Google Answers Logo
View Question
Q: Entomology ( Answered 5 out of 5 stars,   4 Comments )
Subject: Entomology
Category: Science > Biology
Asked by: jamesveall-ga
List Price: $3.00
Posted: 30 Jul 2005 13:10 PDT
Expires: 29 Aug 2005 13:10 PDT
Question ID: 549843
Why do spiders have 8 legs?
Subject: Re: Entomology
Answered By: justaskscott-ga on 30 Jul 2005 14:12 PDT
Rated:5 out of 5 stars
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

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.]
jamesveall-ga rated this answer:5 out of 5 stars and gave an additional tip of: $5.00
Thank you very much - I think this is a wonderful answer and more
importantly my 5 year old boy went to sleep very happy.

Subject: Re: Entomology
From: pinkfreud-ga on 30 Jul 2005 13:25 PDT
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.
Subject: Re: Entomology
From: myoarin-ga on 30 Jul 2005 14:30 PDT
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,

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

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
Subject: Re: Entomology
From: ckr333-ga on 30 Jul 2005 16:40 PDT
A millipede or centipede has many legs but I've never seen it walk sideways!
Subject: Re: Entomology
From: karizma-ga on 09 Sep 2005 06:58 PDT
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.

Important Disclaimer: Answers and comments provided on Google Answers are general information, and are not intended to substitute for informed professional medical, psychiatric, psychological, tax, legal, investment, accounting, or other professional advice. Google does not endorse, and expressly disclaims liability for any product, manufacturer, distributor, service or service provider mentioned or any opinion expressed in answers or comments. Please read carefully the Google Answers Terms of Service.

If you feel that you have found inappropriate content, please let us know by emailing us at with the question ID listed above. Thank you.
Search Google Answers for
Google Answers  

Google Home - Answers FAQ - Terms of Service - Privacy Policy