Washington State University

Ask Dr. Universe

Not everything has a reason

January 6th, 2012

Dear Dr. Universe,
Why do spiders have eight legs?
Kingston, Ontario

Tup Wanders/Flickr

Tup Wanders/Flickr

Phylogenetic inertia, that’s why.

Though I guess that’s not really WHY. So another way of answering your question is “because their ancestors had eight legs.” That’s about all I could squeeze out of Pat Carter, who studies evolutionary physiology here at Washington State University. That means he studies how animals came to work the ways they do.

Spiders belong to a large group of animals called the Chelicerata (kuh-LIH-suh-RAH-da), says Professor Carter. They are named for the snappers on their heads, their jaws, their chelicera. Some chelicera snap up and down, and some snap sideways.

The other thing that animals in the Chelicerata group have in common is four pairs of legs.

Though that doesn’t explain WHY they have four pairs of legs, does it?

Well, let’s think about horseshoe crabs, which also belong to the Chelicerata group and are actually more closely related to spiders than crabs. They also have four pairs of legs. But they also have other leg-like appendages on their abdomens. (Appendages are things that stick out from the body.) Horseshoe crabs, which haven’t changed much for hundreds of millions of years, and spiders probably developed from the same ancient relatives.

But spiders lost those extra appendages. Spiders DO, however, have a pair of appendages surrounding their chelicera. These PEDIPALPS help the spider grab food and shove it in her mouth. PEDI means foot, by the way. Get my drift?

Clearly, says Professor Carter, the Arachnids (spiders, scorpions, mites and ticks, all of which are Chelicerates) are pretty successful. They’ve been around for millions of years and show no sign of disappearing. But the same could be said for insects. In other words, six legs seem to work pretty well for insects, and eight legs seem to work pretty for spiders and their relatives.

No, I’m NOT dodging your question, though it must seem like it. It’s just, says Professor Carter, that maybe there really isn’t any REASON that spiders have eight legs. They just do. And maybe different appendages that different relatives developed came to be used differently. Just as a reminder, this process took place over millions and millions of years.

SO—for some random genetic reason, some ancient relative of the spider developed eight legs. Or maybe he developed ten legs, two of which eventually developed in a later relative into pedipalps.

People tend to think about evolution in terms of adaptation, says Professor Carter. Adaptations are features that organisms develop through genetic mutations that happen to help them adapt to their environment.

It’s NOT that these adaptations develop in ORDER to help these guys survive. Rather, these chance mutations help them survive better than similar organisms that didn’t develop the adaptations. This is part of what is called NATURAL SELECTION. Those best adapted for survival survive. Got it?

But lots of traits are NOT adaptive, says Professor Carter. They just happen. Neither are they NON-adaptive, which means the trait would make the organism LESS able to survive and reproduce.

The reason we don’t know more about spider evolution in general is that fossils of spiders are relatively rare. The first spider probably appeared around 400 million years ago. But finding a fossil that old is rare.

Scientists HAVE found many less ancient spider fossils, many of them preserved in amber, which is hardened tree sap. More than 300 species of spiders have been described from about 40 million years ago.

However, these so closely resemble modern spiders that they really don’t tell us much about spider evolution.

But back to your question. We don’t know WHY spiders have eight legs, says Professor Carter. They just do. There is no WHY. That’s part of what “phylogenetic inertia” is all about. And a big part of how evolution works.