Two New Phoretic Species of Heterostigmatic Mites (Acari: Prostigmata: Neopygmephoridae and Scutacaridae) on Australian Hydrophilid Beetles (Coleoptera: Hydrophilidae)

Simple Summary Heterostigmatic mites are globally very diverse, and many are generally phoretic or parasitic on insects. However, the Australian fauna of phoretic heterostigmatic mites is almost unexplored. Here, we describe two new species of Allopygmephorus and Archidispus (Acari: Prostigmata: Heterostigmata) found phoretic on semiaquatic beetles. Our findings report both genera for the first time from Australia. Abstract Many heterostigmatic mites (Acari: Prostigmata: Heterostigmata) display a wide range of symbiotic interactions, from phoresy to parasitism, with a variety of insects. Australia is expected to harbour a rich diversity of heterostigmatic mites; however, its phoretic fauna and its host associations remain mainly unexplored. We conducted a short exploration of Australian insect-associated phoretic mites in summer 2020 and found two new phoretic heterostigmatic species on a semiaquatic hydrophilid beetle species, Coelostoma fabricii (Montrouzier, 1860) (Coleoptera: Hydrophilidae). Here, we describe these two new species, Allopygmephorus coelostomus sp. nov. (Neopygmephoridae) and Archidispus hydrophilus sp. nov. (Scutacaridae), which both belong to the superfamily Pygmephoroidea. Both species are distinct from their congeners, with a plesiomorphic character, bearing a median genital sclerite (mgs). Our study reports both genera for the first time from Australia.


Introduction
Heterostigmatic mites (Acariformes: Prostigmata) are morphologically diverse, and numerous species are associated with a variety of insects [1,2]. Their associations with insects vary from facultative or obligate phoresy to parasitism [1]. Many species are freeliving, and sometimes hitchhike on insects to new environments, or reside in nests of social insects [2]. Some of them are highly host-specific and they sometimes exhibit special attachment site preferences on the hosts' body, such as beneath the abdomen or thorax, between leg coxae, on cervix or wings, or under elytral spaces [3][4][5]. Females in phoretic species have generally evolved several morphological adaptations, such as a robust pair of legs I with consolidated tibiotarsi I associated with enlarged claws, which help them to firmly attach to their hosts during phoretic dispersal [1,6].

Differential Diagnosis
The new species is readily distinguishable from all other congeners (listed in Table 1) due to the presence of mgs. The remarkable shape of seta 1b (with a notch or slit in the distal half) has also never been described in any Allopygmephorus, but the same character was detected in A. persicus following a detailed microscopy (A. Khaustov; personal communication), although this was not depicted in its original description [36].

Differential Diagnosis
This species is readily distinguishable from all other species of the genus due to the shape of seta 1a (thickened, blunt-ended and smooth) and presence of mgs; regardless, A. hydrophilus sp. nov. is most similar to A. kazuyoshikurosai Khaustov based on setae 3b, 4a and 4b being thickened in the basal half and thin in the distal half; however, it differs from the new species as all dorsal setae are moderately pointed (most dorsal setae are distinctly blunt-ended in A. kazuyoshikurosai), and setae 1a being short, spiniform and blunt-ended

Discussion
The host beetle genus Coelostoma shows the greatest diversity in the Afrotropical and Oriental regions, with a few species in the Palearctic and Australian realms [49][50][51][52]. Although many hydrophilid beetles are aquatic, some are semiaquatic, including Coleostoma, which resides in wet grasslands near riverbanks and ponds. Here, they live under wet rocks or among grass roots, where they feed on decomposing organic matter [51], which is also an ideal substrate for the growth of fungal mycelia-the likely food source for the phoretic Allopygmephorus and Archidispus mites. Many Coelostoma species are nocturnal: during the day, they usually hide under moss or the roots of plants growing next to the watercourse, and may be found feeding on wet and submerged surfaces, including those of wet rocks and artificial concrete surfaces, at night. Some species may be collected from mud or from underneath wet leaf litter; few species are only found in interstitial habitats under stones and among gravel at the sides of stony riverbeds [51] Allopygmephorus is well-known from sodden and rotting vegetation matter or from the beetles that frequent these habitats, especially Hydrophilidae and Heteroceridae (Table 1). The genus is widespread, occurring in the Afrotropical, Indo-Malayan, Neotropical and Palearctic regions. Its discovery in the Australian realm is, thus, not surprising, although its presence on C. fabricii, the sole Australian representative of this large Afrotropical-Oriental beetle genus, suggests it may have arrived, together with its carrier, from the Oriental realm. However, this hypothesis makes the unconvincing assumption that A. coelostomus has a close host relationship with its carrier. Previous collections of Allopygmephorus suggest a low host specificity in the group, with mites riding on several families of beetles that presumably visit the same habitat (Table 1).
Scutacarid mites generally use their insect hosts for phoretic dispersal between ephemeral habitats; however, some can be inquilines of their hosts' nests. It is hypothesized that some species may play a sanitary role in their hosts' nests [24]. Some genera show strong preferences towards special insect groups, while others are comparatively less host-specific [24]. Archidispus is a group of scutacarid mites with a strong preference for phoresy on carabid beetles [24]. Records from other beetle species are rare but do include a record of A. bembidii from Coelostoma hydrophilid beetles [9]. Similarly to Allopygmephorus, numerous species of Archidispus utilize several carrier species and it remains Idiosomal dorsum (Figures 6a and 8a): Tergite C weakly dimpled, other tergites smooth; prodorsal shield (PrS) entirely covered by tergite C, with one pair of clavate trichobothria (sc1) (Figure 5c (30)(31)(32)(33)(34).

Differential Diagnosis
This species is readily distinguishable from all other species of the genus due to the shape of seta 1a (thickened, blunt-ended and smooth) and presence of mgs; regardless, A. hydrophilus sp. nov. is most similar to A. kazuyoshikurosai Khaustov based on setae 3b, 4a and 4b being thickened in the basal half and thin in the distal half; however, it differs from the new species as all dorsal setae are moderately pointed (most dorsal setae are distinctly blunt-ended in A. kazuyoshikurosai), and setae 1a being short, spiniform and blunt-ended (setae 1a longer and beak-like in A. kazuyoshikurosai), setae pl" on tarsus IV with blunt-tipped rod (pointed in A. kazuyoshikurosai).

Etymology
The species epithet 'hydrophilus' means water-loving and refers to the humid environment in which the hydrophilid host beetle of this species and most likely the species itself dwell.

Discussion
The host beetle genus Coelostoma shows the greatest diversity in the Afrotropical and Oriental regions, with a few species in the Palearctic and Australian realms [49][50][51][52]. Although many hydrophilid beetles are aquatic, some are semiaquatic, including Coleostoma, which resides in wet grasslands near riverbanks and ponds. Here, they live under wet rocks or among grass roots, where they feed on decomposing organic matter [51], which is also an ideal substrate for the growth of fungal mycelia-the likely food source for the phoretic Allopygmephorus and Archidispus mites. Many Coelostoma species are nocturnal: during the day, they usually hide under moss or the roots of plants growing next to the watercourse, and may be found feeding on wet and submerged surfaces, including those of wet rocks and artificial concrete surfaces, at night. Some species may be collected from mud or from underneath wet leaf litter; few species are only found in interstitial habitats under stones and among gravel at the sides of stony riverbeds [51] Allopygmephorus is well-known from sodden and rotting vegetation matter or from the beetles that frequent these habitats, especially Hydrophilidae and Heteroceridae ( Table 1). The genus is widespread, occurring in the Afrotropical, Indo-Malayan, Neotropical and Palearctic regions. Its discovery in the Australian realm is, thus, not surprising, although its presence on C. fabricii, the sole Australian representative of this large Afrotropical-Oriental beetle genus, suggests it may have arrived, together with its carrier, from the Oriental realm. However, this hypothesis makes the unconvincing assumption that A. coelostomus has a close host relationship with its carrier. Previous collections of Allopygmephorus suggest a low host specificity in the group, with mites riding on several families of beetles that presumably visit the same habitat (Table 1).
Scutacarid mites generally use their insect hosts for phoretic dispersal between ephemeral habitats; however, some can be inquilines of their hosts' nests. It is hypothe-sized that some species may play a sanitary role in their hosts' nests [24]. Some genera show strong preferences towards special insect groups, while others are comparatively less host-specific [24]. Archidispus is a group of scutacarid mites with a strong preference for phoresy on carabid beetles [24]. Records from other beetle species are rare but do include a record of A. bembidii from Coelostoma hydrophilid beetles [9]. Similarly to Allopygmephorus, numerous species of Archidispus utilize several carrier species and it remains unknown if A. hydrophilus is specific to C. fabricii or is opportunistically phoretic on various semiaquatic beetles. Broader surveys are needed to provide more information and uncover more of Australia's vast undescribed fauna of phoretic and other mites.