[Animals] Why do spiders have 8 legs?

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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. 

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.

"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. 

https://www.livescience.com/animals/spiders/why-do-spiders-have-8-legs