Lemurs are members of a class of primates known as prosimians and make up the infraorder Lemuriformes. This type of primate was the evolutionary predecessor of monkeys and apes (simians). The term "lemur" is derived from the Latin word lemures, which means "spirits of the night". This likely refers to many of the nocturnal lemur species and their large, reflective eyes. The term is generically used for the members of the four lemuriform families, but it is also the genus of one of the lemuriform species. The two flying lemur species are not lemurs, nor are they even primates.