Parasitic lice comprise over 5000 species of insects that parasitize birds and mammals. Parasitic lice are now generally classified into a single order (Phthiraptera), because it has long been recognized that sucking lice are derived from chewing lice. Understanding the origins of parasitic lice has been more difficult, because their simplified body form makes identifying morphological homologies with other insects difficult. Morphological and recent molecular evidence indicate that parasitic lice are derived from within the bark lice (Psocoptera). Even more surprising is a result from recent morphological and molecular analyses indicating that parasitic lice may be derived two times independently from within this group of insects. The origins of parasitism and the diversification of the major louse lineages appears to have occurred before the Cretaceous-Tertiary boundary. Within avian Ischnocera, there has been repeated convergence of microhabitat specializations of lice within various groups of birds.