Two new pseudoscorpion species (Pseudoscorpiones, Chthoniidae, Cheiridiidae) from the Tonga Islands, Polynesia, with a redescription of the genus Nesocheiridium

Abstract The genera Tyrannochthonius Chamberlin, 1929 and Nesocheiridium Beier, 1957 are recorded from the Tonga Islands, Polynesia, for the first time. Tyrannochthonius euasp. nov. is described from the island of Eua. Nesocheiridium onevaisp. nov. is described from the island of Onevai. This is the first discovery of a representative of the genus Nesocheiridium in more than 60 years. The holotype of the type species, Nesocheiridium stellatum Beier, 1957, is redescribed, allowing a better understanding of this poorly known genus. The genus Nesocheiridium is diagnosed by the following combination of characters: integument coarsely granulate, dorsally granulo-reticulate; vestitural setae either relatively long, with a leaf-like outline, or arcuate with a small spine; cucullus short; only 10 abdominal tergites visible in dorsal view; cheliceral rallum of four blades; venom apparatus present in both chelal fingers; fixed chelal finger with granulate swelling mesally and seven trichobothria; trichobothria ib and ist located distad of granulate swelling; eb and esb situated close together at the base of the finger; moveable chelal finger with two trichobothria.


Introduction
Polynesia is a subregion of Oceania, comprising more than a thousand islands spread across the central and southern Pacific Ocean. The small size of the islands and their isolation promote strong evolutionary selection (Filin and Ziv 2004) and high endemism of the fauna (Udvardy 1965). During an expedition to collect invertebrates in Oceania in 1980, a few pseudoscorpions were collected on the Tonga Islands. The Kingdom of Tonga comprises 169 islands, stretching approximately 800 km in a north-south line in Polynesia, flanked by Fiji to the northwest and Samoa to the northeast. Tonga, like much of Polynesia, is poorly known in terms of its pseudoscorpion fauna. Except for New Zealand (Harvey 2013), only a few works have dealt with the pseudoscorpions of this region (With 1907;Kästner 1927;Beier 1932Beier , 1940Chamberlin 1939a, b;Muchmore 1979Muchmore , 1983Muchmore , 1989Muchmore , 1993Muchmore , 2000Harvey 2000). Only a single species, Geogarypus longidigitatus (Rainbow, 1897), had been recorded from the Tonga Islands before now (Harvey 2000). Two species are added here, belonging to the genera Tyrannochthonius Chamberlin, 1929 andNesocheiridium Beier, 1957.
The genus Tyrannochthonius is widely distributed in tropical and subtropical regions of the world. It is one of the largest chthoniid genera, with 130 described species. Most of these have restricted distributions, known from only a few locations. The available data indicate a tendency for short-range endemism of its species (Edward and Harvey 2008;Harvey 2013). In Polynesia, Tyrannochthonius species have only been recorded from Hawaii (Muchmore 1983(Muchmore , 1989(Muchmore , 1993(Muchmore , 2000 and New Zealand (Chamberlin 1929;Beier 1966Beier , 1967Beier , 1976. The Tongan specimen belongs to a new species, which is described here. The genus Nesocheiridium was erected in the Cheiridiidae by Beier (1957), with Nesocheiridium stellatum Beier, 1957 as its only included species. Until now, the holotype of N. stellatum, from Saipan, Marianna Islands, Micronesia, has been the only known specimen of the genus. A re-examination of that species and the new species described here allow a better characterization of the genus.

Methods
All specimens examined for this study had been preserved in 75% ethanol. They were studied as temporary slide mounts, prepared by immersing the specimens in lactic acid for clearing. After study, they were rinsed in water and returned to 75% ethanol, with the dissected portions placed in microvials.
Morphological and morphometric analyses were performed using a Leica DM1000 compound microscope with an ICC50 Camera Module (LAS EZ application, 1.8.0). Measurements were taken from digital images using the AxioVision 40LE application. Reference points for measurements follow Chamberlin (1931), except that the pedicel was included in the measurements of the lengths of the chela and chelal hand. Drawings were made using a Leica DM1000 drawing tube. Digital photographs of new species (Figs 2, 4) were taken using a Canon EOS 5D camera attached to a Zeiss Axio Zoom.V16 stereomicroscope. Image stacks were produced manually, combined using Zerene Stacker software, and edited with Adobe Photoshop CC. Photographs of N. stellatum were taken at the Collaborative Invertebrate Laboratory, Field Museum, Chicago, USA (FMNH) using a Digital Microptics system consisting of a Nikon D5100 camera, a flash lighting system, P-51 Camlift with controller and software including Base plate, on a computer workstation.
The types of new species are deposited in the zoological collections of the Naturhistorisches Museum Wien, Austria (NHMW).

Cheiridiidae Hansen, 1894 Cheiridiinae Hansen, 1894
Nesocheiridium Beier, 1957 Diagnosis. Small species, with adult body length ranging from 0.85 to 0.94 mm. Integument coarsely granulate, dorsally granuloreticulate. Vestitural setae relatively long, arcuate with a small spine, often covered by a fine exudate, giving them a leaflike shape. Carapace narrowed towards anterior end, with short cucullus and a deep, submedian, transverse furrow. One pair of eyes. Cheliceral hand with four setae (seta ls absent), all acuminate. Galea long and slender, simple in male, with three terminal rami in female. Rallum of four blades, distal one enlarged and dentate. Ten abdominal tergites visible in dorsal view, I-IX divided. Ventral anal opening large and circular. Pedipalps densely and strongly granulate, including hand and the base of the fixed fingers, femur pedicellate. Fixed chelal finger with granulate swelling mesally, most distinct from ventro-lateral view. Chelal fingers slightly shorter than hand without pedicel. Venom apparatus present in both chelal fingers. Seven trichobothria present on fixed chelal finger, situated mainly in its basal half. Trichobothria ib and ist located distad of the granulate swelling, eb and esb situated close together subbasally. Moveable chelal finger with two trichobothria, situated in its basal half.
Remarks. Some of the morphometric values given here differ slightly from the original description (Beier 1957 Description. Adult female (Figs 4B,6). Integument coarsely granulate, dorsally granuloreticulate (Fig. 6B, C). Vestitural setae arcuate with a small spine, often covered by a fine exudate, giving them a leaf-like shape. Carapace (Fig. 6A): 0.72 × longer than broad, subtriangular; cucullus short; two distinct eyes with lenses; two lateral lighter sections at the level of the eyes (this is not due to damage); submedian transverse furrow deep (carapace slightly damaged in middle); anterior disk laterally with two protuberances, posterior disk with a shallow medial depression in its middle (Fig. 6A); with 30 leaf-like setae (20 before furrow, 10 behind); with one pair of lyrifissures in ocular area. Chelicera (Fig. 6D): 1.80 × longer than broad; four setae on hand, all acuminate; moveable finger with one short seta; fixed finger with two teeth near tip; with two lyrifissures on hand; galea long, slender, with three apical rami; serrula exterior with 10 blades; rallum consisting of four blades (Fig. 6E). Coxae (Fig. 6F): coarsely granulate; chaetotaxy: manducatory process two or three acuminate setae, rest of palpal coxa with four acuminate and two leaf-like setae, situated in anterior half; pedal coxae I 5-7 acuminate setae, II 6-8 acuminate setae, III seven or eight acuminate setae, IV eight or nine acuminate setae. Lyrifissures: one on coxa III, one on coxa IV; maxillary lyrifissures not visible.
Remarks. The two species currently placed in the genus are easy to distinguish from each other by the form of the carapace (N. stellatum lacks a medial depression on the posterior disk, whereas N. onevai sp. nov. has a weak medial depression on the posterior disk); the shape of the palpal patella (not broadened in N. stellatum, versus markedly broadened mesally in N. onevai sp. nov.); the number of setae on the carapace (43 in N. stellatum, 30 in N. onevai sp. nov.); the number of marginal teeth on the fixed chelal finger (16 in N. stellatum, 10 in N. onevai sp. nov.); the shape of the setae on sternite VIII (leaf-like in N. stellatum, as opposed to acuminate in N. onevai sp. nov.); and the lengths of the palpal segments (femur 0.33 mm in N. stellatum, 0.26 mm in N. onevai sp. nov.; patella 0.26 mm in N. stellatum, 0.21 mm in N. onevai sp. nov.; hand with pedicel 0.28 mm in N. stellatum, 0.21 mm in N. onevai sp. nov.).

Discussion
The original description of the genus Nesocheiridium was based on a single male of N. stellatum. The discovery of a new, congeneric species affords the opportunity to clarify the diagnostic characters of this inadequately known genus. The presence of a granulate swelling on the base of the fixed chelal finger is considered to be the main diagnostic character of Nesocheiridium. However, it is worth noting that, although this character has not previously been mentioned in descriptions of other Cheiridiinae, some drawings, such as those published for Neocheiridium corticum (Balzan, 1877) by Mahnert and Aguiar (1986) and for N. africanum Mahnert, 1982by Mahnert (1982, indicate its presence in Neocheiridium. Thus, despite a better understanding of the species of Nesocheiridium, doubts remain about the validity of the genus. One obstacle to clearly defining the genus within Cheiridiinae is the fact that many of the other genera of Cheiridiinae remain inadequately diagnosed. The need for revisionary work in this subfamily Cheiridiinae has previously been mentioned by other authors (e.g., Mahnert and Aguiar 1986;Judson 2000;Sammet et al. 2020).