Which four lines mark the middle latitudes?

One is the equator and maybe the tropic of Capricorn and the tropic of Cancer.

Ok that is totally incorrect! the middle lat. are not that at all. the two tropics and the equator are called the lowlat.! I know that doesn't sound right but if you think about it... they are called the low lat. because they are the smallest degree (23.5), or closes to the equator. The Arctic circle and Antarctic circle would be the correct answer. (66.5 degrees)