Description of Nilgiriuspygoprominulus sp. n. (Opiliones, Assamiidae, Trionyxellinae) from China, with notes on its sexual dimorphism

Abstract Background A new species of Nilgirius, N.pygoprominulus sp. n. (male and female) in the family Assamiidae from Yunnan Province, China, is described and illustrated herein. Sexual size dimorphism (male larger than female) is inconsistent with most assamiids. Other sexually dimorphic features (body shape, leg IV and pseudonychium) are reported. New information Nilgiriuspygoprominulus sp. n. is described as a new species of Trionyxellinae. Information about sexual dimorphism of the species is reported.


Introduction
Assamiidae Sørensen, 1884 is one of the largest families of Opiliones, consisting of more than 450 described species (Kury et al. 2022b).They are widely distributed in the Afrotropics, Australasia and the Indian subcontinent, with the greatest diversity of genera and species found in sub-Saharan Africa.The classification of Assamiidae has gone through several stages, from three subfamilies (Assamiinae Roewer, 1912, Dampetrinae Sørensen, 1886and Trionyxellinae Roewer, 1912) to 17 subfamilies by Roewer (1912) and Roewer (1935).Martens (2022) proposed a new subfamily, Filopalpinae Martens, 2022, which includes five new species.Thus, currently, Assamiidae comprises 18 subfamilies.
Assamiids exhibit a wide range of morphological diversity.Some species are completely blind, such as Irumuinae Kauri, 1985, while others have a hypertrophied fourth leg coxa and scutum in males (Paktongius Suzuki, 1969 andMysorea Roewer, 1935).The body size of these arachnids can range from 2 to 8 mm.The carapace of the species has a row of five sharp spines on the anterior margin and the pedipalpi are flattened and crossed in front of the body.Recently, Palmieri et al. (2023) conducted a molecular phylogeny of Assamiidae using a ten-locus Sanger dataset.They sampled key taxa from both the Afrotropics and Australasia and recovered the Assamiidae as a monophyletic group, including Trionyxellidae.However, the sister group of Assamiidae remains uncertain.
The genus Nilgirius Roewer, 1915 (Trionyxellinae) is considered monotypic and is known only from a single specimen of the nominate species N. scaber Roewer, 1915.This specimen was collected from the region of Nilgiri Hills, located in the south-eastern State of Tamil Nadu, India (Roewer 1915).One of the distinguishing characteristics of this species is its peculiar feature of a pair of spines on the interocular mound.As additional specimens were collected from various regions in India, Roewer (1923), Roewer (1929), Roewer (1935) and Roewer (1939) subsequently recorded this species on multiple occasions (Fig. 1).
In the past two decades, taxonomists have described some species of assamiids (Bauer and Prieto 2009;Santos and Prieto 2010;Zhang et al. 2010;Lotz 2011;Zhang and Zhang 2015;Martens 2022), but more attention needs to be given to the detailed descriptions of the functional morphology of male genitalia.To enhance the knowledge of the morphological features of Assamiidae, particularly Trionyxellinae, we describe and illustrate the expanded and unexpanded male genitalia of Nilgirius pygoprominulus sp.n.Furthermore, the sexual dimorphism of the new species is also recognised.

Materials and methods
The specimens were preserved in 75% ethanol, examined and drawn under a Leica M205A stereomicroscope, equipped with a drawing tube.Photographs were taken using a Leica M205A stereomicroscope, equipped with a DFC 450 CCD.Tarsal claw of leg III and leg IV were photographed using an Olympus microscope, equipped with a KUY NICE CCD camera.Individual images were compiled into a composite image using Helicon Focus (htt p://www.heliconsoft.com/helicon/heliconfocus.html) and edited using Adobe Photoshop CS3.The male genitalia were initially placed in hot lactic acid (40-50℃) for about 1-2 min, then transferred to distilled water; the movable parts of the glans will mostly expand within 1 min (Schwendinger and Martens 2002).The terminology of genital structures follows Martens (1986) and Macías-Ordóñez et al. (2010) and the macrosetae terminology of male genitalia follows Kury and Machado (2021).Terminology for the outline of the dorsal scutum follows Kury and Medrano (2016).Type specimens of the new species are deposited in the Museum of Hebei University, Baoding, China (MHBU).All measurements are given in mm.

Notes
Only one species was included in the genus before the description of this new species.convex, narrower than mid-bulge.Anterior margin of carapace with two spines at the lateral portion and a single median spine, all similar spines directed horizontally, the middle one is the smallest.Interocular mound oval and armed with a spine median longer than the height of interocular mound (Fig. 3A, lateral view).Scutal areas I-V each with a row of acuminate tubercles and a longitudinal row of similar tubercles on the lateral margins.Free tergites and anal operculum also armed with a row of tubercles that are more slender and dapper than scutal areas I-V.

Nilgirius pygoprominulus
Venter (Fig. 2C; Fig. 5C).Genital operculum and free sternites with hair-tipped granules.Spiracles concealed.Coxa I-IV with a row of marginal tubercles at the border and tuberculated on the ventral surface.Coxa IV enlarged.
Chelicerae (Fig. 2G; Fig. 3B-D).Basichelicerite elongate, with distinct bulla, no prominent armaments, except five small tubercles.Movable finger with twelve moundshaped teeth; fixed finger with six teeth.Pedipalpi (Fig. 3E and F).Trochanter ventrally with an enlarged setiferous tubercle and a small one.Femur dorsally with a row of six small setiferous tubercles, ventrally with a row of six setiferous tubercles; meso-apically with one setiferous tubercle.Patella ventro-mesally with three and ventro-ectally with one tubercle.Tibia ventromesally with five and ventro-ectally with an enlarged tubercle and three small tubercles.Tarsus ventro-mesally and ventro-ectally each with six setiferous tubercles and tarsal claw slightly curved, shorter than tarsus.
Legs (Fig. 2E; Fig. 6A and B).All segments rounded in cross section, with scattered small tubercles.Femora III-IV slightly curved, distitarsi I with two tarsomeres and  of truncus (pars distalis) with 18 macrosetae distributed symmetrically.Five pairs of macrosetae inserted in both sides of ventral plate, the distal C1-C2 form a vertical line comparatively with the transversal A1-A3 proximally.Three pairs of macrosetae ventrally close to the centre of distal section (Fig. 4A), one pair of proximal B proximally and two pairs of E1-E2 distally.One pair of D near the basal glans.
Nilgirius pygoprominulus sp.n.Measurements of the pedipalpus and legs of the male holotype, length/depth.

Sexual dimorphism.
(1) body of male much larger than that of females; (2) dorsal scutum slightly piriform in male and approximately trapezoidal in females; (3) coxa of leg IV and pseudonychia of leg III-IV enlarged in male; (4) male leg IV much longer than female's; (5) teeth on cheliceral finger in male a bit more robust than those of females.

Diagnosis
The distal portion of male genital truncus (pars distalis) markedly enlarged and with two rounded lobes (Fig. 4A).Penial macroseate A clustered transversally on the lateral vetral plate (Fig. 4B).Dorsal scutum of male outline of type beta (Fig. 2A; Fig. 5A).Scutal areas I-V with scattered tubercles, free tergites each with transverse rows of acuminate tubercles.Interocular mound with one single large spine (Fig. 3A; Fig. 5B  and E).Pedipalpal femur meso-apically with a setiferous tubercle and tibia ventroectally with an enlarged tubercle and three small tubercles (Fig. 3E and F).

Etymology
The specific epithet combines the Greek word pyge, meaning "rump, buttocks", plus the Latin adjective prominulus, meaning "tubercle" or "spine".Pyge refers to the shape of penis and prominulus refers to the tubercle of the dorsal scutumn and legs of the new species.

Distribution
Known only from the type locality.
Nilgirius pygoprominulus sp.n.Measurements of the pedipalpus and legs of the female paratype, length/depth.

Taxonomy
The limited ability of Opiliones species to disperse and their narrow distribution are the primary reasons for studying their biogeography.Nilgirius scaber has been mentioned repeatedly by Roewer (1915Roewer ( , 1923Roewer ( , 1929Roewer ( , 1935Roewer ( , 1939) and all specimens have been found in southern India (Nilgiris Hills, Kodaikanal, Vandaravu, Mariyanshola Forest, Kukkal, Coorg).By contrast, N. pygoprominulus sp.n. was collected from southern China (Baoshan City, Yunnan Province).The two localities are nearly 2600 km apart (see Fig. 1), indicating a disjunction between the two species.Additionally, the altitude of the habitats for both species is different, with N. pygoprominulus sp.n. being recorded at 164 m, while some specimens of N. scaber were found at altitudes of 1800-2350 m ( Roewer 1929: 621).The disjunction and the differences in habitat suggest the possibility of two different evolutionary lineages.Note that Palmieri et al. ( 2023) found a sister-group relationship in a clade composed of "Indian Region" and "Thai-Malay Peninsula" which is replicated here in the species of Nilgirius, considering that Yunnan is more Thai-Malay than "Eastern Palearctic".
Nilgirius pygoprominulus sp.n. has distinct external morphological characteristics compared to N. scaber.The interocular mound of the new species is armed with a single spine, whereas N. scaber has two spines.Additionally, the anterior margin of the carapace of N. pygoprominulus sp.n. has two spines at the lateral portion, whereas N. scaber has an extra small spine between the two spines.Furthermore, N. pygoprominulus sp.n. has five tubercles on the dorsal basichelicerite instead of being smooth like N. scaber.
As the genus Nilgirius was erected in 1915, Roewer (1915) ) placed it in the subfamily Trionyxellinae Roewer, 1912.There are still inconsistent opinions on whether the 'Trionyxellinae' is a family or a subfamily (Kury 2018;Palmieri et al. 2023).We tentatively place Trionyxellinae as a subfamily of Assamiidae s.l.
Within the subcontinental Trionyxellinae, there are eight genera with very close geographical locations.Five genera (Balnissa Roewer, 1935, Brysma Roewer, 1935, Calloristus Roewer, 1935, Trionychiperna Roewer, 1929 and Nilgirius) were recorded from the Deccan (India).Three genera (Kandyca Roewer, 1915, Nuwaria Roewer, 1915and Trionyxella Roewer, 1912) in Sri Lanka.Additionally, Nuwaria possessed the most similar external morphology with Nilgirius according to the limited original description.There is a supposition that the eight genera may be the most likely to converge into a molecular clade.
Obviously, our understanding of the Assamiidae s.l. in China is still in its early stages.Currently, only one species, Nilgirius pygoprominulus sp.n., has been identified.While there is a lack of information on the genital morphology of most assamiids, we can classify this new species as belonging to the group of 'the Sri Lankan/Indian pseudonychiate genera' (Kury 2007).This classification is based on the presence of a pseudonychium on tarsi III-IV of the legs, which is a typical morphological characteristic of these legs.The presence of the pseudonychium in the legs III-IV may be an ancestral trait of the group.
Nilgirius pygoprominulus sp.n. also possesses enlarged appendages in the male (coxa IV and leg IV).However, there is no difference in sexual dimorphism in the pedipalpi and chelicerae, except for variations in the number of teeth on the cheliceral fingers.The most conspicuous sexual dimorphism is that the male body is larger than the female's, instead of the female being larger than the male.Additionally, the pseudonychium of legs III-IV in the male are stronger than those of females.From the morphological charateristics, especially the larger body and reinforced leg IV in male, we tentatively suppose that it is more likely to help Nilgirius pygoprominulus sp.n. win in male-male competition and are more likely to be preferred by females.
A great variety of forms of sexual dimorphism in Opiliones suggest that sexual selection, which acts by intersexual selection (female choice) and intrasexual selection (male-male contest), may have played an important role in their evolution (Buzatto et al. 2014).
Sexual dimorphism in the leg armature appears in diverse forms in Opiliones and is associated with functional meaning.A study focusing on Neosadocus bufo (Giltay, 1928) showed that the leg armature was used as a weapon in contests between males (suggested fight for specific sites and individual females; Willemart et al. 2009).Territorial behaviour in males may be associated with a greater development of proximal musculature of leg IV, causing their coxa to be largely developed.

Figure 1 .
Figure 1.Distribution map of the genus Nilgirius.

Male holotype .
Habitus as presented in Fig.2A, C, Fig.3Aand Fig.5A-C.Colouration in ethanol: entire body dorsally rusty yellow with dark brown patches; median area of prosoma with dark brown reticulations before and behind the interocular mound; both lateral ridges of scutum with dark brown stripes; dorsal scutal areas I-V and free tergites dark brown with transverse paler interspaces.Coxae with light brown patches on edges and surface.Chelicerae, pedipalpi and legs reticulated with light to dark brown.Dorsum (Fig.2A; Fig.5A).Dorsal scutum (DS) pyriform, with the widest portion at scutal area II, as a typical beta shape(Fig.2A;Fig.5A).Mid-bulge symmetrical, centred around mid-DS, covering two-thirds of scutum length.Carapace narrower than mid-bulge, subparallel, widening posteriorly.Posterior border of scutum moderately

Figure 5 .
Figure 5. Photographs of male (A-C, holotype) and female (D-F, paratype, MHBU-Opi-17ZC1101f) of Nilgirius pygoprominulus sp.n..A Male body and parts of appendages, dorsal view; B Same, lateral view; C Same, ventral view; D Female body and parts of appendages, dorsal view; E Same, lateral view; F Same, ventral view.Scale bars: 1 mm.