A new genus and species of Digamasellidae (Acari: Mesostigmata) displaying some extraordinary gnathosomal structures

A new genus and species of digamasellid mites is described based on adult females and males. Bulbolaelaps bossei n. gen., n. sp. was collected from the fruiting bodies of gilled fungi on a tree trunk in Zanjan province, Iran. The new genus can be distinguished from the other genera of Digamasellidae by having a bulbous swollen protuberance on the venter of the palptrochanter; corniculi weakly formed and epistome weakly sclerotised; hypostomal setae h1 and h2 positioned more or less transversely; deutosternal groove narrow with limited number of denticles; ventral base of female cheliceral digit with a spine-like projection; and setae ad1 and pd1 tarsi II–IV narrowly lanceolate apically.

Members of the family Digamasellidae are mainly found in decaying organic material, on the soil surface, in galleries of bark beetles, under bark of trees, manure, and in bracket fungi where they feed on small arthropods, nematodes and even fungi. Deutonymphs of some species are phoretic on insects (Karg, 1993; Lindquist et al., 2009. The purpose of this study is to described a new genus and species of digamasellid mite found in association with gilled fungi from Iran.

Material and methods
Mites were collected by direct removal from gilled fungi and then preserved in 70% ethanol. Specimens were cleared in a mixture of Nesbitt and lactophenol solutions 1:1, and mounted in modified Hoyer's medium as described by Faraji & Bakker (2008). Specimens were examined using a Leica DM 2500 light microscope equipped with differential interference contrast (DIC) and drawings were made with the aid of a camera lucida (drawing tube) attached to an Olympus phase contrast microscope. All measurements are in micrometres (μm). Fixed cheliceral digit measured from the anterior part of dorsal lyrifissure to apical hook. The mean of the measurements is given followed by the range in parentheses. The setal notations for the idiosoma follow Lindquist & Evans (1965), and leg chaetotaxy follows Evans (1963). Notations of idiosomal solenostomes (pore-like structures) as well as lyrifissures (poroids) follow Athias-Henriot (1975).

Results
Type species: Bulbolaelaps bossei Diagnosis Podonotal and opisthonotal shields separated in both sexes; in males, opisthonotal shield fused with opisthogastric shield; scleronoduli present; almost all dorsal shield setae barbed (except S5, long, simple and mainly coiled), most of them apically bulbous; sternal seta St3 migrated inwards; palps with bulbous swollen protuberance on the venter of the palptrochanter; corniculi weakly formed and epistome weakly sclerotised and fimbriated dorsally; hypostomal setae h1 and h2 aligned more or less transversely; deutosternal groove narrow; ventral base of female cheliceral digit with a spine-like projection; setae ad1 and pd1 on legs II-IV narrowly lanceolate apically.
Etymology The specific name bossei is after Mr. Theo Bosse, the facility manager of Eurofins MITOX BV, who has kindly supported and provided the senior author with research tools, equipment and chemicals during many studies.
Type material and depository Holotype female, 22 July 2011, from fruiting bodies of wet gilled fungi (possibly a species of Pleurotus sp., fam. Pleurotaceae) on a tree trunk, Chiyar, Zanjan Province, Iran (36°43 ′ 47.1 ″ N 48°18 ′ 28.6 ″ E), deposited in JAZM, collector: Mohsen Zare; six females and eighteen males, same data as holotype: one paratype female and four paratype males deposited in JAZM; two paratype females and four paratype males deposited in ANIC; one paratype female and three paratype males deposited in SMNG; one paratype female and three paratype males deposited in BMNH; one paratype female and three paratype males deposited in OSAL.
Remarks The new genus mainly differs from the other genera of Digamasellidae by the following apmorphic attributes: (1) a bulbous (membranous) swollen protuberance at the venter of the palptrochanter. This feature is unique in the family Digamasellidae. A similar feature "membranous flap" was reported by Costa (1968), Moraza (2019) and Nemati et al. (2019) for the genus Reticulolaelaps Costa (Laelapidae). By dissecting the palps, Nemati et al. (2019) showed that membranous flap was connected to the inner part of the palptrochanter, more or less the same location as for the new genus here. The swollen protuberance in Bulbolaelaps is threedimensional and kept its shape even after re-mounting and for a dissecting the specimen. For Reticulolaelaps the membranous flap might have been three-dimensional in living individuals but after mounting and dissecting it became flattened. (2) Corniculi weakly formed and epistome weakly sclerotised. Even with a DIC microscope and with dissected specimens, these two features are not easy to observe. Corniculi of the other genera of digamasellids are well sclerotised and mainly horn-like (bifid in the male of Longoseius Chant, 1961 and with a curved groove in the male of Panteniphis Willmann, 1949). The epistome of Bulbolaelaps n. gen. is triangular and fimbriated dorsally while the other genera have epistome mainly triramous or occasionally biramous (in Longoseius). Orientolaelaps Bregetova & Shcherbak, 1977 shows a triangular shape but in fact it can be considered triramous with reduced median and lateral tines; (3) Hypostomal setae h1 and h2 more or less transversely aligned. In all other genera of digamasellids, seta h1 is positioned far anterior to seta h2; (4) Deutosternal groove narrow and weakly developed with four transverse rows of denticles, the most distal one smooth, the three middle ones each unidentate, the proximal row much wider and multidenticulate. In all other genera of digamasellids the deutosternum has a relatively wider groove with five (rarely six) rows of denticles with the proximal row much widened; (5) Ventral base of female cheliceral digit with a spine-like projection. In the other genera of Digamasellidae this projection does not exist; (6) Setae ad1 and pd1 on tarsi II-IV narrowly lanceolate apically. This feature is not present in any other digamasellid genera or other closely related families; and (7) All dorsal shield setae barbed, and most knobbed apically. This feature does not exist in any other digamasellid genera except Dendrolaelaspis Lindquist, 1975, which shows some posterior opisthonotal setae stout, spatulate and a few setae serrate and Dendrolaelaps quadritorus (Robillard) shown to have two pairs of opisthonotal setae stout, spatulate (Lindquist 1975; Robillard 1971.

Discussion and conclusion
According to the key to families of order Mesostigmata (Lindquist et al., 2009) as well as definitions provided by Lindquist (1975), Shcherbak (1980) and Castilho et al. (2012), we have placed Bulbolaelaps bossei n. sp. in the family Digamasellidae based on the following key morphological characters: Dorsal shield divided; two pairs of scleronoduli present; S5 long and coiled; females with four pairs of setae and three pairs of poroids on the sternal shield; palptarsal apotele 2-tined; basal row of deutosternal denticles greatly widened and more multidenticulate; 13 setae on femur I; 12 setae on each of genu and tibia I, and seven setae on each genu and tibia IV; males with setae St5 on separate triangular parapodal shields; ventrianal shield of males merged laterally and posteriorly with opisthonotal shield; and femur, genu, tibia and tarsus of leg II in males each with ventral setae thickened and robust.
There are some key morphological characters in Digamasellidae to separate genera and to propose sister groups. These are: the basal row of deutosternal denticles being greatly widened and more multidenticulate, the male setae St5 on separate triangular parapodal shields, dorsal shields separated, presence of scleronoduli, location of spermathecal opening (coxa III or IV) and anterior migration of the opisthonotal shield without median notch. The first two characters seem to be strong apomorphic traits at the family level. Unfortunately, males of a few genera have not been described yet. Also, the location of spermathecal solenostome in Bulbolaelaps n. gen. is not known. Considering these characters, Bulbolaelaps n. gen. can be grouped with Insectolaelaps Shcherbak, 1980, Multidendrolaelaps Hirschmann, 1974and Oligodentatus Shcherbak, 1980 Because only the adults were collected from the fruiting bodies of gilled fungi, we are not confident about the true habitat of this species. The apparent absence of the juveniles from the fungi might suggest another habitat for development of juveniles. If immatures also inhabit fungi, their absence may be explained by two reasons: the collector picked up the large individuals (adults) which with naked eyes were easily detectable or almost all immatures had already reached to adulthood before the moment of sampling. The presence of a bulbous (membranous) swollen protuberance on the venter of the palptrochanter may be an adaptation for optimal feeding. This protuberance is located exactly in front of hypostomatic setae h1 and h2. Migration of seta h1 backwards might be an adjustment for the presence of this protuberance. It will be interesting to find whether juveniles show this feature as well. The reduced form of corniculi, which are to the contrary stronger among some gamasines associated with fungi, may indicate this species as a liquid feeder. As a matter of fact, some gilled fungi exude beads of moisture, called guttation (Parmasto and Voitk, 2010). Interestingly, this species was collected from gilled fungi completely covered by moisture and individuals were walking easily on the wet surface. It is possible that the gnathosoma has evolved in a way to consume guttated drops. On the other hand, the chelicerae of this species look very robust suitable for crushing fungal hyphae rather than sucking fluids. Catching live individuals and rearing them in the laboratory would solve feeding habits of this species.
Having most of dorsal shield setae relatively long, barbed and knobbed apically as well as modified form of apical setal pair d-1 of tarsi II-IV may give the mite an advantage to move freely among fungal gills as well as acting as tactile sensory giving information about the location of gills.
Dispersal of this species is another interesting topic that needs a further investigation, with the deutonymph being the phoretic instar in digamasellids. Thunes et al. (2000) found 36 species of beetles associated with a fungus (Fomitopsis pinicola) in a Norwegian spruce forest, and Yamashita et al. (2015) reported 82 coleopteran species associated with bracket fungi in a Bornean tropical rain forest. It is possible that deutonymphs of this new species uses adult coleopteran species for dispersal to new habitats.
A revision of the genera of Digamasellidae is still required especially for the status of the two enigmatic genera of Panteniphis and Pontiolaelaps. The definition for the family Digamasellidae should also be widened based on the new discoveries in this study. A comprehensive study including molecular research is needed to determine the phylogenetic relationship among digamasellid genera.