Males with no “head”: evolutionary audacity in a remarkable new podapolipid mite genus (Acari: Heterostigmata) parasitising hydrophilid beetles

Mites of the family Podapolipidae (Heterostigmata: Acariformes) are among the most morphologically diverse parasitic arthropods, exhibiting extreme specialisation associated with insect hosts from several orders. Current records substantially underestimate their host range; despite high diversity on Coleoptera, these mites are recorded from only a handful of terrestrial beetle families. Here, we describe a remarkable new genus and species, Hydrophilopolipus hajiqanbari gen. nov., sp. nov., from the hydrophilid beetle Coelostoma fabricii (Montrouzier) in Australia, expanding podapolipid host lineages to include semi-aquatic beetles. The new genus exhibits numerous apomorphic features, several of which appear to be unique within Podapolipidae, most strikingly in males, which show a drastic reduction of the gnathosoma and complete loss of the chelicerae. Moreover, in all life stages, idiosomal setae v1 and 3a are reduced to alveoli, setae c1 are absent, and the tibial solenidion ( φ ) on leg I is lacking. Males also lack plate EF and setae f , and larvae have separate plates C, D, and EF but lack plate H and setae h 2. Despite these reductions, we argue a sister-group relationship with the carabid-associated genus Eutarsopolipus , the only other podapolipid genus with both hexapod females and hexapod males bearing terminal genitalia. We discuss the evolution of “headless” males, and other reductive features, in the context developmental progenesis, suggesting that similar evolutionary trajectories may be more widespread within Podapolipidae and other parasitic mites than currently recognised, particularly among lineages associated with poorly explored non-terrestrial hosts. • New podapolipid mite genus discovered on semi-aquatic beetles • First record of Podapolipidae associated with Hydrophilidae • Multiple apomorphic reductions are shared across all life stages • Males exhibit extreme gnathosomal reduction and loss of chelicerae • Comparative morphology predicts close relationship with Eutarsopolipus