Syringophilid Quill Mites Obey Harrison’s Rule

Harrison’s Rule (HR) postulates a positive allometry between host and parasite body sizes. We tested HR for Syringophilid quill mites parasitizing birds. Using host body mass and parasite body length as size indices, this pattern was absent in the Syringophilidae family and the Syringophilinae subfamily as a whole. However, when considering the parasite genera as units of study, as proposed originally by Harrison, we found that host body mass positively correlates with both male and female parasite body length in seven genera (Aulobia, Aulonastus, Neoaulonastus, Picobia, Neopicobia, Syringophilopsis, and Torotrogla). Most of these relationships were non-significant. On the contrary, male and female Syringophiloidus mites exhibited negative relationships with host mass (both non-significant). This apparent contradiction disappeared when we applied wing length as an index of host body size. Since species of this genus are specific to the host flight feathers (secondaries and also primaries), wing length is a more meaningful index of host body size than body mass. Overall, most cases corresponded to the positive direction predicted by Harrison when examined on the genus level. This finding also implies a surprising reliability of the genus concept, at least in this group of ectoparasites.