﻿A new species of Arrhopalites Börner (Collembola, Symphypleona, Arrhopalitidae) from China, with a key to the Asian species of the caecus group

﻿Abstract The second species of Arrhopalites from China is described and illustrated and an identification key to the Asian species of the caecus group is presented. Arrhopalitesbrevicornissp. nov. is the eleventh species of the caecus group recorded in Asia and it can be clearly differentiated by the unguiculus III with 2 internal teeth (0–1 in all other species). Also, the combination of antennae less than 2 times the size of the head, antennal segment IV without annulations, 1+1 eyes, dorsal head with 9 spines, 2+2 regular spines per side on the anal valves, circumanal chaetae without basal serrations, subanal appendage long and apically serrated, manubrium with 5+5, and dorsal dens with 16 chaetae is unique among the Asian species of the caecus group.


Introduction
Arrhopalitidae Stach, 1956 comprises species of Arrhopalites Börner, 1906, Pygmarrhopalites Vargovitsh, 2009and Troglopalites Vargovitsh, 2012. Currently this family gathers 141 species described worldwide, with 41 of them belonging to Arrhopalites (Bellinger et al. 1996(Bellinger et al. -2022. Vargovitsh (2013) divided the genus into three species groups based on the ventral (anterior) dental chaetotaxy: diversus group, with the chaetal formula of 3, 2, 1, 1 from the apex to the basis of the structure; caecus group, with 3, 2, 1, 1, 1 chaetae; and harveyi group with 3, 2, 2, 1, 1 chaetae. This division, as well as the support for the family and genera, have not been tested yet with the use of molecular phylogenetics, which could clarify different points of view about the systematics of Arrhopalitidae internal systematics (Zeppelini 2011;Vargovitsh 2013). Also, such kind of study could verify the phylogenetic signal of the dental chaetotaxy within Arrhopalites, which is widely used among the Symphypleona to separate species groups, but may be, at least in few genera, an arbitrary feature to gather unrelated taxa (see Cipola et al. 2021: 37-38). Nevertheless, Vargovitsh's groups of Arrhopalites currently provide clear data to quickly compare species within the genus (Vargovitsh 2013).
Despite its extensive territory, only one species of Arrhopalites was recorded from China so far, A. pukouensis Wu & Christiansen, 1997, described from Jiangsu Province, in the eastern region of the country. Another species (A. nanjingensis Lin & Chen, 1997) was originally described as Arrhopalites, but it was transferred to Pygmarrhopalites by Vargovitsh (2009). So, herein we describe in detail a second species of Arrhopalites from China and provide an identification key to the Asian species of the caecus group.

Materials and methods
Specimens were collected in the field with entomological aspirators and transferred to plastic containers in the laboratory of Entomology, Nanjing Agricultural University (NJAU), China, where they are being cultured. Specimens used for description were sorted in September 2021 and transported to Shanghai Natural History Museum, where the following steps were developed. Under a stereomicroscopy Teelen XTL-207, specimens were bleached and diaphanized, first in 5% KOH and after in 10% lactophenol for three minutes/each. Hoyer's liquid was used to mount the specimens between a slide and a glass coverslip. Slides were dried in an oven at 50 °C for 10 days Bellinger 1980, 1998). A Leica DM2500 microscope with a drawing tube was used to draw the illustrations, which were posteriorly vectorized with Corel Draw 2018 v20. Habitus of the species was photographed in 70% ethanol under a Leica S8AP0 stereomicroscope attached to a Leica DMC4500 camera, using Leica Application Suite software. Slides with type specimens mounted in Hoyer's liquid along with 78 specimens preserved in 98% ethanol are deposited at the collection of Shanghai Natural History Museum (SNHM).
The terminology used in descriptions follows Fjellberg (1999) for the labial palp papillae, Cipola et al. (2014) for the labral chaetotaxy, Nayrolles (1988) for the proximal tibiotarsi chaetotaxy, Betsch and Waller (1994) for head and anterior large abdomen chaetotaxy, Vargovitsh (2009Vargovitsh ( , 2012Vargovitsh ( , 2013 for the posterior large abdomen chaetotaxy and Betsch (1997) for the small abdomen chaetotaxy. On the dens we considered as the dorsal chaetae the sum of the dorsal, dorso-internal and dorso-lateral rows. Drawings and observations were made based in the entire type series.
The abbreviations used in the text and drawings are: Abd = abdominal segment(s); Ant antennal segment(s); and Th = thoracic segment(s). Type material. Holotype on slide "SNHM00001": female, Jilin Province, China, 44°33'N, 123°31'E, 2013, in soil samples from the Ecological Research Station for Grassland Farm, July 2013, Bing Zhang leg. Paratypes on slides: 9 females on slides, same data as holotype. Besides the type material, 78 specimens are kept in 98% ethanol at the SNHM, plus several paratype slides are kept at the laboratory of Entomology, NJAU, China.
Description. Female. Body (head + trunk) length of type series (females, N = 4) ranging between 0.71 and 0.81 mm, average 0.74 mm, holotype with 0.75 mm. Habitus as in Fig. 1. Specimens pale yellowish with brownish spots of pigment on frontal and dorsal head and dorso-lateral large abdomen. Body chaetae smooth and acuminate, with the exception of the subanal appendage.
Distribution and habitat. The new species was collected and only recorded in Jilin Province, Changling County, at the Ecological Research Station for Grassland Farm (ERSGF). This region is characterized by a semi-arid continental monsoon climate, with cold, dry winters and warm, rainy summers. Annual mean temperature in the region ranges from 4.68 to 6.48 °C, and annual precipitation is 280 to 400 mm with about 70% falling in the June-August period (Changling County Climate Station, Jilin Province). Changling County is located at a transitional zone of cropping and grazing, with high economical potential. However, drastic environmental disturbances are happening in this region, like sand and dust storms, emergence of saline-alkali soils, and land over-utilization.
Concerning the species recorded from localities closer to Jilin Province, China, the South Korean A. coreanus, A. gul and A. minor share a similar color pattern, number of eyes, the presence of dorsal spines on head and number of dorsal dens chaetae with the new species. However the later differs from them by: the absence of Ant IV annulations (7 of A. gul); antennae less than 2 times the size of the head (at least two times in A. coreanus and A. gul); all ungues tunicate (without tunica on A. gul); female's subanal appendage apically serrated (pointed in A. coreanus and A. minor, and blunt in A. gul); dorsal anal valve chaeta ms1 not forked (forked in A. coreanus); circumanal chaetae without basal serrations (with in A. gul) and manubrium with 5+5 chaetae (4+4 in A. gul, 9+9 in A. minor).
The only other species of the caecus group registered from China is A. pukouensis, from Nanjing, Jiangsu District, approximately 1800 km distant from the type location of the new species. Both species are vastly different as A. pukouensis is unpigmented (vs. pigmented), has no eyes and dorsal head spines (vs. 1+1 eyes and 9 spines, respectively), its ungues are devoid of tunica (vs. present); its female's anal valves have no cuticular spines and their subanal appendage is short (vs. 2+2 spines per side and the    subanal appendage is long, respectively) and its dorsal dens shows 15 chaetae (vs. 16 in the new species). A detailed comparison of the morphology and the known distribution of all the cited species is presented in Table 1. We also provide a key of all Asian species of caecus group below.
Identification key to the Asian species of caecus group

Discussion
The current knowledge on the Chinese Symphypleona is still incipient, despite the recent efforts from different research groups in describing the local springtail fauna and studying its systematics. So far only 17 species of the order were recorded from China, mostly from dicyrtomids of the genera Papirioides Folsom, 1924 (6 spp.) and Ptenothrix Börner, 1906 (5 spp.) (Folsom, 1924;Denis, 1929;Lin and Xia, 1985;Itoh and Zhao, 1993;Chen and Christiansen, 1996;Guo and Chen, 1996;Wu and Chen, 1996;Li et al. 2007). The other records are from Arrhopalitidae (3 spp., including A. brevicornis sp. nov.) and Sminthuridae (2 spp.) and there is a single species of Bourletiellidae (Lin and Chen 1997;Wu and Christiansen 1997;Li et al. 2008;Chen et al. 2019). Due to the vast area of the country and its many different terrestrial habitats, it is likely these numbers are very far from representing the real richness of the Symphypleona from China, and further efforts should be done to better comprehend this particular fauna. Also, adequate strategies to manage the grazing intensity in Chinese grasslands are crucial to preserve endemic species from these regions.