A new species and new record of the cryptobiotic ant genus Ponera Latreille, 1804 (Hymenoptera, Formicidae) from Hong Kong

Abstract Despite its small size, Hong Kong hosts a surprising level of ant diversity. Through faunal studies on arthropods conducted in Hong Kong over recent years, a new record and species of the genus Ponera have been discovered, which are introduced here. Ponera guangxiensis Zhou, 2001 is reported for the first time from Hong Kong, and Ponera tudigongsp. nov. is here described as a new species, easily distinguishable from other Ponera species and unique within the genus for its four mandibular teeth.


Introduction
The cryptic ant genus Ponera Latreille, 1804, contains 59 valid species at present (Bolton 2019;Leong et al. 2019). Known mostly from the Old World, the bulk of its diversity occurs in the tropical to subtropical Oriental realm (Janicki et al. 2016;Guénard et al. 2017). The tropical diversity of Ponera in the Oriental realm and further south into the Oceanian and Australian realms contrasts with its absence as native species in the Afrotropical and Madagascan realms, and its limited distribution within the Neotropical realm, where it is limited to two species in the northern part of Mesoamerica (Branstetter and Longino 2019). Relatively little is known of the biology and ecology of the species of Ponera, owing to its cryptobiotic lifestyle and paucity of collection. In general, Ponera has small colonies (ca. hundreds of individuals), is most commonly found underneath rocks and rotting logs, and is likely a generalist predator of small arthropods (Schmidt and Shattuck 2014).
At present only one species of Ponera, P. sinensis , is known from Hong Kong and the surrounding Guangdong province. Through faunal studies conducted in Hong Kong using pitfall traps and sifted leaf-litter, two newly recorded species have been found: P. guangxiensis Zhou, 2001, and P. tudigong sp. nov., newly described in this paper. We include here accounts of the two species, the description of P. tudigong sp. nov., and an update to the key to East Asian species of Ponera presented by Leong et al. (2019).

Material examined
Twenty-nine individuals collected from Hong Kong were examined ( Figure 1). All nontype material has been deposited in the Insect Biodiversity and Biogeography Laboratory (IBBL) at the University of Hong Kong. The type specimens of P. tudigong have been deposited at the California Academy of Sciences (CAS). Images and specimen data were uploaded to AntWeb.org (2018) and are available in the supplementary specimen file.

Morphology
Morphological measurements were done using a Leica S8AP0 stereomicroscope at 80 × magnification. Measurements used are detailed below and follow those used in Leong et al. (2019), with the exception of the addition of mandible length (MaL) and change of total length (TL) which are described below. Weber's length (WL) has been changed to mesosomal length (ML). Measurements are reported in millimeters to two decimal places. Repeated measurements on a single individual resulted in an accuracy of 0.01 mm. Individuals of Ponera guangxiensis from different regions of Hong Kong were measured to account for population variability. Terminology follows Taylor (1967), Harris (1979), Bolton (1994), and Keller (2011).

Imaging
Specimen images were taken using a Leica DFC450 digital camera mounted on a Leica M205C dissecting microscope, and processed using Leica Application Suite V4 software.

ATL
Abdominal tergum III length. Maximum length of 3 rd abdominal tergum (= 1 st gastral tergum), measured from the center of the anterior margin to the center of the posterior margin, in dorsal view.

ATW
Abdominal tergum III width. Maximum width of the 3 rd abdominal tergum (= 1 st gastral tergum), measured as a straight line from one lateral margin to the other, in dorsal view.

HL
Head length. Maximum length of the head, measured as a straight line between the center of the posterior cephalic margin and the center of the anterior clypeal margin, in full-face view.

HW
Head width. Maximum width of the head, measured as a straight line from one lateral margin to the other, in full-face view.

MaL
Mandible length. Maximum length of the mandible, measured as a straight line from the mandibular insertion to the tip of the apical tooth, with the mandible in dorsal view.

ML
Mesosomal length. Maximum length of the mesosoma, measured as a straight line from the anterior-most point of the pronotum to the posterior basal angle of the metapleuron, in profile view.

PeH
Petiole height. Maximum height of the petiole, measured as a straight line from the ventral margin of the subpetiolar process to the dorsal margin of the petiole, in profile view.

PeNL
Petiole node length. Length of the node of the petiole, measured as a straight line from the anterior margin of the petiole immediately above the dorsal base of the anterior petiolar tubercle to the posterior margin of the petiole, in profile view.

PeW
Petiole width. Maximum width of the petiole, measured as a straight line from one lateral margin of the petiole to the other, in dorsal view.

PrW
Pronotal width. Maximum width of the pronotum, measured as a straight line from one lateral margin of the pronotum to the other, in dorsal view.

SL
Scape length. Maximum length of the scape, measured as a straight line from the base of the scape (excluding the basal neck and condyle).

TL
Total length. Maximum horizontal length of the specimen from the anterior-most extent of the mandibles to the posterior tip of the gaster, measured in profile view.

ATI
Abdominal tergum III (= 1 st gastral tergum) index. ATL/ATW × 100.  Geographic range. Hong Kong S.A.R., China. Diagnosis (worker). Ponera tudigong can be distinguished from the other species in the genus by the following characters: masticatory margin of the mandible with a small denticle and four enlarged teeth (Figure 2), an apomorphic character for this new species (see Leong et al. 2019); petiole relatively thick in lateral view (LPeI 60) with the anterodorsal margin of the node of the petiole projecting slightly forward; metanotal groove forming clearly incised suture in dorsal view (Fig. 4A) Description of worker. Head. In full face view, head subrectangular (Fig. 3), longer than broad (CI: 85) with shallowly concave occipital margin, and slightly convex lateral margins. Anterior clypeal margin nearly straight, with a low and blunt medial tooth. Eyes absent. Mandibles subtriangular, with a small denticle (t5) and four triangular enlarged teeth (t1 to t4) on the masticatory margin ( Fig. 2A). Antennal

Funicular segments of antennae incrassate, increasing in length and breadth distally.
Mesosoma. In profile view, mesosomal dorsum slightly convex. Promesonotal suture and metanotal groove both present and clearly incised. Mesopleuron distinct, separated from mesonotum by a clearly incised promesonotal articulation. In dorsal view, lateral margins of pronotum and mesonotum rounded, lateral margins of propodeum slightly concave. Propodeal corner in lateral view forming a blunt angle.
Metasoma. In profile view, petiole sub-rectangular and remarkably thick compared to other species in the genus (LPeI = 59.5), with anterior and posterior margins straight and parallel; anterodorsal corner nearly right-angled, posterodorsal corner blunt; anterodorsal corner higher than posterodorsal corner. Subpetiolar process sub-rectangular in shape, with a large and circular anterior fenestra, extending across the entire ventral margin of the petiole. Posterior portion of subpetiolar process with a well-developed pair of blunted teeth. In dorsal view, petiolar node sub-oval, clearly wider than long (DPeI = 138.1), with a slightly convex anterior margin and slightly concave posterior margin. In dorsal view, abdominal tergum III (= 1 st gastral tergum) wider than long (ATI = 91.8), and in profile view with a bluntly rounded anterodorsal corner.
Sculpture. Entirety of head punctate, covered by closely and evenly spaced fine puncturing. Mandibles smooth. Sculpturing of mesosoma and metasoma relatively light, ranging from puncticulate to imbricate. Mesosomal and metasomal dorsum nearly smooth.
Pilosity. Head and antennae covered by fine golden-colored pubescence. Mandibles with sparse, short, erect to sub-erect filiform hairs. Pilosity of mesosoma consisting of a fine pubescence, similar to that of the head. Fine and sparse pubescence covering each metasomal segment, becoming sparser after the fourth abdominal segment ( = 2 nd gastral segment). Short, golden, sub-erect to erect filiform hairs present on all metasomal segments, becoming longer and denser posteriorly from the fifth abdominal segment (= 3 rd gastral segment).
Color. Coloration of entire body of the individual, excluding the legs, ranging from light to dark reddish-brown. Legs light brown.
Description of queen. The description of the queen of P. tudigong follows that of the worker caste, with the following differences: Body size larger than that of the worker caste, with large, sub-circular compound eyes, 0.15 mm in diameter, present on the anterior part of the head, roughly one-third the distance to the posterior margin. Three distinct ocelli present on the posterior part of the head. Mandibles similar to worker caste (Fig. 2).  Sub-petiolar process of the queen with a subquadrate anterior fenestra, a low and blunt downward-projecting anterior angle, and a downward-projecting square medial tooth. Abdominal tergum III (= 1 st gastral tergum) as long as wide (ATI: 101).
Sculpturing, coloration, and pilosity of the queen is similar to that of the worker caste, with sculpturing slightly more pronounced.
Etymology. This new species is named in honor of Tudigong (土地公), the lord of the soil and the ground, a widely venerated Chinese deity.
Comments. Ponera tudigong is known from a single location in Tai Lam country park, Hong Kong SAR. The worker and queen were collected in separate pitfall traps 5 meters apart during the same sampling period. They were initially recognized as being distinct from the other local Ponera species, P. sinensis and P. guangxiensis, based on the mandible and shape of its petiole and the absence of eyes, and was later determined to be a new species. It is the only known Ponera species with four mandibular teeth, all other species having either three (most of Ponera species), five (P. pentodontos Xu, 2001), or seven (P. taylori Bharti & Wachkoo, 2012).
Nothing is known about the biology or life history of P. tudigong. Like most species of Ponera, it is likely cryptobiotic and subterranean, as the absence of eyes might suggest. The fact that our specimens were captured in surface pitfall traps may suggest that the species does come to the surface at least occasionally, maybe at night, and possibly to forage or for reproduction purposes, given that a wingless queen was captured in addition to the worker. Of course, soil disturbance during trap emplacement may also be responsible.
Ponera tudigong is known from only a single collection event, despite a significant amount of leaf-litter and pitfall-trap sampling done throughout Hong Kong in recent years. This suggests that the species is indeed quite rare. The habitat it was collected from is a restored secondary forest, and it was collected within one meter of a well-used hiking trail.  represent the newly recorded P. guangxiensis. Whether this represents an underlying ecological change or simply an artefact of sampling remains unclear, and future sampling in Hong Kong may yield new specimens of P. sinensis.

Update to the identification key of East Asian Ponera species
The following is an update to the identification key to East Asian species of Ponera originally provided by Leong et al. (2019), which contains 31 key couplets for 32 species. Ponera tudigong sp. nov., described here, is placed in a new couplet inserted after the 4 th couplet in Leong et al. (2019), which has been slightly modified to reflect this. The new couplet has been numbered as 4', to discern it from couplet 4 in the original key, and to leave the remainder of the original key unaffected.