A century ago Lancelot Harrison (1915) concluded that in host-parasite relationships a bigger host has bigger parasites than the small ones. This positive correlation, known as Harrison’s rule, was tested in many studies and seems to be true in many different parasite taxa. This hypothesis was supplemented by Robert Poulin. He proposed that not only the mean, but also the variance of parasite body size should increase with the increasing host body size, because small parasites species can live in both small- and large-bodied hosts. This latter hypothesis has never been tested. The aim of this study was to test Harrison’s rule and Puolin’s hypothesis across the louse families Ricinidae and Philoteridae (Insecta: Phthiraptera).