Why are the legs of insects not always identical?

Why do insects have six legs?

• Answer:

  Why six, why not four or eight? The easy answer to this question is that there is no reason why 6, and not 8 or 4; after all spiders have eight legs and land vertebrates (amphibians, lizards, mammals, birds) have four legs, so it is just an accident of evolution that the ancestor of all insects had six legs, the ancestor of spiders had eights legs and the ancestor land vertebrates had four. Why an even number (6) why not an odd number (5 or 7)? You may notice that all those animals mentioned above have an even number of legs (4, 6, 8) never odd (3, 5, 7). This is probably because it we are all descended from creatures that are 'bilaterally symmetrical;' that is we are symmetrical split down the middle with a head at the front of our body, pairs of limbs on each side of our body and a tail at the end of our body; and we are not descended from 'radially symmetrical' animals that are wheel-shaped or star-shaped like starfish. Maybe if starfish had evolved to walked on land they would have evolved into animals with five legs and wheel shaped bodies, but they didn't. Since we are bilaterally symmetrical, we have pairs of legs, and that odd fifth or seventh leg, where would it come from? where would it attach to? It's interesting to note that kangaroos, although they jump on two legs (and use two arms to grab things, like people), use their tail as a 'third leg' when they are creeping around slowly; some monkeys and chameleons use their tail like a hand as they climb trees as if they had 'five legs' so it is not impossible for a creature to use odd numbers of limbs to walk. It's just that all land animals that use legs evolved from creatures that were bilaterally symmetrical and so had even numbers legs arranged in pairs What's so special about 6? So it was an accident that insects ended up with six legs, but six is a good number and insects have several advantages from their six legs. For example, you ever tried to balance a table or stool on only two legs? or only one leg? The most stable table or stool would be a table with three legs. When people walk, we put only one foot on the ground and the other foot is moving forward; when we stand we have only two feet on the ground. That is unstable and requires constant effort not to fall over. Mammals with four legs move only leg at a time to walk and the other three are always on the ground, that is more stable, but it is a complicated process because each leg has to be controlled independently. But with six legs, insects always have three legs to stand on and three legs to move forward for that next step. That is a very stable and simple way to walk. Also, some insects don't use all six legs to walk, praying mantises use only four legs to walk (like mammals) and still have an extra pair to use as 'hands' to grab food to eat.