[eli5] Why do insects have lots of legs while larger animals have 4 or 2? Also, why aren’t there any animals (that I know of) with 3 legs?
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Almost all animals are bilaterally symmetrical, so outside of things like starfish and jellyfish you’re pretty much only going to see even numbers of legs.
All land vertebrates evolved from an ancestor that had two pairs of legs. Gaining or losing a leg on one side but not the other would be a very strange leap to make, evolutionarily speaking.
Arthropods, on the other hand, mostly evolved from animals with segmented bodies; gaining or losing a segment may not require as large of a mutation, and if each segment has one or more pairs of legs (as in centipedes and millipedes) you can end up with more variation between species. That said, most arthropod groups have a fixed number of legs, e.g. all insects have six legs, all arachnids have eight, all decapods (crabs and lobsters) have ten, etc.
The closest you’re likely to find to a three-legged animal is the kangaroo, which walks with its tail.
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