Corroboration of Harrison's rule for the body size evolution of cleptoparasitic cuckoo bees (Hymenoptera: Apidae: Nomadinae) following their host shifts

Harrison’s rule, which postulates that the body size of parasites and host covary, has been reported in a range of taxa. However, whether the rule applies to cleptoparasitic insects is poorly known. Subfamily Nomadinae, the largest group of cleptoparasitic bees, intrudes the nests of various host bees. In this study, we reassess the phylogeny of Nomadinae to examine the evolution of host use over time at the family level and the evolution of host-parasite body size, including the expanded sampling of the genus Nomada, which exploits the most diverse hosts within the subfamily. Our phylogenetic results are mostly consistent with previous studies, but we infer the tribe Hexepeolini as a sister taxon to the tribe Nomadini. Additionally, the results reveal discrepancies with the traditional species-group concept of Nomada. Ancestral state reconstruction of host use indicates that parasites used closer relatives early in their evolution before attacking less related groups later. Lastly, we confirm Harrison’s rule in Nomadinae, supporting that body size dynamics influence the host shifts of cleptoparasitic bees.