Close relationship between the genera Sinhomidia and Homidia (Collembola, Entomobryidae) revealed by adult and first instar characters, with description of a new Sinhomidia species

Abstract A third species of the genus Sinhomidia is described from South China: S. unisetasp. nov. This new species can be distinguished from the two other species of the genus by the following characters: colour pattern, single labial chaeta M, chaetotaxy on terga and ventral tube, unguis with three inner teeth, and 15 clypeal ciliated chaetae. Also, the chaetotaxy of the first instar of Sinhomidia is described for the first time in the present paper, and confirms the close relationship between Sinhomidia and Homidia. A key to species of Sinhomidia is provided.


Introduction
The genus Sinhomidia was defined by Zhang et al. (2009), based on the type species Acanthocyrtus bicolor (Yosii, 1965). This genus is characterised by 8+8 eyes; scales pointed with coarse striations; dental spines present on inner dens; clavate tenent hairs; mucro bidentate with subapical tooth larger than apical one, and basal spine reaching subapical tooth; abdominal segment II/III/IV with 2/3/2 bothriotricha, and bothriotrichal complex with slightly modified accessory microchaetae; abdominal segment IV with anterior eyebrow-like macrochaetae (Zhang et al. 2009). This genus has scales present on the appendages according to S. bicolor (Zhang et al. 2009), however, Jin et al. (2017) proposed "scales present or absent on appendage" based on Sinhomidia guangxiensis Jin et al., 2017, which lacked such scales.
The genera Sinhomidia and Homidia Börner, 1906 are closely related, the scaled Sinhomidia being recognised as sister group of unscaled Homidia (Zhang et al. 2014a). This close relationship is supported by morphological, molecular and ecological evidence. Firstly, these two genera share several morphological characters, such as cephalic chaetotaxy on the dorsal side, eyebrow-like macrochaetae on abdominal segment IV, dens with inner spines in adults, smooth labial chaetae e and l 1 , the subapical mucronal tooth larger than the apical one, a bilobed bulb on antennal apex (Zhang et al. 2009, Zhang et al. 2014b, bothriotricha formula (2/3/2 on abdominal segment II/III/IV), and sensory chaetotaxic formula on tergal segments (3, 2/2, 2, 3, ? (sensory chaetae on abdominal segment IV variable), 3). They are easily misidentified as Homidia in the field in view of their colour pattern and body form. Secondly, recent molecular phylogenetic analyses revealed a close relationship between them based on mitochondrial, ribosomal and nuclear gene fragments (Zhang et al. 2014a(Zhang et al. , 2016. Also, Ding et al. (2019) placed Sinhomidia within the clade of Homidia based on mitochondrial COI, 16S rRNA and nuclear 28S rRNA D1-2. Thirdly, these two genera live in similar habitat, and are usually found in leaf litter in tropical to subtropical bioclimates. Sinhomidia with two known species and the new species described here is much less diversified than Homidia which has about 37 species in China (Zhang et al. 2009, Jin et al. 2017, Ma and Pan 2017. Sinhomidia can be easily discriminated morphologically from Homidia by scales on the terga and slightly modified accessory microchaetae of the bothriotrichal complex (Zhang et al. 2009).
The genus Sinhomidia is endemic to China. The type species (S. bicolor) was first recorded from Taiwan (Yosii 1965), and subsequently a female specimen was found in Anhui Province (Zhang et al. 2009) and male specimens in Guangxi Province (Jin et al. 2017). To date, only two species of this genus have been described (S. bicolor and S. guangxiensis), and the chaetotaxy of larvae has not been revealed. Here, a third species from Guangdong Province is described, together with its first instar larva. We provide a detailed comparison between the new species and the two known species, and we compare the first instar chaetotaxy among nine species of family Entomobryidae. In addition, a key to the recorded species of Sinhomidia is provided.

Materials and methods
Specimens were sieved from leaf litter onto a tray in the field, collected by an aspirator, and stored in 99% ethanol at -20 °C. Specimens were photographed using a Nikon DS-Fi1 camera mounted onto a Nikon SMZ1000 stereomicroscope, then cleared in lactic acid, mounted in Hoyer's medium under a coverslip, and examined under phase contrast using a Nikon 80i microscope. Lengths were measured from specimens on slide by NIS-Elements Documentation 3.1 software. Photos, illustrations and labels were enhanced by photoshop CS5 (Abode Systems).
Etymology. Specific epithet refers to the single chaeta M on labial basis (uni + seta). Remarks. Sinhomidia uniseta sp. nov. can be easily distinguished from the other two species of the genus by the dark pigment present on the lateral and anterior margins of Th. II, posterior margin of Abd. II and whole Abd. III, labial single M chaeta, three mac and ms external to sens on dorsal side of Abd. I, 5 (7) mac in postero-median area of Abd. IV, middle sens posterior to m3 on Abd. V, and three teeth on inner side of unguis. Detailed differences between the three species of Sinhomidia are listed in Table 1.

Close relationship revealed by the chaetotaxy of adults and first instar between Sinhomidia and Homidia
The genus Sinhomidia was named referring to many features shared with Homidia (Zhang et al. 2009). Sinhomidia is regarded as sister group of Homidia (Zhang et al. 2014a), and is considered to be scaled Homidia (Zhang et al. 2014b). The genera Sinhomidia and Homidia share many features, such as colour pattern; chaetotaxy of head, labium, labrum, terga; S-chaetotaxic pattern, bothriotrichal pattern; and morphology of Ant. IV bulb, claw and mucro of adults (Pan et al. 2011, Zhuo et al. 2018).   m1  mac  mac  mac  mac  mac  mac  mic  mac  mac  m2  mic  mic  mic  mic  mic  mic  mic  mic  scale  p4  mic  mic  mic  mic  mic  mic  mic  mic  mic  p5  mic  mac  mac  mic  mac  mac  mic  mic  mac  P6  mic  mic  mic  mic  mic  mic  mic  mic a1  mic  mic  mic  mic  mic  mic  mic  mic  mic  a3  mic  mic  mic  mic  mic  ----m2  mac  mac  mac  mac  mac  mac  mac  mic  mac  m3  mac  mac  mac  mic  mac  mac  mac  mic  mac  a5  mic  mic  mic  mic  mic  mac  mac  --m5  mac  mac  mac  mic  mac  -mic  -mac  a6  mic  mic  mic  mic  mic  mic  mic  -mic  p1  mic  mic  mic  mic  mic  mic  mic  mic  mic  p2  ------- Phylogeny supports this close similarity (Zhang et al. 2014a, b, 2016, Zhang and Deharveng 2015, Ding et al. 2019). Here, we show that chaetotaxy of the first instar larva is also more similar between Sinhomidia and Homidia than between Sinhomidia and the species of the other six genera within family Entomobryidae where it has been described, including the number, morphology and relative location of primary tergal chaetae (Tab. 2). These two genera differ nevertheless by several characters, such as scales (present in Sinhomidia versus absent in Homidia) (Figs 6-8), number of guard chaetae on labial papilla E (3 in Sinhomidia versus 4 in Homidia) (Fig. 19), bothriotrichal complex (slightly modified accessory mic of Sinhomidia versus not modified in Homidia) (Fig.  7), the relative position of posterior mac (p series) of Th. II-III (close to m series in Sinhomidia, versus close to posterior margin in Homidia) (Fig. 21), the number of mac on the dorsal side of Abd. I (3-5 in Sinhomidia versus 9-11 in Homidia), and length ratio Abd. IV/Abd. III (11-15 in the new species, but less than 10 times in Homidia, generally) (Zhang et al. 2009, Pan et al. 2011, Pan and Shi 2015, Jin et al. 2017, Zhuo et al. 2018.

Are scales present on appendages of Sinhomidia?
Scales are intuitively considered to have evolved from ordinary chaetae, present in many species, and are important diagnostic characters for classification at the subfamilial and tribal levels of the family Entomobryidae. The tribe Willowsiini is well defined by the absence of dental scales (Zhang et al. 2009). However, the presence or absence of body scales for classification is not valid for Willowsiini (Zhang et al. 2014a). Sinhomidia is a member of Willowsiini by the absence of scales on dens, and differs from other genera by its dental spines. Two recorded species of Sinhomidia and the new species described here are consistent in the morphology of the scales and tip pointed and fusiform with coarse striations, but they do not agree well between them in whether the scales are present on appendages. They are present on appendages of S. bicolor and the new species, but absent on S. guangxiensis (Zhang et al. 2009, Jin et al. 2017). Referring to the examined specimens of the new species, a few scales are present on the basal segments of antennae and legs, and ventral side of manubrium; furthermore, scales on the manubrium are narrower and longer than on the dorsal side of the terga, and similar to normal chaetae (Fig. 8). Additionally, scales easily fall off after clearing, and their sockets are difficult to distinguish from those of normal chaetae when checked by light microscope. To confirm if Sinhomidia has scales present on appendages in all species, it would be necessary to check the holotype of S. guangxiensis.