Discovery of a most remarkable cave-specialized trechine beetle from southern China (Coleoptera, Carabidae, Trechinae)

Abstract Xuedytes bellus Tian & Huang, gen. et sp. n. is described from a limestone cave in Du’an Karst of Guangxi, a kingdom of cavernicolous trechine beetles in southern China. From a morphological point of view, Xuedytes Tian & Huang, gen. n. seems to be the most extremely cave-adapted trechines in the world. Superficially, it looks much like Giraffaphaenops Deuve, 2002 in general body shape, in particular the structure of the prothorax, but simultaneously it is similar to Dongodytes (s. str.) Deuve, 1993, based on elytral characters, including chaetotaxy. Hence the new genus seems to represent a lineage intermediate between Giraffaphaenops and Dongodytes (s. str.).


Introduction
The globe's largest and highly varied karst landscapes that blanket nearly the entire area of southern China (Deharveng and Bedos 2012) are long known to host the richest diversity of cave-dwelling trechine beetles in the world. To date, 48 genera of cavernicolous trechines containing over 130 species have been recorded there (Deuve and Tian 2016, Zhao and Tian 2016, Fang et al. 2017, Huang et al. 2017). Many of them are morphologically highly cave-adapted, such as Giraffaphaenops Deuve, 2002, Dongodytes Deuve, 1993, Uenotrechus Deuve & Tian, 1999, Pilosaphaenops Deuve & Tian, 2008, Sinaphaenops Uéno & Wang, 1991and Shuangheaphaenops Tian, 2017. As the cave beetle fauna of China is still poorly-known, it is hardly surprising that another highly peculiar species representing another new genus has been revealed in the country (Tian 2017).
In early August 2017, a cave biological survey carried out in Du'an Karst of northern Guangxi, southern China led to the discovery of a very peculiar species belonging to the subfamily Trechinae, family Carabidae. Moreover, it shows a number of most remarkable troglomorphic features amongst subterranean trechines generally. Superficially, its strongly elongated and slender body looks very much like that of a Giraffaphaenops species, especially due to the extremely elongated prothorax. However, Giraffaphaenops species are known from the Leye-Tianlin karsts of northwestern Guangxi, about 200 km away from Du'an Karst (Deuve 2002, Tian andLuo 2015). In contrast, the elytra in the new species are quite similar to those observed in Dongodytes (s. str.) Deuve, 1993, yet being much more strongly elongated. In addition, it has many other particular morphological characteristics, such as: (1) Head comparatively short, but sufficiently long and aphaenopsian, bearing multisetiferous pores in frontal areas; (2) Right mandible edentate; (3) Prothorax extremely elongated; (4) Lateral margins of pronotum visible in fore part from above, while long erect setae on disc and two pairs of latero-marginal setae in middle portion present; and (5) Elytra extremely elongated, smooth and glabrous, with striae completely obliterated.

Materials and methods
The beetles were collected in the cave using an aspirator, and kept in vials with 50% ethanol before study, except for a specimen put in a vial with 95% ethanol for molecular analysis.
Techniques and terminology are the same as in Tian (2017).

Taxonomic treatment
Xuedytes Tian & Huang, gen. n. http://zoobank.org/C8AACC92-F265-45CD-9F50-09DAB01ADFD0 Type species. Xuedytes bellus Tian & Huang, sp. n. Generic characteristics. Highly modified aphaenopsian trechines, body shape, in particular prothorax, similar to that in Giraffaphaenops, but elytra generally like in Dongodytes (s. str.) (Fig. 1); large-sized, with body (especially prothorax and elytra) and appendages thin and extremely elongated, eyeless and unpigmented; fore body part (head including mandibles, plus prothorax) much longer than, or as long as (excluding mandibles) elytra, respectively; body smooth; three pairs of frontal setiferous pores present on head; mandibles thin and elongated, feebly curved apically, longer than head width, right mandible edentate; labial suture completely missing; mentum bisetose on either side of tooth at base, base broadly concave; mental tooth simple, short and blunt at tip; submentum 8-setose; ligula bisetose at apex (Fig. 2); antennae very long, antennomeres 10 and 11 extending beyond elytral apices. Prothorax similar to that of Giraffaphaenops, wider than head, very strongly elongated, much longer than head including mandibles, propleura distinctly tumid in basal 1/3, visible from above; pronotum barrel-shaped, thin and distinctly elongated, lateral margins visible throughout from above, slightly narrower than head; hind latero-marginal setae absent, but two long latero-marginal setae plus two or three additional short setae present from middle to front. Elytra similar to those in Dongodytes (s. str.), narrow anteriorly and dilated posteriorly, side margins narrowly bordered throughout, shoulders lacking; striae virtually missing, only weakly traceable; two dorsal and the pre-apical setiferous pores present, each with a very long seta; chaetotaxy similar to that in Dongodytes (s. str.). Protibia smooth, without longitudinal sulcus; protarsomeres not modified in male. Ventrites VII bisetose apically in male, but quadrisetose in female. Male genitalia moderately sclerotized, small, strongly curved ventrally in lateral view, with a quite large and thin sagittal aileron; apical lobe wide and broad in dorsal view; parameres much shorter than median lobe, yet well-developed.
Discussion. Xuedytes Tian & Huang, gen. n. is undoubtedly the most remarkable cavernicolous trechine genera as regards the extremely elongated prothorax and elytra. It may be considered as a lineage intermediate between Giraffaphaenops and Dongodytes (Fig. 3). Superficially it resembles Giraffaphaenops because of the similarly thin and strongly elongated body, especially the prothorax. Its elytra, however, are quite similar to those of Dongodytes (s. str.). The most striking character states of Xuedytes are as follows: (1) Prothorax much longer than head; (2) Elytra very narrow and strongly elongated; (3) Three pairs of frontal pores present on head; and (4) Right mandibular tooth obsolete.
Although Xuedytes is similar to Giraffaphaenops, there are several important differences: (1) Prothorax and elytra much more strongly elongated in Xuedytes than in Giraffaphaenops; (2) Head subquadrate, slightly convex laterally, not contracted posteriad in Xuedytes (vs. inversed triangular, with a well-marked neck constriction in Giraffaphaenops); (3) Entire lateral margins of pronotum visible from above in Xuedytes (vs. invisible from above in front half in Giraffaphaenops); and (4) Two pairs of latero-marginal setae present behind middle of pronotum in Xuedytes (vs. absent in Giraffaphaenops) (Fig. 3).
Apart from the differences in prothoracic features, Xuedytes is easily distinguished from Dongodytes (s. str.) by the following characteristics: (1) Head thicker and broader, not narrowed posteriad in Xuedytes (vs. thinner and evidently narrowed posteriad, forming a long and distinct neck constriction in Dongodytes); (2) Three pairs of frontal pores present in Xuedytes, instead of only one pair in Dongodytes; (3) Elytral striae completely obliterated in Xuedytes (vs. partially visible in Dongodytes) (Fig. 3).
Furthermore, differences between the new genus and both Giraffaphaenops and Dongodytes are also evident regarding the structure of the male genitalia (Fig. 4). The median lobe of the aedeagus is shorter in Xuedytes, but thicker, especially so at the base, with a thinner, almost transparent sagittal aileron.

Diagnosis.
A large-sized, blind, cave-adapted trechine, remarkably modified morphologically, with both prothorax and elytra highly elongated and slender so that body five times longer than wide, antennae slightly shorter than body including mandibles, extending beyond elytral apices; head, pronotum and base of elytra covered with sparse erect setae. Habitus as in Figs 1 and 3.
Legs slender and long, bearing short pubescence; fore and middle femora sparsely setose; fore tibia smooth, with neither a longitudinal furrow nor a sulcus; 1 st tarsomere shorter than, slightly longer than, and much longer than 2 nd -4 th tarsomeres combined in fore, middle, and hind legs, respectively.
Male genitalia (Fig. 4a, b). Aedeagus quite small and short, distinctly curved ventrally in middle portion in lateral view, then broad at apex; inner sac with a fairly large copulatory piece, the latter about 1/3 as long as median lobe; base quite large, open ventrally, with a hyaline sagittal aileron; in dorsal view, apical lobe fairly stout, slightly sinuate on right side, rounded at apex. Parameres short and quite elongated, each bearing four long apical setae.
Distribution. China (Guangxi: Du'an). Known only from Cave II. This cave maintains a natural condition, opening on a small hill on the northern bank of the Hongshui River. The entrance is surrounded by dense bushes and not readily accessible (Fig. 5a, b). The total length of the cave is still unknown, but said to be about 200 m, according to local people. It is sufficiently wet inside the gallery and is good for cave fauna. The beetles were found running on walls and stalactites (Fig. 5c, d), sympatric with spiders (Fig. 5e, f ), millipedes (Fig. 5g, h), woodlice (Fig. 5i) and crickets (Fig. 5j).
the Nanjing Institute of Environmental Sciences, Ministry of Environmental Protection, through a biodiversity conservation project which focused on cave biodiversity, and a project of the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant no. 20134404110026).