[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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Great question! We don’t know why vertebrates got a second pair of legs, since the front limbs evolved from fins, which evolved from gills, but a mutation happened and a second pair of lateral fins showed up near the tail for no reason. More fins might be better, but fish can’t just get more fins.
Insects are the opposite: they, like miriapods and crustaceans, evolved from animals with multiple segments and a pair of legs on each segment who could add segments and legs to their body easily.
And insects don’t always have 6 legs, they have many! As caterpillars insects have many legs, which they lose as the segments fuse together during the final stage of metamorphosis. The head segments get antennae and mouth parts, the thorax segments keep their legs and the abdominal segments lose them all.
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