This document discusses the diversity, physical characteristics, distribution, and coevolution of lice (Phthiraptera). It notes that there are over 4,000 species of chewing lice that parasitize birds and over 400 species that parasitize mammals. Lice are wingless, flattened insects that feed on feathers, fur, skin or blood. Their distribution mirrors that of their hosts worldwide. The document examines the cophylogeny, cospeciation and coevolution between lice and their hosts.