Phthiraptera systematics: past, present, and future

Parasitic lice (Psocodea: Phthiraptera) are small, wingless parasites of birds and mammals that have been at the forefront of coevolutionary studies since the 1990s, and at the forefront of human–insect interactions since the dawn of humankind. Despite this, an unfortunate overreliance on a co-radiation model of evolution between lice and their hosts (“Fahrenholz’s Rule”) has left a muddled legacy in their classification and taxonomy. In recent decades, the classifications of lice at the order, suborder, family, genus, and species levels have all been examined by a mixture of morphological, genetic, and genomic data, which have challenged many traditional taxon limits and relationships. Here, we discuss the traditional classification of parasitic lice, the data that challenged it, and our current understanding of the group. We give an overview of the morphological characters that identify the major radiations of lice, and point out a number of areas that need further work.