Here is a continuation of our discussion - all messages
which I received during the last half day. Enjoy.

Krystyna Gielo-Perczak, Co-moderator Biomch-L
From: "Michael A. Riley"
>To paraphrase George Orwell: "Even legs good, odd legs bad".
>Tom Jenkyn

The current discussion (and especially the paraphrased Orwell
statement) remind me of a television story recently about a dog that
lost two legs, both on one side of the body (I believe on the right
side). Remarkably, the dog was eventually able to walk, run, and even
jump to catch a frisbee, using only the front and back legs on the
other side. The dog's movements were amazingly fluid and athletic;
much more so than those of dogs that have lost only one leg (and
rivaling those of a normal dog). Of course, with only one case one
can't make generalizations, but for dogs or other quadrapeds
unfortunate enough to lose a limb or limbs, might losing 2 actually
be better (for locomotion, at least) than losing only 1?

Regarding the odd-legged crabs & other arthropods--experiments have
been conducted in which 2 legs (one on each side) of an insect have
been amputated (I'm not aware of any experiments involving amputation
of only 1 leg). The insects sometimes switched from the normal
'alternating tripod' gait (3 legs--the front and back on one side and
the middle on the other side--are synchronized and in swing phase,
while the other 3 are synchronized and in stance phase, at a given
point in time) to a 'diagonal' gait in which (if I remember
correctly) ipsilateral limbs moved in-phase with one another, with a
180 degree phase difference between contralateral limbs. This type
of result raises a question--might the prominence of even-numbered
limbs and the observed switches in gait patterns have something to do
with symmetry principles in networks of coupled oscillators?

From: Simon Roe

>Here's another thought.

What if one gets damaged?

If the gait relied on 3 limbs, it would be much more difficult to manage
with 2 than it is for a 4 legged animal to manage with 3.

Simon Roe
From: "Daniel P. Ferris"

Michael LaBarbera wrote an interesting article about wheeled locomotion in
nature for those interested in the topic:
LaBarbera M (1983) Why the wheels won't go. AMERICAN NATURALIST=20
I also wanted to suggest that one reason why there may be a lack of animals
with an odd number of legs is related to the process of evolution. Animals
do not create an appendage out of nothing. They build on the form and
structure that already exists. As such, they are constrained by the
morphology of their ancestors. If all of the creatures that made it to
land from the sea had bilateral symmetry, it is more likely that bilateral
symmetry would be passed down to future species than not.
There are some good discussions about evolutionary contraints on morphology
and the relationship between morphology and ecological performance in:

Koehl, M.A.R. (1996) When does morphology matter? Ann. Rev. Ecol. Syst.

Wake D.B., Roth G., eds. (1989) Complex Organismal Functions: Integration
and Evolution in Verbrates. New York: Wiley & Sons

Wake M.H. (1992) Morphology, the study of form and function, in modern
evolutionary biology. In Oxford Surveys in Evolutionary Biology, ed. D.
Futuyma, J. Antonovics,pp. 289=96346. New York: Oxford Univ. Press

Dan Ferris
From: Chuck Pell

Wheels *are* found in nature --- real ones, not just
rolling spiders or tumbleweeds. True rotary joints
are found in bacteria that possess flagella. The cilia
and flagellum of big eukaryotes actually bend, but
the bacterial flagellum is a rigid helix, a corkscrew-
shaped protein called flagellin. The joint where the
corkscrew goes into the bacterial membrane has
thrust bearings, radial bearings, and is a true rotary
motor. The flagellin, once formed, is thought not
to require functions much more like
a screw through cold molasses, than like a propeller
through water... (Re of