Why do spiders have 8 legs?

Hello jamesveall,

Pinkfreud makes a good argument that spiders have eight legs because
their ancestors did, not because they must.  Nevertheless, I believe
I've found a good answer in this post concerning spiders and octopi:

"Usually the presence of similar structures in unrelated groups of
animals shows that there is a strong functional reason for its
evolution.  In general the possession of 8 legs allows these animals
to move with greater facility in directions other than forwards.  Both
spiders and octopi can often be seen moving backwards or sideways
rather than forwards."

"Re: Why do both octopi and spiders have eight legs?" by Trevor
Cotton, Grad student, Palaeobiology Research Group, Department of
Earth Sciences, University of Bristol (Jul 28, 1999)
MadSci Network
http://www.madsci.org/posts/archives/jul99/933165320.Zo.r.html

I presume that moving backwards, forwards, and sideways is especially
useful when navigating a spider web.

I hope that this information is helpful.

- justaskscott


Search strategy --

Searched on Google for:

spiders "eight legs" "six legs"

[I also tried some other searches; but this one led me to the page I have cited.]

Thank you very much - I think this is a wonderful answer and more
importantly my 5 year old boy went to sleep very happy.

I may be wrong, but I don't think this is a question which has a
simple, scientific answer. Science isn't always good at "why"
questions. Spiders have eight legs because their ancestors had eight
legs. Nature's tendency toward bilateral symmetry makes an even number
of legs a "given," but the abstract concept of a functional spider
doesn't absolutely require eight legs, rather than six or ten or
twelve. Eight legs is just what they've got.

PInkfreud is seldom wrong.  Without her comment, I would have clicked
on, but then thought:  two-legged - birds; four-legged - mammals;
six-legged - insects; eight-legged - spiders ...  Ta-ra!  Spiders have
eight legs because that is the way we define them!
And then?  Ten-legged - ?  Trusty Google:  search: ten-legged.  Pages
of sites for "ten-legged fish band".   Search: "ten legs".  Ta-ra
again! "Decopoda", apparently all crustaceeans: crabs, lobsters,
shrimps:

http://en.wikipedia.org/wiki/Decapoda

The what about twelve legs?  Google, mostly types of speed races with
twelve legs , but then this:

http://www.oum.ox.ac.uk/children/fossils/palarth.htm

about the horseshoe crab, a living fossil.

So spiders are just the eight-legged group in a continuum of the
critter with less or more legs.    They don't have eight legs becaus
we call the spiders; we call them spiders because they have eight
legs.

A millipede or centipede has many legs but I've never seen it walk sideways!

It is considerably easier to walk on 6 or more legs than on 2 or 4;
with six legs (or eight), you can stay balanced on the legs that you
keep in contact with the ground (which would then be 3 or 4), without
having to worry about falling over.

This I guess doesn't explain muchother than if insects (or spiders)
had 4 legs, they would have to have bigger brains to handle their
balance as well as their direction.