Contributions to the knowledge of Eylaoidea (Acari: Hydrachnidiae) from China

. This paper enriches the taxonomic study of Eylaoidea Leach, 1816 from China. Eylais ( Meteylais ) hamata Koenike, 1897 is described to improve the taxonomy of adult Eylaidae Leach, 1816. Meanwhile, a new genus and a new species Pentachares sinensis Li & Guo gen. et sp. nov. is described and illustrated, which belongs to the subfamily Rhyncholimnocharinae Lundblad, 1936 in the family Limnocharidae Grube, 1859. The diagnosis of Rhyncholimnocharinae is modified. Rhyncholimnocharinae is first reported for Chinese fauna. A new key is provided for the subfamilies, genera and subgenera of Limnocharidae.


Material and methods
A 250 μm mesh size net, two stacked sieves (mesh size: 4 mm above, 250 µm below), a 2 mL dropper and a white tray were used to capture water mites. Specimens were preserved in Koenike's fluid and mounted in gelatin mounting fluid (gelatin 8-10 g, phenol 0.8 g, glycerin 50 mL, distilled water 50 mL) (Jin 1997).
Specimens were observed and illustrated under a Leica DM3000 microscope, and the illustrations were edited with Adobe Photoshop CS6. Specimens were measured using Nikon DS-Ri2 (Gu et al. 2020).
In the SEM observation, specimens are kept overnight in 2% glutaraldehyde at 4ºC. The following day, the samples are dehydrated through a graded ethanol series of 30%, 50%, 75%, 85% and 95% (for 30 min each), then fully dehydrated in 100% ethanol (two times for 6 hours each). After drying (40ºC) in the electric blast drying oven (WGLL-125BE) for 30 min, the specimens are attached to a holder using electric adhesive tape, sputter-coated with gold, examined and photographed with a JCM6000 Desktop SEM (at 15.0 kV).

Description
Male (n = 3) Body. Dull red color. Idiosoma extremely soft and oval; integument with fingerprint-like striae. No sclerites except eye-plate in dorsal view (Fig. 1A). Eye-plate relatively long; eye bridge almost uniformin-width, straight or slightly curved; O 2 inserted into a pair of sclerotic loops on eye bridge; a pair of humps existed sometimes (Figs 1B, 2A-B).
Genitalia. Genital field with numerous tapered setae; a pair of sclerotic genital flaps with many fine setae and two small and narrow transverse genital sclerites surrounding gonopore (Fig. 2C); V 1 free and close to rear side of ACG; acetabula rounded and stalked, occupying all over the integument; excretory pore with a sclerotized ring; E 4 behind the middle of Cx-IV and at the level of the excretory pore (Fig. 1E).
Gnathosoma. Cuticle porous; basal segments of chelicera large, cheliceral claws short and blunt, projecting through the center of the filled wheel-like membrane; pharynx with a pair of bulges at the lateral bottom, two strongly sclerotized rings on pharynx (Figs 1C, 2D). PalP. Five-segmented; P-I with 1-2 dorsal setae, apical setae feathered or smooth; P-II with three setae on dorsum, three lateral setae in a line (feathered or not) on outer side, and 4-5 distal setae (feathered or not) and 1-2 lateral setae on inner side; P-III with six ventrodistal feathered setae on inner side, 5-6 setae in a line (feathered or not) and one anteroventral seta (feathered or not) on outer side; P-IV relatively long, with one seta at about ⅓ of the total length of the segment on outer side, and numerous setae on inner side; P-V with two dorsal and 3-4 lateral setae, and 1-2 heavy setae on outer side, 2-3 setae on inner side, and with a six-toothed claw at distal end (Figs 3C-D, 4A-B).

Remarks
Eylais (M.) hamata is widely distributed all over the world (Piersig 1897(Piersig -1900Halbert 1903;Sezek & Özkan 2000). Lundblad (1936) had reported this species from Northwest China, while the previous descriptions were relatively simple (Uchida & Imamura 1951). Current specimens, which were also collected from Northwest China (Qinghaihu National Nature Reserve), are similar with the description mentioned above, especially their eye plate, which is considered as the most distinguishing feature. The only difference with the Turkish population is the body size: males ranging from 1859 to 2539, females ranging from 1725 to 1824 in our specimens, but with the Turkish population, 2250 for the male and 4200 for the female (Sezek & Özkan 2000). This problem can be interpreted by Lanciani (1969Lanciani ( , 1970 that the size of newly emerged eylaid mites could increase continuously in both sexes, even during their adult stages, and the gravid females could grow twice as long. In summary, we attribute our specimens to E. (M.) hamata. Subfamily Rhyncholimnocharinae Lundblad, 1936Rhyncholimnocharinae Lundblad, 1936: 203. Rhyncholimnocharinae -Cook 1974: 41. -Viets 1987 Diagnosis (after Cook 1974, modified) Character of the family Limnocharidae; gnathosoma attached to a long tubular extension of integument; palp segments 3 (but terminal segment may be greatly reduced giving the appearance of a two-segmented palp), occasionally 2, 4 or 5; dorsum with or without paired sclerites (Tuzovskij & Gerecke 2020).

Habitat
Running waters (sediment of organic detritus, dead wood and leaf litter of pools, or in mosses at stronger flow velocity).

Distribution
Nearctic, Neotropical, Australian and Oriental realm.

Remarks
There are a total of 24 species in this subfamily including the new species herein described (Tuzovskij & Gerecke 2020). Most species have a three-segmented palp, while Rhyncholimnochares tapiarum Tuzovskij & Gerecke, 2020 presents a two-segmented palp and R. expansiseta Cook, 1980 shows the phenomenon of its dorsal and ventral edges of P-I merged with the second segment (Tuzovskij & Gerecke 2020). The palp of Austrolimnochares womersleyi (Lundblad, 1952) has 4 or occasionally 3 segments (Harvey 1990

Etymology
Five-segmented palp is the main diagnosis feature of this new genus. "Pent-" means five, just represents the five segments of palp in this new genus.

Diagnosis
Characters of Rhyncholimnocharinae; palp five-segmented, P-V attached to the middle surface, rather than the distal end of P-IV; dorsalia absent; without swimming setae on legs.

Habitat
Same as subfamily.

Distribution
Oriental realm.

Remarks
The new genus presents the diagnostic features of both two subfamilies in Limnocharidae: a long tube of protrusible integument proves that it belongs to Rhyncholimnocharinae, meanwhile the palp with five segments indicates it is from Limnocharinae. The phylogenetic value of the palp-fused as a character has been questioned (Cook 1974;Gerecke et al. 2020). Therefore, we arrange the new genus in Rhyncholimnocharinae, according to the feature of a tubular extension of integument.

Diagnosis
Capitulum attached to a long tubular extension of integument; palp five-segmented, P-V attached to membranous protuberance at medioventral surface of P-IV, heavy seta and a pair of setae at distal end; dorsum without sclerites except for the frontal plate; on frontal plate, median eye invisible, rod-like O 1 and A 1 sitting at anterior part, A 2 at the middle of eye capsules, bifurcated O 2 near the middle of posterior part; 50-60 acetabula per side of genital field; excretory pore with a sclerotized ring; legs without swimming setae, claws simple.

Etymology
Named after the country where the specimen collected. "Sina-" means China.

Description
Female (n = 1) Body. Amber in color, lateral eyes red, gnathosoma invisible in dorsal view when alive (Fig. 6). Idiosoma extremely soft and extensible, integument papillate. Dorsum without sclerites in addition to frontal plate (Fig. 7A). Gnathosoma porous, in the ventral view rounded, rostrum relatively short; mouth disk with a loop of fine setae; porous basal segment of chelicera relatively large, proximal part of cheliceral stylet connected with basal segment vertically (Fig. 7B). Anterior part of frontal plate longer than eye capsule region; anterior margin concave, sclerotized inward; median eye invisible; O 1 rod-like and terminal expanded; A 2 at the middle of eye capsules; O 2 on the convex lateromedial plate and bifurcated; posterior part of frontal plate long, lateral margins covered by small tubercles densely; posterior margin straight; keel approximately as long as the frontal plate (Fig. 7C). PalP. Five-segmented; P-I without seta; P-II with one dorsal and 3-4 ventral setae; a dorsodistal and 2 ventrolateral setae on P-III; P-IV with three ventral setae and one lateral heavy seta; P-V attached to a membranous protuberance at medioventral surface of P-IV, and bearing three setae at distal end ( Fig. 8A-B).  Coxae. Cx-I inner-apical angles connected by a narrow bridge. Cx-I anterior margins concave, with uniformly stout setae. E 1 and E 2 on the membranous interspace between coxal groups. Cx-III triangular; Cx-IV elongated and triangular; all coxae porous (Fig. 8C).
leGs. With numerous stout setae except by the swimming setae, terminal segments of all legs with a loop of short setae; claws simple ( Fig. 9A-D).

Male
Unknown.

Eylaidae: about taxonomy of Eylaidae
There are some problems with the current taxonomy of Eylaidae.
(1) The classification is mixed. Some taxa are based exclusively on the larval stage (e.g., subgenus Spineylais Wainstein, 1968 and about 20 species), and some others on the adult stage, in consequence, it is not comparable (Lanciani 1969(Lanciani , 1970Cook 1974;Smith 1986).
(2) The classification of the adults relied on few features, such as eye plate. But polymorphic phenomena often occur on eye plate within the same species (Halbert 1903). This classification system is relatively confused and consequently arbitrary. More detailed features of adults are needed to identify and achieve the correspondence between the two systems, which is crucial.

Limnocharidae: about Laterolimnochares and Pentachares Li & Guo gen. nov.
Laterolimnochares Jin, 1999 and Pentachares Li & Guo gen. nov. are both from Oriental realm. Their morphological similarities are in the following aspects: 1) the shape and the chaetotaxy position of frontal plate; and 2) the structures of palp, especially P-V attached to the middle surface, rather than the distal end of P-IV. Meanwhile, their similar characters are obviously distinct from other Limnocharid genera.
According to Tuzovskij & Gerecke (2020), the frontal plate is of great significance in the taxonomy of Limnocharidae as a special feature. And as for palp, it plays an important role in the identification of mites. All mentioned above indicate that Laterolimnochares and Pentachares Li & Guo gen. nov. could have a close systematic relationship. But, they are attributed to different subfamilies due to the latter presents a long protrusible tube of soft integument. We have to consider if the protrusible integument of Laterolimnochares has degenerated, or the phylogenetic value of this feature should be questioned. For this problem, the future work should be focus on discovering more new species, improvement of larval stage taxonomy and applications of molecular technology.