The multiformity of antennal chaetae in Troglopedetes Joseph, 1872 (Collembola, Paronellidae, Troglopedetinae), with descriptions of two new species from Thailand

Abstract Two new species of the genus Troglopedetes Joseph, 1872 (T. meridionalissp. nov. and T. kaesp. nov.) are described from caves of the Thai peninsula. This is the first report of the genus south of the Kra Isthmus. The two new species have two rows of dental spines shared by all Thai Troglopedetes. They differ from other members of the genus mainly in the arrangement of dorsal chaetotaxy on head. The antennal chaetotaxy of the two species is analysed in detail in the second part of the paper. All types of antennal chaetae of both new species and their distribution patterns are described for each antennal segment: scales, ordinary chaetae, S-chaetae and subapical organite of Ant. IV. Twenty different types of chaetae are recognised and all except one are present in both species. The total numbers of ordinary chaetae and S-chaetae and their patterns of distribution on antenna are very similar between the two species (483 vs. 518 ordinary chaetae; 207 vs. 208 S-chaetae). Each type of chaetae has its own distribution pattern, markedly contrasted between dorsal and ventral side of antennae, and between antennal segments. This diversity of morphologies and distribution patterns and their similarity between the two species, as well as differences with other species of the same family, suggest that antennal chaetotaxy could provide powerful new characters for the taxonomy of Troglopedetes and related genera.


Introduction
The genus Troglopedetes Joseph, 1872 is present in both edaphic and subterranean environments (Deharveng 1987) in three regions of the world: the Mediterranean basin, Africa and tropical continental Asia. The genus includes 31 species (Bellinger et al. 1996(Bellinger et al. -2020, of which 12 are present and described from Thailand, all from caves of the northern and western part of the country (Deharveng 1988a(Deharveng , 1990Deharveng and Gers 1993;Jantarit et al. 2016). Thailand is today the richest country of the world for Troglopedetes, which is diversified both in caves and in soils, with many undescribed species (Jantarit et al. 2016). The genus was only known from localities north of the Kra Isthmus, and replaced further south by the closely related genus Cyphoderopsis Carpenter, 1917, with most of its species still undescribed (Deharveng and Gers 1993;Jantarit et al. 2013Jantarit et al. , 2016. The genus Troglopedetes was erected in 1872 by Joseph for T. albus Joseph, 1872 from caves in Slovenia. Later, Absolon (1907) also used the genus name Troglopedetes for his type T. pallidus Absolon, 1907 from a cave in Slovenia, without acknowledging Joseph's publication. This is currently accepted today, although in fact T. albus is not a nomen nudum, "however meager the description may be" according to Ellis and Bellinger (1973), who then placed Troglopedetes Absolon, 1907 as a junior homonym of Troglopedetes Joseph, 1872. Hence, Troglopedetes Joseph, 1872 is not a nomen oblitum and should be considered as the valid name for the genus in order of precedence.
The genus was subsequently recorded from Thailand by Deharveng (1988a) with T. fredstonei, a species from Chiang Dao cave in northern Thailand, and eleven species were later described from northern and western Thailand (Deharveng 1990;Deharveng and Gers 1993;Jantarit et al. 2016). The diagnostic characters for discriminating the different Thai species were primarily based on the dorsal chaetotaxy of head and on a set of adaptive (pigmentation, eye number, antenna length, claw complex) and nonadaptive (spines on dens) characters. Interspecific differences also exist for the length of chaetae of the labial basis and macrochaetotaxy.
Troglopedetes is one of the best-defined genera of Paronellidae by the subdivision of its 4 th antennal segment, a feature unique in the family. However, other important taxonomic characters have not been described in detail in many species, like the dorsal chaetotaxy of head and body, S-chaetal pattern on antennae and tergites, trichobothria complex, arrangement of pseudopores, and furcal structure. Regarding the antennae, like for other Entomobryoidea, only a few chaetae have been considered in published descriptions, mostly those of the sensorial organ of the third antennal segment. The complexity of antennal chaetotaxy in Entomobryoidea, with its large number of chaetal types and strong polychaetosis, may explain why authors have been so reluctant to analyse these organs in great detail. Recent works, however, by Fanciulli et al. (2003) and Soto-Adames et al. (2014) depicted more details of antennal chaetal patterns, focusing on a group of subcylindrical S-chaetae on antennal segments II and III of European species. Also, attempts to categorise antennal phaneres by Lukić et al. (2015Lukić et al. ( , 2018 for Heteromurus (Verhoeffiella) and Jantarit and Sangsiri (2020) for Alloscopus point to the potential interest of more thoroughly investigating the antennal chaetotaxy of Troglopedetes.
In this study, we describe two new species, T. meridionalis sp. nov. and T. kae sp. nov., from caves of peninsular Thailand. These records are the first report of the genus south of the Kra Isthmus, considered so far as a probable biogeographical transition zone between the genus Troglopedetes and the closely related genus Cyphoderopsis (Deharveng and Gers 1993;Jantarit et al. 2013). In a second part of the paper, we describe in detail the morphological diversity of antennal chaetae of the two species as well as their arrangements. The objective is to provide a reference framework for homologising as far as possible chaetae and chaetal patterns across different genera of Entomobryoidea, and for evaluating the importance of antennal chaetotaxy for the taxonomy of Entomobryoidea.

Materials and methods
The specimens of the two Troglopedetes species described here were found in the dark zone of two caves, Tham Don Non, Lang Suan district, Chumphon Province and Tham Kae, La-ngu district, Satun Province (Fig. 1). Collembola were collected with aspirators and Berlese extractors. They were stored in 95% ethanol and were mounted on slides in Marc Andre II medium after clearing in lactic acid. Morphological characters were examined using Leica DMLB and Leica DM1000 LED microscope with phase-contrast. Photos of the habitus were taken by a Canon EOS 6D with Canon EF 100mm f/2.8 Macro lens and optimised by Helicon Remote software. Stacking was performed under Helicon Focus 6. Drawings were made using a drawing tube, and figures were improved with Photoshop and Illustrator CC/PC (Adobe Inc.).

Conventions for describing chaetotaxic and pseudopore patterns
Pseudopore arrangement follows Deharveng et al. (2018). The formula of tergite pseudopores is given by half-tergite from Th. II to Abd. IV (Jantarit et al. 2013). The formula for labium basis chaetae follows the system of Gisin (1967) with the uppercase letter used for ciliated and lower-case letter for smooth chaetae. Labial chaetotaxy follows Fjellberg (1999). Dorsal chaetotaxy and chaetal areas of head follow Deharveng and Gers (1993), Jordana and Baquero (2005), and Mitra (1993). Dorsal macrochaetotaxy description combines notation of individual chaetae derived from Szeptycki (1979) with chaetal group notation (rationale explained below). Formula for dorsal macrochaetae and trichobothria are given by half-tergite from head to Abd. IV; for S-chaetae by half-tergite from head to Abd. V. Homologising ordinary chaetae between different taxa is not an easy task within Entomobryoidea where chaetotaxy has been strongly modified compared to the simple chaetotaxy of Poduromorpha, due to oligochaetosis, polychaetosis, variability, and usually unequally sized tergites (Yoshii 1989, Soto-Adames et al. 2014) as well as the important shift and secondary grouping of chaetae. It has been stressed for many years that homologies should be rooted in the analysis of first instars (Deharveng 1979;Szeptycki 1979;Zhang et al. 2011;Soto-Adames and Bellini 2015), but this has been done for a very few species only. Mitra (1973) recorded the development of tergite chaetae in the different stages of a species of Callyntrura; for Troglopedetinae, the only information we have in this respect is the description of the first instar of Campylothorax sabanus (Wray) by Soto-Adames (2016). For Troglopedetes, which is clearly oligochaetotic with several displaced mac, we opted for a cautious approach, using homologies by chaetal groups, less precise but more robust than chaeta-to-chaeta homologies, in cases of uncertainty. S-chaetae terminology of tergites follows Zhang and Deharveng (2014). The S-microchaeta (ms) of Th. II corresponds to S-chaetae type 1 of Jantarit et al. (2013) and microsensillum of Soto-Adames et al. (2014). The S-chaeta "sens" corresponds to S-chaetae type 2 of Jantarit et al. (2013) and to lateral sensillum of Soto-Adames et al. (2014).

Subfamily Troglopedetinae
In the concept of Soto-Adames et al. (2014), the tribe Troglopedetini is considered a synonym of Paronellini. This is plausible, but not supported by the last detailed redescription of Paronella fusca Schött, 1893, type species of the genus Paronella, by Mitra (1992), who gives 2+2 trichobothria on Abd. IV (vs. 3+3 in Troglopedetini), the main character separating the two tribes. Re-examination of the type material of the species is needed before accepting such a taxonomic decision.
Dental spines. Dens of Troglopedetes is elongated with either one or two rows of spines, those of the external row larger and more serrated than those of the internal one that are rather short and smooth. Soto-Adames et al. (2014) mistakenly stated that Troglopedetes has a single row of dental spines. In fact, species of the Mediterranean region, Africa, south-west and central Asia known so far have only one row of 8-45 dental spines, but all Thai species have two rows of dental spines (internal row with 9-45 spines, external one with fewer spines).
Chaetal types. Four types of chaetae on somites, appendages (except antennae) and mouthparts: scales, present on antennae I and II, head, body and furca, absent on legs and ventral tube; ordinary chaetae on all body parts; S-chaetae and trichobothria on tergites; hairs devoid of sockets on outer maxillary lobe and labial papilla. Chaetal types on antennae are much more diverse and described further separately.
Mouthparts. Labral formula 4/5,5,4 (Fig. 2G); prelabral chaetae short, bent and ciliated, labral chaetae thinner, longer, smooth and acuminate, those of the distal row slightly shorter than those of the median row. Ventro-distal complex of labrum well differentiated, asymmetrical, with 2 distal combs (a larger one with 6-8 teeth on the left side, a smaller one with more than ten minute teeth on the right side) and an axial pair of sinuous tubules as in Cyphoderopsis (Jantarit et al. 2013) (Fig. 2F). Distal part of labrum not adorned with spines dorso-distally. Labial palp similar to that described by Fjellberg (1999) for Troglopedetes sp., with strong papillate chaetae. Indicative number of guards for each major papillate chaetae: A (0), B (5), C (0), D (4) and E (4); lateral process subcylindrical, reaching slightly above the apex of papilla E (Fig. 2C); five smooth proximal chaetae. Chaetae of labial basis as M1M2REL1l2, with M1, M2, E and L1 subequal and ciliated, R shorter than others and ciliated, l2 short, smooth and acuminate (Fig. 2I). Outer maxillary lobe with one papillate chaeta, one basal chaeta and two sublobal hairs, shorter than others (Fig. 2B). Maxillary head with a 3-toothed claw, several stout shortly ciliated lamellae not observed in detail and three thin elongate structures (two dorsally and one ventrally) (Fig. 2D, E). Mandible head strong, asymmetrical (left side with four teeth, right side with five teeth); molar plate with three strong pointed basal teeth, and other two or three inner distal teeth, identical in both mandibles (Fig. 2H).
Ventral chaetotaxy of head (Fig. 2I). Head densely covered with oval scales (40-50 µm), postlabial chaetae along the linea ventralis as three mes anteriorly, one mac and an oblique line of five mes posteriorly on each side.
Antennae . Antennae shorter than body, 2.2 × (n = 6) as long as cephalic diagonal. Ant. IV subdivided into two subequal segments, without apical bulb (Fig. 5). Lengths of antennal segments I-IV (IVa+IVb) as 1:1.9:1.3:2.5 (average, n = 6). Two other specimens with Ant. II and III fused (Figs 6, 7). Antennal chaetae (scales, five types of ordinary chaetae, 14 types of S-chaetae and subapical organite) described separately. Antennal scales oval, present dorsally only on Ant. I and II and ventrally on Ant. II, absent ventrally on Ant. I, and absent on Ant. III and IV ( Head with 12-13 peri-antennal mac in line on each side, with 4+4 central mac (chaetae A, B, E, F of Deharveng and Gers (1993), absence of the chaetae C, D and G, cephalic mes short, feebly serrated, equal, symmetrically arranged (not analysed). One lateral cephalic trichobothrium much shorter than closest mac on each side; su-   ture zone not visible ( Fig. 2A). Head dorsally densely covered with round to oval scales (20-35 µm). Body densely covered with oval scales (15-50 µm). Th. II with a collar consisting of a few rows of mac along its anterior and anterolateral margins, a compact group of six central mac on each side ("P3 complex" of Soto-Adames et al. (2014) and two antero-lateral mac; one antero-lateral ms; one antero-lateral sens; two or three short mic laterally, and a few others not counted centrally (Fig. 8A).
Th. III with four mac by side (a group of three central and one anterior to them), one sens at antero-lateral margins, and ca. eight or nine mac or long mes at lateral margins (Fig. 8A).
Abd. I without central mac, with one ms laterally on each side, and three mes laterally, mic not counted (Fig. 8A).
Abd. II with two tric on each side and six or seven modified mes around them (three or four around the internal tric and three near external tric), two mac (one near internal tric and one near external tric), one sens near internal tric (Fig. 8A, B), three mic (one close to internal tric and two close to external tric), others mes sockets internally visible, not counted.
Abd. III with three tric on each side (one internal, two external) and eight or nine modified mes around tric (two near internal tric, six or seven near the two external tric); four mac (one near internal tric and three near external tric); one sens anterior to internal tric and one ms posterior to the two external tric; several mes at lateral margins, not counted ( Fig. 8A, C).
Abd. IV with three tric on each side (two antero-lateral, one postero-lateral) and ca. 7-9 modified mes around the two antero-lateral tric; postero-lateral tric without modified mes. Mac distributed as three central on each side (one antero-external to pseudopore, two anterior to posterior tergite margin), one near postero-lateral tric, and at least four external, mixed with many mes or smaller mac on lateral to posterior margins (not counted); probably three sens anteriorly; at least six S-like chaetae sensu Lukić et al. (2015) anteriorly, and several mes or S-like chaetae uniformly distributed (not counted); six serrated mes in line in the posterior row, four near axis and two along pseudopore line, from medium to short size ( Fig. 8A, D).
Abd. V with only one sens detected on each side, and several ordinary chaetae from mes to mac, not counted (Fig. 8A). Abd. VI chaetotaxy not analysed.
Ecology. Troglopedetes kae sp. nov. is only known from a small chamber in the dark zone of a cave, accessible by a very low and narrow passage. Specimens were found as small populations in an oligotrophic habitat, i.e., on wall and ground with very humid and wet environment, without any trace of organic matter.
Etymology. The species name is taken from the type locality (Tham Kae). Remarks. Troglopedetes kae sp. nov. has four medial head macrochaetae, three medial Abd. IV macrochaetae, one inner teeth of claw and mucro with five teeth. It is near T. centralis Deharveng & Gers, 1993 from a cave in Doi Chiang Dao, Chiang Mai province in the absence of eyes, chaetotaxy of labial basis and of outer maxillary lobe, dorsal macrochaetotaxy from head to Abd. IV, chaetotaxy of anterior side of ventral tube and claw morphology. However, T. kae sp. nov. differs from T. centralis by its smaller size    (1.3-1.6 vs. 1.7-2.1 mm), body colour with spots of orange pigment (vs. white), longer antennae (0.5-0.6 × as long as body vs. 0.4), thinner claw with an internal tooth at 30 vs. 50-65% from the basis, chaetae on lateral flap of the ventral tube (6+6 vs. 7+7), higher ratio dens:mucro (9.3-13.7 vs. 8.8), and mucro with five teeth (vs. four).
In the same cave and same habitat, we found another morphotype with very different claw complex; thick and clavate tenent hair, two strong inner teeth on claw at 74% and 91% of inner edge, and external edge of unguiculus with two or three minute outer basal teeth, sometimes inconspicuous (Fig. 9B). However, material was not enough to describe it in detail.
The locality where T. kae sp. nov. was collected is located 400 km south of the Isthmus of Kra, i.e., more south than other described Troglopedetes from Thailand. The Isthmus of Kra is well-known to be a biogeographical transition zone between Indochinese and Sundaic fauna (Fig. 1). It was previously considered to be the southern distribution limit for the genus Troglopedetes, and the northern limit for the closely related genus Cyphoderopsis in Thailand Gers 1993, Jantarit et al. 2013). This discovery provides evidence that the two genera may actually overlap in southern Thailand, and questions the role of the Isthmus of Kra as biogeographical barrier for cave fauna.  Table 2.
Description. Habitus. Slightly troglomorphic, slender, with elongate legs, furca and antennae (Figs 1, 11A). Body length 1.3-1.5 mm. Fourth abdominal segment 3 × as long as the third one along dorsal axis. Furca well developed, ca. 2 × shorter than body length. Body colour white with spots of orange pigment. Eyes absent, no ocular patch.
Mouthparts. Labral formula 4/5,5,4; prelabral chaetae short, bent and ciliated, labral chaetae thinner, smooth and acuminate, those of the distal row slightly shorter than those of the median row. Ventro-distal complex of labrum well differentiated, asymmetrical, with two distal combs (a larger one with 6-8 teeth on the left side, a smaller one with more than ten minute teeth on the right side) and an axial pair of sinuous tubules. Distal part of labrum not adorned with spines dorso-distally. Labial palp similar to that of T. kae sp. nov. (Fig. 11B). Chaetae of labial basis as M1M2REL1l2. Chaetae M1, M2, E and L1 subequal and ciliated, R shorter than others and ciliated, l2 short, smooth and acuminate (Fig. 11D, G). Outer maxillary lobe with one papillate chaeta, one basal chaeta and two sublobal hairs, shorter than others (Fig. 11C). Maxillary head with a 3-toothed claw, several stout shortly ciliated lamellae not detailed here; special structures present on the maxilla head, i.e., a thin elongate structure, arising from the basis of the maxilla head and reaching claw basis; close to it a spiny structure and a thin structure in an opposite side (Fig. 11F). Mandible heads strong, asymmetrical (left side with four teeth, right side with five teeth); molar plate with three strong pointed basal teeth, and other two or three inner distal teeth, identical in both mandibles (Fig. 11E).
Ventral chaetotaxy of head (Fig. 11D, G). Head densely covered with oval scales (40-50 µm), postlabial chaetae along the linea ventralis as three mes anteriorly, one mac and one oblique line of four or five mes posteriorly on each side.
Th. II collar consisting of a few rows of mac along its anterior and antero-lateral margins, a compact group of six central mac on each side ("P3 complex") and two antero-lateral mac; one antero-lateral ms; one antero-lateral sens; two or three short mic laterally, and a few others not counted centrally (Fig. 15A).
Th. III with four mac on each side (a group of three central and one anterior to them); one sens at antero-lateral margins; one mic laterally; and ca. 11+11 mac or long mes at lateral margins (Fig. 15A).
Abd. I without central mac, with one ms laterally on each side; two or three mic arranged in line externally to pseudopore and two larger lateral mic; three mes laterally (Fig. 15A).
Abd. II with two tric on each side and six or seven modified mes around them (two around the internal tric and four or five near external tric), two mac (one near internal tric and one near external tric), one sens near internal tric; three mic (one close to internal tric and two close to external tric), others mes sockets internally visible, not counted (Fig. 15A, B).
Abd. III with three tric on each side (one internal, two external) and nine or ten modified mes around tric (two near internal tric and seven or eight near the two external tric); four mac (one near internal tric and three near external tric); one sens anterior to internal tric and one ms posterior to the two external tric; two mic/mes external to the external mac; several mes at lateral margins, not counted (Fig. 15A, C).
Abd. IV with three tric on each side (two antero-lateral, one postero-lateral); and ca. 6-9 modified mes around the two antero-lateral tric; postero-lateral tric without modified mes. Mac distributed as three central on each side (one antero-external to pseudopore, two anterior to posterior tergite margin), one near postero-lateral tric, and four or five external, mixed with many mes or smaller mac on lateral to posterior margins (not counted); probably three sens anteriorly; at least seven S-like chaetae sensu Lukić et al. (2015), and several mes or S-like chaetae uniformly distributed (not counted); six serrated mes in line in the posterior row (one near axis and five along pseudopore line, from medium to short) (Fig. 15A, D).
Abd. V with only two sens detected on each side, and several ordinary chaetae from mes to mac in size (Fig. 15A). Abd. VI chaetotaxy not analysed.
Ventral tube (Fig. 16C). Ventral tube 4 × longer than wide; with 3+3 long serrated mac anteriorly and six mes (two ciliated and four smooth) on each lateral flap (Fig. 16C); posteriorly with many long ciliated mes, not suitable for observation in available specimens.
Mucro rather stout, short, 12-15 × shorter than dens (Fig. 16E, F), with four main teeth, the apical one blunt and strong, the two subapical ones smaller, the dorso-basal one slightly longer with one or two toothlets basally (Fig. 16F).
Genital plate. Genital plate not seen. Ecology. Troglopedetes meridionalis sp. nov. was found in small populations in the dark zone of a cave, foraging on a small patch of old and humid bat guano.
Etymology. The name of the species, meridionalis, means southern in Latin, referring to the location of the species in the southern part of peninsular Thailand.
Remarks. Troglopedetes meridionalis sp. nov. is the only species of the genus with one medial head macrochaeta. It is similar to T. convergens Deharveng & Gers, 1993 from a cave of Ratchaburi province in the absence of eyes, two rows of dental spines, labial basis chaetotaxy, similar dorsal chaetotaxy from head to Abd. IV, anterior ventral tube chaetotaxy, and claw morphology. It can be distinguished by the combination of characters listed in Tables 3A and 3B, in particular the absence of the chaetae "E" on head, larger size (1.3-1.5 vs. 0.95-1.3 mm), antennae relatively shorter (0.35-0.4  vs. 0.45-0.5 × as long as body), tenent hair acuminate vs. usually clavate, each lateral flap of ventral tube with six chaetae (vs. seven), dental spines of the internal row more numerous (30-37 vs. 19-23) and higher ratio of dens:mucro (12-15 vs. 9). The pair of mac immediately ahead A and that ahead the uneven anterior mac on head are not figured in Deharveng and Gers (1993), as they were not considered as mac, but long mes. Their sockets in the new species are smaller than those of mac, but clearly marked. Diversity of antennal chaetae in T. kae sp. nov. and T. meridionalis sp. nov.
The work presented below is an attempt to build a comprehensive list of phanere types found on the antennae of an Entomobryoidea, to describe them morphologically, and to explore their distribution on antennal surface. Antennae bears many useful taxonomic and phylogenetic characters in Poduromorpha and has been the object of many studies (Deharveng 1981). In Entomobryoidea, antennal chaetotaxy is much more complex and variable, and has never been thoroughly explored for this reason. Only a few characters easy to observe are routinely used in taxonomy. Three recent papers, however (Lukić et al. 2015(Lukić et al. , 2018Jantarit and Sangsiri 2020) provide detailed information on the chaetal investiture of Verhoeffiella and Alloscopus species (family Entomobryidae). Here, we carried out a detailed analysis of antennal chaetae of the adult of the two new Troglopedetes species described above, one female (normal antennae,

Types of phaneres
There are five main types of phaneres (sensu Deharveng 1983) in Troglopedetes, like in all scaled Entomobryoidea: scales, ordinary chaetae (including spines), trichobothria, S-chaetae and subapical organite of Ant. IV. All, except trichobothria, are present on the antennae. We list phanere types that we recognise within each of these broad categories, mentioning putative homologies from the literature, and we summarise their arrangement pattern on each antennal segment.

Scales
Three types of scales were recognised in Thai Troglopedetes, but only one type was found on antennae of T. kae sp. nov. All scales are roundish, oval, variable in size on different organs, devoid of ridges but adorned by a dense and homogeneous cover of minute spicules (Fig. 12E).

Ordinary chaetae
Ordinary chaetae are the most numerous chaetae on the body and appendages of Troglopedetes. They are well-diversified on antennae (Fig. 12F), where five types have been recognised. In Verhoeffiella and Alloscopus, Lukić et al. (2015) and Jantarit and Sangsiri (2020) recognised three types of ordinary chaetae on the antennae.

Distribution patterns of antennal phaneres
In this study, ordinary chaetae on a single antenna numbered 483 for T. meridionalis sp. nov. and 518 for T. kae sp. nov. and were assigned to the 5 categories described above; 208 S-chaetae were numbered for T. meridionalis sp. nov. and 207 for T. kae sp. nov. and were assigned to the 14 morphological categories described above. (Tables 4,5). The ventral side of antenna is richer in both S-chaetae and ordinary chaetae than the dorsal side (ordinary chaetae = 278 vs. 240 in T. kae sp. nov., and 251 vs. 232 in T. meridionalis sp. nov.; S-chaetae = 115 vs. 92 in T. kae sp. nov., and 134 vs. 76 in T. meridionalis sp. nov.) (Tables 4, 5). The distribution of the different types of Schaetae along the antennal segments is arranged in more or less clearly defined patterns which are described in the following paragraphs, summarised in Tables 4 and 5, and illustrated in Figures 3-7, 11H-I, 12A-D, 13, 14.
First antennal segment: eight types of S-chaetae are recognised (Figs 3A, B, 11H, 12A-D): type 2, type 3, type 5, type 6, type 8, type 9, type 10, and type 12. Only one type is present on the dorsal side (type 6), the others are located on the ventral side.
Third antennal segment: six types of S-chaetae are recognised (Figs 4A-C, 11H, I, 13A-C): type 1, type 4, type 5, type 7 type 8, and type 10. All are present on the dorsal side and three types are present on the ventral side (type 7, type 8, and type 10). Table 4. Detailed distribution of antennal chaetae. Presence in Verhoeffiella and Alloscopus by comparison with chaetal morphologies described by Lukic et al. (2015Lukic et al. ( , 2018 and Jantarit and Sangsiri (2020) respectively, ? = no information. Fourth antennal segment: five types of S-chaetae are recognised (Figs 5A, B, 7A, B, 11H, 14A-C): type 7, type 8, type 10, type 11, and type 13. All are present on the dorsal side, but only two types are present on the ventral side (type 8 and type 11). They are distributed as follows on each subsegment.

Type of chaetae Distribution on antenna
Fourth antennal segment I (a): five types of S-chaetae are recognised: type 7, type 8, type 10, type 11 and type 13 (T. meridionalis sp. nov. has only three types while T. kae sp. nov. has all types), all present on the dorsal side.
Fourth antennal segment II (b): three types of S-chaetae are recognised: type 8, type 10, and type 11, all present on the dorsal side. Two types are present on the ventral side (type 8 and type 11).
The most frequent S-chaetae are type 8 and type 10 that are present all along antennal segments, followed by type 7 that was found on three antennal segments (Ant. II, III, IVa), but not on Ant. I. Type 2, type 3, and type 9 were found on only two segments (Ant. I and Ant. II). Type 5 was found on only two segments (Ant. I and Ant. III). Other types are all limited to a single antennal segment: Ant. I (type 6 and type 12), Ant. II (type 14), Ant. III (type 1, type 4) or Ant. IV (type 11, and type 13 in T. kae sp. nov.) (see Tables 4, 5 for details).
With regard to the abundance of S-chaetae along antennal segments, type 8 is the most common followed by type 11 (Tables 4, 5). Ant. IV has the highest number of S-chaetae in both species followed by Ant. I, III, and II respectively in T. meridionalis sp. nov. and Ant. III, I and II respectively in T. kae sp. nov. (Table 5), while ordinary chaetae abundance ranks as Ant. IV, II, III and I (Table 5). The distal subsegment of antenna IV (Ant. IVb) is richer in S-chaetae than the proximal subsegment Ant. IVa, but their respective number of ordinary chaetae is rather similar (Table 5).

Discussion
The diversity of chaetal types was very similar in the two studied species, and betweenspecies differences in the relative numbers of each chaetal type were limited, probably only reflecting individual variability. Similarities with Verhoeffiella and Alloscopus have been noted, but there was also many differences regarding the types of chaetae. It is not clear whether chaetal morphologies which seem special to one of the three genera are really taxon-specific receptors, or the result of undetected homologies due to different morphological evolution of some chaetae in the two genera. The only way to test these hypotheses will be to investigate thoroughly Entomobryoidea of other lineages.
The antennal phaneres of T. meridionalis sp. nov. and T. kae sp. nov. are arranged in a complex pattern. On the 20 morphological types of chaetae that we recognised (five types for ordinary chaetae, 14 types for S-chaetae and subapical organite of Ant. IV), 12 types were located at a fixed position on antennal segments (four types of ordinary mes; S-chaetae type 1, type 4, type 6, type 11, type 12, type 13, and type 14; subapical organite of Ant. IV) (Tables 4, 5). It would be expected that the number of chaetae should directly vary with the length of antennal segments, ranked as Ant. IV > II > III > I. This applies only to the overall result and to ordinary chaetae (Table 5) but not or only partly to S-chaetae. In particular, Ant. IV (a and b) is longer than other segments, but less rich in types of S-chaetae (Tables 4 and 5). Interestingly, however, the distal subsegment of antenna IV (Ant. IVb) possesses more kinds and a higher density of Sand ordinary chaetae than the proximal one (Ant. IVa), with a complex arrangement of type 11 S-chaetae (Figs 5A, B, 7A-B, 11H, 14A-C). Ant. II, though longer than Ant. I or Ant. III, has a similar or lower number of S-chaetae than Ant. I and III, but it bears more ordinary chaetae than the others (Tables 4, 5). Ant. I, the shortest segment, is proportionally the richest in diversity and number of S-chaetae (eight different types, Tables 4, 5), highlighting the importance of this segment for sensory reception. Ant. III, as common in Collembola, bears a complex sensorial structure (AIIIO) with typically five S-chaetae, conserved across most species of Collembola and widely used in taxonomy (Figs 4A-C, 11I). Antennae appears therefore as a mosaic of sensorial areas, with probably different sensorial functions which remain undocumented in Entomobryoidea.
It is rather common that Ant. II and III fuse together ( Fig. 6A-C). It can be found asymmetrically in a single antenna or in both antennae, making the antenna(e) a little shorter (Deharveng and Gers 1993;Lukić et al. 2018), with a chaetotaxic pattern strongly modified. For example, in T. kae sp. nov., S-chaetae type 13 is found only in the specimens with fused Ant. II and III. Such fusing may be due to regeneration after the loss of antennal segments following predator attack as observed by Ernsting and Fokkema (1983) in Orchesella.
S-chaetae on antennal segments vary in number and probably type diversity, depending on size, age, and sex of Collembola, but this remains to be documented. They are also related to species ecology. Cave adapted species in particular are said to have more developed sensory structures than surface species (Deharveng 1988b;Thibaud and Deharveng 1994;Lukić 2019). A group of subcylindrical S-chaetae (type 10) on antennal segments II and III is for instance present in three Mediterranean cave species of the genus (T. ruffoi in Delamare-Deboutteville 1951and Fanciulli et al. 2003, and T. absoloni and T. ildumensis in Soto-Adames et al. 2014). This character, however, is unknown in other Mediterranean species. It is absent in the described Thai species, all of which are so far cave-restricted, where chaetae of type 10 never clusters, and are in significantly lower number (Figs 3C, D, 4A, B, 13A-F). The multiplication of type 10 S-chaetae cannot be therefore considered a troglomorphic character at the moment. This character has evolved in caves for Mediterranean lineages cannot be ruled out, but this could only be confirmed by examination of antennal morphology of surface species in the region, which has not been done so far. Nevertheless, antennal elongation observed in most cave Troglopedetes is associated to an increase of the number of antennal receptors.
Antennal chaetotaxic characters are widely used for the supraspecific taxonomy of Poduromorpha (Deharveng 1981), but much less in Entomobryoidea (Chen and Christiansen 1993;Deharveng and Bedos 1996;Lukić et al. 2015Lukić et al. , 2018, due to their complexity. The diversity and pattern of antennal chaetae described in this work are intended serve as reference for further comparisons in this respect. The recognised chaetal types need to be homologised in morphology and distribution across more genera of Entomobryoidea, and limited data available in the literature or from personal observation already indicate that this is possible in many cases. Knowledge of antennal S-chaetae in Entomobryoidea may prove to be as taxonomically significant as it has been in Poduromorpha. In a broader context, the complexity of chaetal types and distribution patterns described illustrate the functional complexity of arthropod antennae. Though information is lacking, chaetal pattern of distribution on antennae is probably mostly related to their function and the abiotic environment (light, soil, water, food) and interaction within communities. Antennal sensilla ultrastructure and functions in Collembola were studied by Altner' team in several papers (e.g., Altner and Kuhn 1989), but very few taxa and antennal sensilla have been examined, e.g., Waldorf (1976), Verhoef et al. (1977), Altner and Thies (1978), Slifer and Sekhon (1978). In a further step, the chaetal types we recognised here, useful for taxonomical purpose, will have to be assigned as far as possible to the categories used by morphologists (Zacharuk 1985), in order to gain insight into their functional organisation on the antenna.