Why Do Spiders Have Eight Legs?

Among the comments, someone raised the question, “Why do spiders have eight legs?”

At the time, I dismissed it. It proved to be one of those things that just kept bugging me (excuse the pun). I had to find out, expecting some revolutionary answer.  I am sure a part of my curiosity came from the fact that I hate spiders.  I find nothing attractive about them. 

If you are now curious as to why spiders have eight legs, read on and satisfy your curiosity.

There is no logic to how many legs are ideal.  Humans have two, dogs and cats have four.  Horses also have four, and we all seem to do OK. The extreme are millipedes.  Contrary to popular belief, they don't have a million legs. Not that I have counted them, but scientists say they have over 1000.

So, how did spiders end up with eight legs, four times that of humans and twice that of animals?  Greedy little buggers!

I could say because it makes them extra creepy or it allows them to move 360 degrees; anyway, they like to help terrify you when they disappear.  No, it is much simpler than that.  Their parents had eight legs!!!

Spiders are not insects.  They are arachnids. They belong to the same class as scorpions and ticks, equally unattractive critters.  The fact that all insects have six legs has nothing to do with it.

It started about 300 million years ago. Creatures started segmenting different body parts. One group ended up with a head and an abdomen.

Now take a look at a spider (ugly little varmints!). Their legs are all part of the head.  The abdomen is just this fat blob that sits behind the head.  Unlike humans and animals of class, where the legs, two or four, are attached to the equivalent of the abdomen.

Insects, crustaceans, and millipedes all came from a common ancestor. This ancestor probably had a segmented body, with appendages on each segment.  With spiders, their legs come from their head, so to speak.  It is likely that in their evolution they had more than eight legs.  Some of these developed into fangs and palps for sensing or assisting with eating.  Even their spinnerets (the appendages that produce the web) are modified legs.

There is no definitive reason why they have eight legs.  They just do.  Have you ever seen a six-legged spider?

Some researchers think that spiders lose legs quite often.  In a normal spider population, as much as 60% of spiders have lost one or two legs.  Having eight legs may be because they carry a few spares, allowing them to lose a few legs throughout their life. 

Some smart scientists discovered that a five-legged spider struggled to build a nest. Spiders may have evolved additional legs from insects as an advantage for web building and escaping predators.  Losing one or two legs isn’t really a problem for them.

Spiders are a bit obsessed with the number eight.  The normal spider also has eight eyes.  A bit like those redneck trucks you see around with a number eight on the front.  As spider ancestors evolved, their eyes kept splitting. Why they stopped at eight, no one knows.

It reminds me of a joke about a scientist who wanted to know how fleas jump.  He got a whole bunch of fleas.  One by one, he diligently pulled a single leg off the fleas in succession.  When he had removed all the legs, the fleas could not jump. He concluded that their brains and thinking, which control jumping, must be in their legs. 

Isn’t science wonderful? We can send men to the moon, but we can’t work out why spiders have eight legs.

Till next time,

Calvin