Lice (singular: louse), are wingless insects that live in hair. They are all external parasites on every species of bird and most mammalian orders. They are not found on Monotremes and a few eutherian orders, namely bats (Chiroptera), whales, dolphins and porpoises (Cetacea) and pangolins (Pholidota). There are more than 3,000 different species; three are classified as human lice.