[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?

Question

[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?

In: 381

Answer 1

Insects always have six legs, because the first insect to evolve happened to have six legs, and there was no evolutionary pressure to change that. Animals have four limbs because the first one did, and therefore so did all its descendants. There has occasionally been evolutionary pressure towards bipedal motion in some species, leading to two legged creatures, but there has to be a huge benefit to using two of your limbs as arms or wings for that to happen, because standing upright on two legs requires much more effort than standing on four legs.