Arthropods seem to come in any even number of legs above 4, e.g. 6 (insects), 8 (arachnids), ... and so on up to the very many legs of the millipedes.

But are there, or have there ever been, any 4-legged arthropods, and if not, why not? (Particularly given that 4 legs or fewer seems to do just fine for all higher animals)

why is it instinctive for us to cross one leg over the other when we arn't sitting beneath a table or desk, but sitting on a chair, such as in a bus, in a theatre, or on a park bench? Do we do it because it's uncomfortable sitting with both legs down?

None of the countless species of animal in existence has three legs. Creatures such as the kangaroo and the meerkat use their tails for balance, but a tail is plainly not the same as a leg. This pattern does not apply only to mammals - other kinds of animal have an even number of legs, too. Why wouldn't having three legs work?

Monika Hofman, London, UK