In many respects the insects can be regarded as the most successful class of animals in evolutionary terms. And one of the most common features of insects is that they (mostly) all have six legs.
Not discounting other traits, is there something about six legs that has helped insects achieve this success?
Can we use physical laws to analyze and determine an optimality of having six legs - perhaps such as stability?
Answer
I can think of two possible reasons: first, you can have half your legs up in the air at one time (as in walking - two on one side and one on the other, then change) and still be perfectly stable (3 legs = most stable, like a tripod); and second, if a predator chews off a leg on either side, you still have two legs (so you can still walk). I think those arguments are borderline biomechanical, rather than physical...
The first argument has some solid scientific backing - see for example http://web.neurobio.arizona.edu/gronenberg/nrsc581/powerpoint%20pdfs/cpg.pdf . It doesn't take a lot of brains to walk with six legs... I fact it can be done almost entirely with "local" neurons. That's a good thing when you don't have a lot of brains.
Quoting from https://answers.yahoo.com/question/index?qid=20090418111020AA75mgR :
Generalizing, insects walk with a metachronal gait and, with speed, a tripod gait - which involves a tripod stance - 2 legs on one side of the body and one on the other remain stationary while the other legs move forward, then the stationary legs walk as the others take a stance. In this way, walking involves maximum stability with a minimum of neural coordination. In fact, ganglia and other nerves and sensors located on each leg may contribute as much to the actual walking movement as the brain does. It's a very easy, stable and adaptable locomotory system which evolved from the basic arthropod body plan with 2 pairs of limbs on each body segment.
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