Fahrenholz’s rule is not a valid methodology for determining species limits in chewing lice (Psocodea, Phthiraptera)

During the 20th century, the taxonomy and classification of chewing lice (Psocodea, Phthiraptera) was under the influence of Fahrenholz’s rule, which states that louse and host phylogenies should mirror each other. Strict implementation of this rule lead to the description of countless taxa based on host associations, with little or no other distinguishing characteristics. Recent data from a multitude of sources indicate that the underlying assumptions of this theory are flawed and that this methodology is therefore not consistent with best practices in louse taxonomy, identification and classification. Here, we summarize the historical development of Fahrenholz’s rule and associated parasitophyletic rules, the evidence for and against these rules, and the pervasiveness of these rules throughout the 20th century. We conclude that the evidence against Fahrenholz’s rule is so overwhelming, that it cannot be recommended as a basis for future investigations, except as a null hypothesis to be tested. Cases where Fahrenholz’s rule applies may exist, but we recommend that each case is examined on its own merits, based on data derived from the lice themselves, and not from preconceived ideas of host specificity or strict adherence to co-speciation between lice and their hosts