A new species of Psilogamasus Athias-Henriot, 1969 from China and redefinition of the genus (Parasitiformes: Parasitidae)

A new species, Psilogamasus pentatideus Yao and Jin n. sp., is described based on adults from Hainan province, China. Taiwanoparasitus Tseng is newly synonymized with Psilogamasus Athias-Henriot. Four species, Taiwanoparasitus pentasetosus Tseng, 1995, Vulgarogamasus brachysternalis Ma and Lin, 2005, V. longascidiformis Ma and Lin, 2005, V. lingulatus Bai and Ma, 2013, are transferred to Psilogamasus as new combinations. A key to the known species of the genus is given and the diagnosis of the genus is revised.


Introduction
Mites of the family Parasitidae Oudemans, 1901 are distributed worldwide and occur in animal dung, compost, leaf litter, moss, rotting seaweed, soil and the nests of small mammals and birds (Micherdziński 1969; Hyatt 1980; Halliday et al. 2005; Witaliński 2017. The family comprises 46 genera in two subfamilies (Hrúzová & Fenďa 2018; JuvaraBals 2019. Bibliographic analysis by the authors shows that about 130 species in 15 genera of Parasitidae have been described from China. The monotypic genus Psilogamasus, belonging to the subfamily Parasitinae, was erected by AthiasHenriot (1969) on the basis of Psilogamasus hurlbutti AthiasHenriot, 1969 collected in Tanzania. The characters regarded as diagnostic by AthiasHenriot (1969) were: podonotum with 18 pairs of setae, of which seven pairs are short, opisthonotum with six pairs of setae, opisthogaster with five pairs of setae, peritrematal shields free posteriorly, endogynium large, sacshaped, movable digit of female chelicera with four teeth, telotarsus IV with a trichocystic seta (AthiasHenriot, 1969). This species has since been recorded in North Carolina (Hennesey & Farrier 1989), so may be more widely distributed through the world.
In this regard, Taiwanoparasitus Tseng, 1995 is very similar to Psilogamasus. It was a monotypic genus when erected, with Taiwanoparasitus pentasetosus Tseng, 1995 as type species. Recently, Hrúzová & Fenďa (2018) transferred two species from the Chinese mainland, Vulgarogamasus brachysternalis Ma & Lin, 2005 and V. longascidiformis Ma & Lin, 2005, to Taiwanoparasitus (Tseng 1995; Ma & Lin 2005. The generic diagnosis of Taiwanoparasitus was derived from only females of the type species T. pentasetosus and was proposed on the basis of its combination of five pairs of opisthonotal setae and stigmatal position . Hrúzová & Fenďa (2018) pointed out that this genus has similarity with Psilogamasus AthiasHenriot, 1969, which Tseng (1995 did not compare when proposing Taiwanoparasitus, and these genera need further work to determine their taxonomic status. Herein, we describe a new species of Psilogamasus, move four species into the genus, and redefine the genus. This broader concept of the genus necessitates the synonymy of Taiwanoparasitus under Psilogamasus, as alluded to by Hrúzová & Fenďa (2018).

Material and methods
Mites were extracted from rotten leaves by Berlese Tullgren funnels, each equipped with a 40 or 60 Watt bulb, and stored in 75% alcohol. Mites were cleared in Nesbitt's solution and then mounted on slides in Hoyer's medium. Specimens were examined using a Nikon DS-Ri2 microscope. All measurements were taken with the software Leica Application Suite V 4.4 for all available specimens and are given in micrometers (μm).
Idiosomal length was measured, in dorsal view, from the anterior margin of the podonotal shield to the posterior margin of the idiosoma and idiosomal width was taken at the widest point. The length of dorsal shields was measured medially from the anterior to the posterior margins, the width was measured at their widest points. Legs were measured from the base of the coxa to the distal end of the tarsus, excluding the ambulacrum (stalk, claws and pulvillus), and the palps were measured from the base of the trochanter to the distal end of the tarsus. Setae were measured from the alveolus to the apex of the seta.

Redefinition of Psilogamasus with four new combinations
Genus Psilogamasus Athias-Henriot, 1969 Type species: Psilogamasus hurlbutti AthiasHenriot, 1969 Diagnosis Both sexes. Dorsal idiosoma with less than 30 pairs of setae, of which seven or eight pairs of podonotal setae very short; setae z5 of dorsal hexagon similar to j5 and j6 in form (smooth), while different in length (z5 longer); presternal platelets absent; tritosternum biramous; opisthogaster bearing five to six pairs of setae; setae ZV1 and gland pores gv2 absent; seta al of palp femur comblike, al1 and al2 of palp genu entire and spatulate distally; corniculi small; epistome trispinate, central prong apically with two to five branches or multidentate, lateral prongs spiculate with bifid apex or two to four apical branches.
Female with separated podonotal shield that with 16-18 pairs of setae, and opisthonotal shield that with five to six pairs of setae; genital shield triangular or subtriangular; peritrematal shield anteriorly fused to podonotal shield and posteriorly free; movable digit of chelicerae with three or four teeth; setae av1 and av2 on femur II, av1 on genu II and tibia II acicular.
Male with holodorsal shield and transverse suture in central region; movable digit of chelicera with five or six teeth; two ventral setae on femur II modified into spurs fused at base; one ventral seta on genu II and tibia II each modified into spur.

Remarks
In Hrúzová & Fenďa (2018), the genera Psilogamasus and Taiwanoparasitus were separated by the number of setae on the opisthonotal shield (six and five pairs, respectively) and the presence of trichocystic seta pd2 on telotarsus IV (present and absent, respectively). However, Table 1 Morphological variations in females of Psilogamasus brachysternalis (Ma & Lin, 2005) n. comb., Psilogamasus hurlbutti (Athias Henriot, 1969), Psilogamasus lingulatus (Bai & Ma, 2013) n. comb., Psilogamasus longascidiformis (Ma & Lin, 2005) n. comb., Psilogamasus pentasetosus (Tseng, 1995)  they also discussed the lack of data on the latter character, leaving the number of opisthosomal shield setae as the critical feature for defining the genera. In regards to the number of opisthosomal shield setae, we explored the constituent species of both genera and found no other character states that were linked with this feature (Table  1). Therefore, we concluded this was best considered an intrageneric feature, as suspected by Hrúzová & Fenďa (2018). Therefore, we consider Taiwanoparasitus a junior synonym of Psilogamasus.
The above action of synonymizing Taiwanoparasitus under Psilogamasus leads to several new combinations, i.e., T. brachysternalis, T. longascidiformis and T. pentasetosus are now included in the genus Psilogamasus.
Psilogamasus brachysternalis (Ma & Lin, 2005) Tichomirov (1969), markedly differs from Psilogamasus by having setae z5 of the dorsal hexagon very similar to j5 and j6 in length, and both podonotal and opisthonotal shields with more than 20 pairs of setae. The female of V. lingulatus differs from the other species of Vulgarogamasus by the following characteristics: z5 longer than j5 and j6, opisthonotal shield with only 6 pairs of setae, peritrematal shield free posteriorly and central prong of epistome multidentate distally and lateral ones spiculate or bifid apically. The features of female V. lingulatus conform with Psilogamasus: peritrematal shield anteriorly fused to podonotal shield, posteriorly free; 16-18 pairs of acicular setae on the podonotal shield, of which 8 pairs are tiny; six pairs of acicular setae on opisthonotal shield; opisthogaster bearing 5 pairs of setae, setae ZV1 and gland pores gv2 absent; central prong of epistome multidentate distally and lateral ones spiculate or bifid apically; endogynial sac large.
The description of P. pentatideus n. sp. and the additions of P. brachysternalis n. comb., P. longascidiformis n. comb. and P. lingulatus n. comb., P. pentasetosus n. comb. permit a revision of the genus diagnosis. Within this genus, the number of setae on the idiosomal shields, dental numbers of cheliceral digits, and the length of the peritrematal groove are variable (Table  1).
However, we add further character states to define the female of Psilogamasus. On the venter, setae ZV1 is absent (erroneously named ZV1 by Tseng (1995) was ZV2) and peritrematal shield free posteriorly; on the dorsum the total number of dorsal setae is less than 30 pairs, and of which z5, j5, j6 are smooth; and setae av1 and av2 on femur II, av1 on genu II and tibia II are acicular.
Legs ( Figure 4E) -Lengths of legs I-IV 879-894, 585-597, 545-559 and 922-968, respectively. Leg IV longer than other legs, Leg Ⅱ stouter than others. Chaetotaxy: Setal complement and arrangement as in female. Two ventral setae on femur II modified into spurs fused at base; one ventral seta on genu II and tibia II each modified into spur. Other characteristics as in female.

Other stage
Unknown.

Etymology
This species is named after its movable digit of chelicera in male with five teeth, what is rare in Parasitidae.

Remarks
The newly described species is morphologically similar to P. pentasetosus in the setal number on the opisthonotal shield and dental number of chelicera movable digit. However, P. pentatideus Yao & Jin n. sp. is different from P. pentasetosus by follows: (1) podonotal shield bearing 18 pairs of setae, vs. 16 pairs in P. pentasetosus; (2) opisthogastric shield bearing five pairs of setae, vs. 6 pairs in the latter; (3) three prongs of epistome each apically divided into two to five branches, vs. central one multidentate distally and lateral ones spiculate or bifid apically in P. pentasetosus.
Key to known species of the genus Psilogamasus