Megisthanus leviathanicus sp. nov. (Parasitiformes: Megisthanidae), the largest known Mesostigmata, a symbiont of the beetle Mastachilus australasicus (Coleoptera: Passalidae).

Megisthanus leviathanicussp. nov. is described from the passalid beetleMastachilus australasicusfrom Australia. The adult male is the largest of all known Mesostigmata with one specimen measuring 5.05 mm in length. The sexes are dimorphic: female mites are smaller, have numerous elongate setae, and a lyriform dorsal shield, while the major males have minute dorsal setae and a subovate dorsal shield. Some minor males (n = 4/48) resemble the female in having numerous elongate dorsal setae. Usually only one major male was found on a beetle, but rarely a minor male also occurred, suggesting that a major male defends a beetle and that minor males share access to females. Both sexes had considerable variation in the secondary sclerotization around their dorsal shields. This variation requires some adjustments to the current taxonomy. HereinMegisthanus caudatusis considered a senior synonym ofM. brachyurussyn. nov.andM. testudosyn. nov., asM. brachyurusis based on a female without secondary sclerotization andM. testudoon the male ofM. caudatus. The Australian speciesMegisthanus thorelli, and the North American speciesMegisthanus floridanus, are diagnosed. The diagnostic character states for megisthanid species are explored, especially the female’s internal genitalia. http://www.zoobank.org/urn:lsid:zoobank.org:pub:FB6B6509-64A6-4454-8B58-F412FA2833C1 [ABSTRACT FROM AUTHOR]