It seems that there is no ideal number of legs. Humans have two, dogs have four, insects have six, and centipedes can have more than 1,000. So what made spiders settle for eight legs?
“I think the best answer and the easiest answer is that spiders have eight legs because their parents had them.” Thomas Hegna, assistant professor of invertebrate paleontology at the State University of New York at Fredonia, told Live Science. “But then it goes into a kind of regression, and somewhere it all had to start.”
If we follow the succession of eight-legged spider parents back to about 500 million years ago, in the Middle Cambrian, we come to the root of the line of chelicerae, the group of arthropods which contains the spiders. If we go back even further, to 541 million years agowe find oceanic lobopods, the ancestors of all arthropods.
The name “lobopod” does not designate a single species but rather a wide variety of species with fairly simple bodies. Basically, they were worm-like creatures with segmented bodies. Each segment featured roughly identical pairs of short, stocky legs, and this pattern continued down the length of their bodies.
As lobopods evolved, they began to specialize their legs and fuse body segments. The first chelicerae seem to have merged their small body segments into two large ones: the head and the abdomen. Scientists don’t know why, but the head kept the legs, and the abdomen lost them. When the spiders appeared 315 million years agothey inherited a body plan that was probably already 150 million years old.
It is unknown what environmental pressures, if any, caused the chelicerae to settle on their eight-legged arrangement. However, we know a lot about the origin of their legs – and it’s weird.
“These legs are actually part of their mouth,” Nipam Pateldevelopmental biologist and director of the Marine Biological Laboratory, affiliated with the University of Chicago, told Live Science.
Because spiders, insects, crustaceans, and centipedes 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 this basic plan. According to Patel, all arthropod appendages — including legs, antennae, and even mandibles (jaws) — can be attributed to a chunky lobopod appendage.
Take a mantis shrimp. It swims with a bunch of small legs on a segmented abdomen. On the cephalothorax (a fused head and thorax) are its walking legs, then near its mouth are small appendages that not only make up its jaws, but also carry 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 thorax, while its head and mouth are basically configured like those of the mantis shrimp.
Then there are the spiders.
“If you look at a spider embryo, it looks exactly like an insect embryo,” Patel said. “Except it only grows paws on its head. But instead of using them as mouthparts, it uses them for walking.”
The reason why spiders walk with appendages on their faces goes back to lobopods and the original body plan of the chelicera. While modern arthropods are spoiled for choice for specialized appendages, lobopods were worm-like creatures with many sets of roughly similar appendages.
“Initially, all the legs were the same,” bruce heather, a research associate at the Marine Biological Laboratory, told Live Science. “But then the first appendages differentiated to be a sensory appendage, like sensing and grasping food.”
From this time the chelicerate ancestors of the spider began to diverge from other groups. In the ancestors of insects and crustaceans, the multitasking front appendages of the lobopod lost their grasping and feeding ability and became specialized sensory structures called antennae. But for the chelicerae, these same appendages have lost their sensory abilities and have become fangs.
During this time, the second pair of legs of the chelicerae evolved into a set of eye-catching appendages called pedipalps; the next 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 is really cool fossils in amber of a species that appears to be an ancestor of spiders and scorpions, so it has traits intermediate between the two. And on this fossil, there are very clear legs hanging from the abdomen.”