephalothorax (combined head and thorax) are walking legs, and then near its mouth are small appendages that not only form its jaws, but also sweep food into its mouth to help it eat.
Compare that to an insect whose abdomen has no appendages. But it has six legs on its chest, while its head and mouth are set in place like a praying mantis shrimp.
Then there are spiders.
“If you look at a spider embryo, it looks exactly like an insect embryo,” Patel said. “Except he only grows legs on his head. But instead of using them as mouth parts, he uses them to walk.”
The reason spiders walk with appendages on their faces is due to the lobes and the original clawed body plan. Whereas modern arthropods are spoiled by specialized appendages, lobopods were worm-like creatures with many groups of nearly identical appendages.
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“In the beginning, all the legs were the same,” Heather Bruce, research associate at the Marine Biology Laboratory, told Live Science. “But then the first appendices became differentiated for being a sensory appendage, like sensing and grabbing food.”
From that point on, the clawed ancestors of the spider began to diverge from other groups. In the ancestors of insects and crustaceans, the multifunctional front appendages of lobules lost their ability to grasp and feed and became specialized sensory structures called antennae. But for chelicerae, those same appendages lost their sensory abilities and became fangs.
Meanwhile, the second leg pair of chelicerates evolved into a group of grasping appendages called pedipalps; The next four sets of legs remained in their role as walking legs, losing all appendages thereafter.
Well, not all of them. “Spindles evolved from spider legs,” said Bruce. “there Really cool fossils in amber of a species that appears to be the ancestors of both spiders and scorpions, so has some intermediate traits between the two. And on that fossil, there are very visible legs hanging off the belly.”