Materials

In total, 63 specimens belonging to the Leptanillinae were included in this study, in addition to Martialis heureka Rabeling & Verhaagh, 2008 (Martialinae) as an outgroup. Ultra-conserved element data are included in this study for 51 taxa of those 63, including M. heureka, a representative of Y. laventa (CASENT0842745), along with representatives of all major subclades of that subfamily. Thirteen of these specimens are newly sequenced in this study. Twelve specimens of Protanilla Taylor in Bolton (1990) and Leptanilla along with one Anomalomyrma Taylor in Bolton (1990) were morphologically examined, but not sequenced (Table 1). Generic assignment of sequenced material follows Griebenow (2020) rather than Borowiec et al. (2019) where sampling overlaps with those studies. Additional collection data for these specimens are provided in a   Supplementary Table S1 (IS22035_AC.PDF).

Table 1. 

Specimens are deposited at the following institutions (abbreviations follow  http://hbs.bishopmuseum.org/codens/, where applicable): the California Academy of Sciences, San Francisco, USA (CAS); the California State Collection of Arthropods, Sacramento, CA, USA (CSCA); the Biodiversity Museum, University of Hong Kong, Hong Kong, PR China (HKUBM); the Jalal Afshar Zoological Museum, Department of Plant Protection, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran (JAZM); the Museum of Comparative Zoology, Harvard University, Cambridge, MA, USA (MCZC); Lund University, Lund, Sweden (MZLU); National Changhua University of Education, Changhua, Taiwan (NCUE); the Okinawa Institute of Science & Technology, Onna-son, Japan (OIST); the Royal Ontario Museum, Toronto, ON, Canada (ROME); the R. M. Bohart Museum of Entomology, University of California, Davis, CA, USA (UCDC); the Museum für Naturkunde der Humboldt-Universität, Berlin, Germany (ZMHB); and the Zoological Museum, University of Isfahan, Isfahan, Iran (ZMUI).

Collection and specimen preparation

Specimens of Yavnella laventa sp. nov. were collected using subterranean sampling devices, i.e. two buried pitfall traps, placed in the MSS. The buried pitfall traps (hereinafter ‘MSS traps’) consisted of a rigid plastic cup, with holes bored in them midway from top to bottom to prevent flooding, and a horizontal stone placed on top. These were set in a clast on the bank of a wadi, opposite to a salt diapir (Fig. 1) (the Khoorab Salt Dome; Abbassi et al. 2015). These MSS traps were placed by slope boring at depths of 60–100 cm, baited with sardines and dates jointly contained in small vials, and half filled with brine. We attempted to measure relative humidity (RH) within the MSS using a Lascar EL-USB-2 data logger buried adjacent to the MSS traps. Brine is not an ideal preservative for purposes of acquiring DNA, but was used in a broad survey of salt karst fauna in the vicinity of Khoorab because of low evaporative rate. Although extraction of a genome-scale molecular dataset was successful when attempted with a single specimen (see ‘Sequencing & data processing’), we recommend that future targeted efforts to collect leptanilline ants in the MSS or in salt caves use ethanol as a preservative, with traps being set for much briefer periods than outlined below.

Fig. 1. 

Layout of MSS traps, and position relative to the adjacent salt diapir (the Khoorab Salt Dome) in which Last Cave is located.

Traps were left for 15 months, from 14 February 2019 to 26 June 2020, at the end of which specimens were transferred to 80 or 95% ethanol, the latter if intended for non-destructive DNA extraction. A few specimens remained in brine and were used for dissection and imaging. In addition, four pitfall traps baited in the same way and containing the same liquid were placed at ground level inside an adjacent salt cave (‘Last Cave’) within the Khoorab Salt Dome, for the same duration as the MSS traps, but no Y. laventa were collected in these traps. We measured RH within Last Cave using the same data loggers as listed above.

Sequencing & data processing

For the specimens newly sequenced for this study, DNA was extracted non-destructively using a Dneasy Blood & Tissue Kit (Qiagen Inc., Valencia, CA, USA) with H₂O at room temperature to elute DNA, or, in the case of CASENT0842745 and several other samples, 56°C buffer AE (Cruaud et al. 2019) in order to increase DNA yield. Genomic concentrations were quantified for each sample with a Qubit fluorometer (ver. 2.0, Life Technologies Inc., Carlsbad, CA, USA). Input DNA was sheared using a Diagenode Bioruptor (Diagenode, Denville, NJ, USA) or Qsonica Q800R3-110 (Qsonica Inc., Newtown, CT, USA). Sheared product was used as input for the modified library preparation protocol of Branstetter et al. (2017), with the ant-specific version of the UCE probe set hym-v2 (Branstetter et al. 2017). Enrichment success and size-adjusted DNA concentrations of pooled libraries were assessed using the SYBR FAST qPCR kit (Kapa Biosystems, Wilmington, MA, USA), and all pools were combined into an equimolar final pool. Final pools were sequenced on an Illumina HiSeq X at Novogene (Sacramento, CA, USA) or prepared, enriched and sequenced using similar protocols at RAPiD Genomics (Gainesville, FL, USA). Refer to Ward and Blaimer (2022) for further details on library preparation and enrichment. For sequencing protocols implemented for the phylogenomic data used in this study that have been previously published, refer to Griebenow (2020).

The FASTQ output was demultiplexed and cleansed of adapter contamination and low-quality reads using illumiprocessor (ver. 2.1, B. C. Faircloth, see  https://github.com/faircloth-lab/illumiprocessor) in the PHYLUCE bioinformatic software package (ver. 1.7.1, see  https://phyluce.readthedocs.io/en/latest/; Faircloth 2016). Raw reads were assembled with SPAdes (ver. 3.12.0, see  https://github.com/ablab/spades; Bankevich et al. 2012). Species-specific contig assemblies were obtained with the ant-specific hym-v2 probe set (Branstetter et al. 2017), aligned with MAFFT L-INS-I (ver. 7.741, see  https://github.com/GSLBiotech/mafft; Katoh and Toh 2010), and trimmed with Gblocks (ver. 0.91, see  https://home.cc.umanitoba.ca/~psgendb/doc/Castresana/Gblocks_documentation.html; Castresana 2000) within a PHYLUCE workflow modified from Faircloth (2016) with min_identity = 80 within phyluce_assembly_match_contigs_to_probes.py, resulting in an alignment 313 498 bp long. This alignment was 76.79% complete, composed of 39.1% parsimony-informative sites; AT content was 57.4%. Summary statistics for this alignment were computed with the summary command in AMAS (see  https://github.com/marekborowiec/AMAS; Borowiec 2016) (Table 1).

Phylogenomic inference

Partitioning to generate subsets of each UCE locus was performed using PartitionUCE (see  https://github.com/Tagliacollo/PartitionUCE; Tagliacollo and Lanfear 2018). Using IQ-Tree (ver. 2.1.2, see  http://www.iqtree.org/; Minh et al. 2020) on the CIPRES Science Gateway (ver. 3.3, see  http://www.phylo.org/; Miller et al. 2010), partition schemes were inferred with ModelFinder in IQ-Tree (ver. 2.1.2; Kalyaanamoorthy et al. 2017) using subsets generated by PartitionUCE for the complete alignment, with the Bayesian Information Criterion (BIC) deciding among available partitioning schemes, followed by maximum-likelihood (ML) phylogenetic inference under these partition schemes (Chernomor et al. 2016) for 1000 ultrafast bootstraps (UFBoot) (Hoang et al. 2018) and SH-like approximate likelihood ratio test (SH-aLRT) (Guindon et al. 2010) replicates. The relaxed hierarchical clustering algorithm was implemented in these analyses (Lanfear et al. 2014), with ModelFinder considering only the most likely 20% of partition schemes. Substitution models with I+G extensions to accommodate among-site rate heterogeneity were permitted, as version 1.4.3 onward of IQ-Tree implements an optimisation heuristic that effectively compensates for the non-identifiability of these models (Nguyen et al. 2018). Bayesian inference was performed in ExaBayes (ver. 1.5.1, see  https://github.com/aberer/exabayes; Aberer et al. 2014) on the CIPRES Science Gateway under the partitioning scheme produced by ModelFinder in IQ-Tree (as described above) with GTR+G imposed across all partitions, all parameters unlinked, with the single Markov Chain Monte Carlo (MCMC) running until the average standard deviation of split frequencies (ASDSF) of topologies <0.05, for 100 000 generations. Convergence of the MCMC with respect to continuous parameters was visually assessed in Tracer (ver. 1.7, see  https://github.com/beast-dev/tracer/; Rambaut et al. 2018).

Nomenclature

Nomenclature for sculpturation follows Harris (1979); setation, Wilson (1955) and Boudinot et al. (2020). Notation of palp and tibial spur formulae follows Bolton (2003). Cephalic nomenclature follows Richter et al. (2021) and Boudinot et al. (2021). Mesosomal nomenclature follows Liu et al. (2019); metasomal, Lieberman et al. (2022). Male genital nomenclature follows Boudinot (2018).

Measurements

Sorting and initial examination of the material was done using an Echo-Lab SM203H stereomicroscope (DEVCO, Milan, Italy). Morphometric data for four specimens of Y. laventa are included in Table 2. Detailed morphological study of these specimens was performed with a Leica MZ75 compound microscope (Leica Microsystems, Oak Grove, IL, USA) at magnifications of up to 50×. Photographs were obtained as image stacks using a Leica DMC2900 camera attached to a Leica MZ16A stereomicroscope or using the Visionary Digital Imaging System (Visionary Digital, Richmond, VA, USA), with z-stepping in the Leica Application Suite (LAS) software (ver. 4.13.0, see  https://www.leica-microsystems.com/products/microscope-software/p/leica-las-x-ls/) and montaged with Helicon Focus Pro (ver. 7.7.4, Helicon Software Ltd, Kharkiv, Ukraine). Scanning electron microscopy was performed with a Hitachi TM4000 (Hitachi Global, Tokyo, Japan). Measurement and index definitions are provided below.

Table 2. 

HW, maximum width of cranium in full-face view.

HL, head length, maximum length of head in full-face view from anterior margin of head to cranial vertex.

SL, scape length, maximum length of scape in medial view, excluding bulbus.

MaL, mandible length, maximum length of mandible from view orthogonal to lateral mandibular margin, measured from ventral mandibular articulation to mandibular apex.

WL, Weber’s length, maximum diagonal length of mesosoma in profile view, measured from most anterior extent of pronotum excluding cervical shield to most posterior extent of propodeal lobes, when present.

PrW, pronotal width, maximum width of pronotum, measured in dorsal view.

MW, mesonotal width, maximum width of mesonotum in dorsal view, measured immediately anterior to mesocoxal foramen.

PTL, petiolar length, maximum length of petiole in dorsal view, not including presclerites.

PTH, petiolar height, maximum height of petiole in profile view, including sternal process and dorsal node, if distinct.

PTW, petiolar width, maximum width of petiole in dorsal view.

PPL, postpetiolar length, maximum length of postpetiole in dorsal view, not including presclerites.

PPW, postpetiolar width, maximum width of postpetiole in dorsal view.

PPH, postpetiolar height, maximum height of postpetiole in profile view, including sternal process and dorsal node, if distinct.

Indices

Class INSECTA Linnaeus, 1758

Order HYMENOPTERA Linnaeus, 1758

Family FORMICIDAE Latreille, 1809

Subfamily LEPTANILLINAE Emery, 1910

Tribe LEPTANILLINI Emery, 1910

Diagnosis (worker-based)

Palp formula 2,1 or 1,1. Mandible without differentiated basal and masticatory margins. Medial mandibular margin without regularly spaced serration (Fig. 3c, d). Peg-like chaetae absent from mandible and labrum. Clypeus without median demarcation from frons by posterior carina, anteroposteriorly compressed anterior to antennal torul; antennal torulus adjacent to, or abutting, anterior margin of cranium (Fig. 3c, d); antennal socket fully exposed. Compound eye absent. Frontal carina absent. Antenna 12-merous. Promesonotal articulation highly flexible. Mesotibia with 0–2 apical spurs. Propodeal lobe absent; propodeal spiracle situated low on propodeum. Abdominal segments II–III with tergosternal fusion. Spiracle of abdominal segment III very large and placed far forward. Abdominal segment III posteriorly constricted, forming postpetiole (Fig. 4). Spiracles of abdominal segments IV–VII concealed by posterior margins of preceding tergites. Abdominal segment IV without tergosternal fusion; stridulitrum absent from abdominal presclerite IV. Abdominal tergite VII large, with simple posterior margin. Sting present. Pretarsal claw without apical tooth on inner margin.

Fig. 3. 

Full-face view of mandibular armature across the Leptanillinae. (a) Protanilla beijingensis (CASENT0842639). Regular serration at mandibular apex outlined in red. (b) Anomalomyrma indet. (CASENT0178553). (c) Leptanilla thai (CASENT0842784). (d) Yavnella laventa (CASENT0842745). PLC, peg-like chaetae. Scale bars: a, b, 0.2 mm; c, 0.04 mm; d, 0.1 mm.

Fig. 4. 

Propodeum and abdominal segments II–IV of Yavnella laventa (CASENT0842746), dorsal view. Scale bar: 0.1 mm.

Genus Yavnella Kugler, 1987

Yavnella Kugler, 1987 [published in a 1986 volume in Kugler 1986], p. 52. Type species: Yavnella argamani, original designation.

Diagnosis (worker-based)

Three mandibular teeth present (Fig. 5a). Palp formula 2,1 (Fig. 5c). Mesotibia with 2 apical spurs. Petiole much longer than wide in dorsal view (PI ≤ 31) (Fig. 4), without distinct anterior peduncle. Abdominal segment IV constricted anteriorly in dorsal view; total length of abdominal segment IV greater than that of abdominal segments V–VII combined.

Fig. 5. 

Mouthparts of Yavnella laventa. (a) Mandibles, dorsal oblique view (CASENT0842789). (b) Mandibles, profile view (CASENT0842748). Putative ‘trigger hairs’ outlined in black. (c) Mouthparts of Yavnella laventa, ventral view (CASENT0842789). Maxillary palp (MXP) outlined in black. Scale bars: a, b, 0.1 mm; c, 0.05 mm.

Yavnella laventa Griebenow, Moradmand & Isaia, sp. nov.

(Fig. 4–11)

ZooBank: urn:lsid:zoobank.org:act:1BCBAA0B-753E-4DD4-8CFA-43CC25BCE68E

Diagnosis (worker-based)

As for genus (see above).

Fig. 6. 

Yavnella laventa, holotype (CASENT0842746). (a) Profile view. (b) Dorsal view. (c) Full-face view. Scale bars: a, b, 0.5 mm; c, 0.1 mm.

Fig. 7. 

Cranium of Yavnella laventa (CASENT0842789), posterior view. OCC, occipital carina; OCF, occipital foramen. Scale bar: 0.1 mm.

Fig. 8. 

Frontoclypeal process of Yavnella laventa (CASENT0842789), full-face view. CAR, lateral carina; FCP, frontoclypeal process. Scale bar: 0.1 mm.

Fig. 9. 

Propodeum and anterior petiole of Yavnella laventa (CASENT0842745), profile view. MLF, metapleural longitudinal flange. Scale bar: 0.05 mm.

Fig. 10. 

Tibial spurs of Yavnella laventa. (a) Antennal strigil, anterior view (CASENT0842745). (b) Protarsus, posterior view; calcar outlined (CASENT0842745). (c) Mesotibia and mesobasitarsus, posterior view (CASENT0842746). (d) Metatibial spur (CASENT0842789), posterior view. CLC, calcar; PTB, protibia; PBS, probasitarsus; MSP, anterior mesotibial spur; LSP, posterior mesotibial spur; MTP, metatibial spur. Scale bars: a, 0.1 mm; b, 0.2 mm; c, 0.05 mm; d, 0.1 mm.

Fig. 11. 

Ventral view of abdominal sternite II of Yavnella laventa (CASENT0842745). Scale bar: 0.1 mm.

Distribution

Yavnella laventa is known only from the type locality, inhabiting the MSS within a debris flow on the bank of an ephemeral stream adjacent to a salt diapir (Fig. 12). The Khoorab Salt Dome is one of ~130 salt diapirs occurring in southern Iran (Talbot and Alavi 1996). It is therefore possible that Y. laventa occurs across this area, at least in microhabitats resembling those present at the type locality.

Fig. 12. 

Map of the type locality within Iran, and the spatial relationship of the MSS traps in which Yavnella laventa was collected relative to each other and to Last Cave. Yellow marks indicate approximate positions of MSS traps, which were located below the surface of the ground. (a) Location of MSS2. (b) Location of MSS4.

Habitat

Mean annual precipitation around the type locality is ~400 mm (Zarei 2010), meaning that moisture is a limiting abiotic factor. Microclimatic conditions in the MSS at the type locality were not directly measured becuase of data logger malfunction. Indeed, RH in the MSS is rarely measured for this reason (Mammola et al. 2016). Contrary to cave habitats, in which RH is generally constant, studies of this parameter in the MSS show seasonal variation, with a drop in spring through summer (Barranco et al. 2013). Using a data logger and measuring RH in the nearby salt cave (the ‘Last Cave’), we found that RH varied seasonally from 50 to 80%, contrasting with the humidity and climatic constancy commonly associated with subterranean habitats (Cigna 2002; Badino 2010). We hypothesise that the hygroscopic property of salt causes a strong drop in humidity inside Last Cave, which may account for the absence of Y. laventa.

Key to genera of the Leptanillinae based on the worker caste

1.

Abdominal segment III not constricted posteriorly (Fig. 13a); occiput visible in full-face view (Yamada et al. 2020, fig. 1a)Opamyrma Yamane, Bui & Eguchi, 2008

–Abdominal segment III constricted posteriorly, forming postpetiole (Fig. 13b, c); occiput not visible in full-face view2

2.

Posterior face of dorsal petiolar node not distinct (Fig. 13c); abdominal segments II-III with tergotergal and sternosternal fusion partial to completeAnomalomyrma Taylor in Bolton (1990)

–Posterior face of dorsal petiolar node distinct (Fig. 13b); abdominal segments II-III without tergotergal or sternosternal fusion3

3.

Mandible with peg-like chaetae on medial face (Fig. 3a); mandible with regularly spaced dorsomedial serration; mandible lacking subapical teeth (Fig. 3a, b)Protanilla Taylor in Bolton (1990)

–Mandible without peg-like chaetae on medial face; serration present or absent from dorsomedial margin (Fig. 3c, d), if present then irregularly spaced; mandible with ≥1 subapical tooth4

4.

Frontoclypeal process present or absent (Fig. 14a), if present apex entire (Fig. 14b) or emarginate, if emarginate then apicolateral margins angular (Fig. 14c); SI < 100; PI > 31Leptanilla Emery, 1870

–Frontoclypeal process present, apex emarginate, apicolateral margins lobate (Fig. 14d); SI ≥ 100; PI ≤ 31Yavnella Kugler, 1987

Fig. 13. 

Profile view of abdominal segments II–III across the Leptanillinae. Profile of abdominal tergite II outlined in magenta; profile of abdominal segment III outlined in blue. (a) Opamyrma hungvuong (AKY05vii17-06) (Yamada et al. 2020, fig. 1C). (b) Protanilla bicolor (CASENT0235341), Estella Ortega (AntWeb, ver. 8.68.7, California Academy of Sciences, see  https://www.antweb.org). (c) Anomalomyrma helenae (CASENT0220220) (Borowiec et al. 2011, fig. 6). Scale bars: a, 0.5 mm; c, 0.2 mm; c, 0.5 mm.

Fig. 14. 

Full-face view of the frontoclypeal margin across the tribe Leptanillini. (a) Leptanilla KE01 (CASENT0842721). (b) Leptanilla boltoni (CASENT0260440). (c) Leptanilla thai (CASENT0842784). (d) Yavnella laventa (CASENT0842789). Scale bars: a, b, 0.02 mm; c, 0.035 mm; d, 0.05  mm.

Morphology

The habitus of Y. laventa is exceptional among worker Leptanillinae in the elongation of the appendages, including the scape (Table 3), flagellomeres, and tarsomeres (Fig. 6c, 15b). The anterior constriction of abdominal segment IV is also unique among the Leptanillinae, exceeding the constriction observed in Leptanilla tanakai Baroni Urbani, 1977 (Baroni Urbani 1977, fig. 33). Protanilla spp. have long scapes (SI > 100) by comparison to Leptanilla (Richter et al. 2021), but these are less elongated than in Y. laventa (Table 3). Elongation of the scapes in Protanilla was hypothesised to be a secondary reversal from the ancestral condition in the Leptanillini (Richter et al. 2021). This implies that the elongation of the scape in Y. laventa is also a secondary reversal.

Table 3. 

Fig. 15. 

Protarsi of selected Leptanilla spp. and Yavnella laventa, lateral view. (a) Leptanilla boltoni (CASENT0842753). (b) Yavnella laventa (CASENT0842745). (c) Leptanilla thai (CASENT0842752). (d) Leptanilla theryi (CASENT0842751). TRC, traction chaetae. Scale bars: a, 0.05 mm; b, 0.2 mm; c, 0.1 mm; 0.05 mm.

The elongation of appendicular articles in Y. laventa is unparalleled in the ‘leptanillomorph clade’, i.e. Martialis + Leptanillinae (Borowiec et al. 2019; Richter et al. 2021), as is the elongation of the petiole (PI = 29–32) (Tables 2, 3). Leptanillomorph workers generally have robust, short limbs, with a submoniliform antennal funiculus; this tendency is most pronounced in the Leptanillini. Along with positioning of the antennal toruli anterior to their ancestral position for the Formicidae, shortening of extremities is associated with motion in confined subterranean conditions (Eisenbeis and Wichard 1987; Richter et al. 2021). By contrast, the extremities of Y. laventa are attenuated and fragile, with the antennal funiculus filiform. This convincingly restricts this species to subterranean voids, as predicted for M. heureka (Rabeling et al. 2008, fig. 2). The sparseness of traction chaetae on the ventral tarsal surface (Fig. 10b, 15b) also implies limited digging capability in Y. laventa compared to examined Leptanilla spp. (Fig. 15).

The emarginate frontoclypeal process of Y. laventa resembles that observed in many Leptanilla spp., mostly distributed in the Indo-Malayan ecoregion. Although regarded as clypeal in origin by previous authors (e.g. Leong et al. 2018), the homology of the frontoclypeal process is unclear, since it is difficult to delimit the clypeus in the absence of the epistomal sulcus.

The mandibular surface of Y. laventa bears sparse, tapering suberect setae of mostly uniform length and diameter. Two pairs of more robust, longer suberect setae are present on the medial mandibular surface, with the distal pair positionally homologous with the putative ‘trigger hairs’ present in Protanilla lini and Protanilla rafflesi Taylor in Bolton (1990) (Richter et al. 2021). This is the first purported example of trigger hairs in the Leptanillini. The definition of trigger hairs has always been functional rather than anatomical, relying upon confirmation of ‘trap-jaw’ behaviour, or assertion by analogy to other ants for which behavioural observations exist (e.g. Creighton 1930; Barden and Grimaldi 2012; Richter et al. 2021). Observations of living Y. laventa, or three-dimensional modelling of mandibular movement in this species based upon micro-CT data, would test the hypothesised function of these mandibular setae (cf. Richter et al. 2021). Consultation of the primary literature (e.g. Man et al. 2017, fig. 5; Aswaj et al. 2020, fig. 2C; Baidya and Bagchi 2020, fig. 1C), photographs on AntWeb (ver. 8.68.7, California Academy of Sciences, see  https://www.antweb.org), and available specimens showed that the subapical mandibular seta is present and robust in all described species of Anomalomyrmini for which this information is available. The presence of a robust subapical mandibular seta was also confirmed in all available undescribed specimens of that tribe (Table 4). There were few available worker specimens belonging to the Leptanillini in which mandibular setation could be assessed. A subapical mandibular seta is present in those that were examined and in O. hungvuong (Yamada et al. 2020, fig. 2E) (Table 4) but is less produced than in the Anomalomyrmini or Y. laventa, leaving its function as a trigger hair doubtful.

Table 4. 

In Y. laventa the mandibles are elongated such that, when closed, these rest in a position subparallel to the anteroposterior axis of the cranium (Fig. 3d, 5b, 6c), resembling the Anomalomyrmini. This is not a condition previously observed in the Leptanillini. Save for the posture of the mandibles at rest, Y. laventa has little morphological commonality with the Anomalomyrmini to the exclusion of other Leptanillini. Anomalomyrmine workers are uniformly distinguished from the Leptanillini, including Yavnella, by the presence of four maxillary palpomeres; the presence of regular serration on the medial mandibular margin, and absence of large teeth from that margin; the presence of at least one peg-like chaeta on the labrum; and the median demarcation of the clypeus from the frons by a carina.

Ecology

Yavnella laventa was collected ≥60 cm. below the surface, setting the workers of this species apart from other Leptanillinae for which soil depth of origin is recorded: these workers approach the depths of Y. laventa only in Leptanilla taiwanensis Ogata, Terayama & Masuko, 1995 and Protanilla beijingensis Man, Ran, Chen & Zhu, 2017 have been collected with unbaited pitfall traps at depths of up to 55 cm (Man et al. 2017).

The biotope of Y. laventa appears to be the MSS, rather than soil as is the case in other leptanilline ants, which is consistent with the strikingly gracile phenotype of this species. The elongated, delicate limbs preclude the endogean (i.e. soil dwelling) biology otherwise observed in this subfamily, instead indicating hypogean habits. This elongation of extremities is consistent with troglomorphism. Worker Leptanillinae lack compound eyes, and so that condition in Y. laventa does not constitute troglomorphism per se, although it corroborates our supposition that this species is exclusively subterranean, as are all other leptanilline ants. Rather, the argument for troglomorphism in Y. laventa rests upon overall elongation of the extremities in conjunction with subterranean biology. This is analogous to troglomorphic Japygidae and Campodeidae (Hexapoda: Diplura), which likewise belong to an ancestrally eyeless, endogean clade, and differ from endogean relatives by larger size and elongation or multiplication of appendicular articles (Sendra et al. 2021). A similar pattern is also recovered in subterranean spiders (Araneae), e.g. Troglohyphantes spp. (Linyphiidae), in which leg length appears to correlate with habitat (pore) size (Mammola and Isaia 2016).

We here follow the definition of Bichuette et al. (2015) for troglomorphism, regarding it as constituting phenotypic traits selectively favoured by subterranean biology and apomorphic relative to non-subterranean relatives of the putatively troglomorphic lineage. By this definition, troglomorphism does not necessarily coincide with habitation in subterranean voids that are considered caves by dint of being ‘commensurable to the human scale’ (Mammola et al. 2016, p. 3). Therefore, that Y. laventa was collected in the MSS does not exclude its being troglomorphic. Indeed, this condition is unsurprising, since troglomorphic organisms are frequently encountered in the MSS (Christiansen 2005; Juberthie and Decu 2006; see Mammola et al. 2016 for further evidence from the literature).

Few ants are known to reside permanently in caves (Pape 2016) or exhibit troglomorphism (Christiansen 2005). Whereas Nylanderia pearsei Wheeler, 1938 (Formicinae: Lasiini) from the Yucatán Peninsula and an undescribed Leptogenys sp. (Ponerinae: Ponerini) from central Texas are subterranean so far as is known (Wheeler 1938; Reddell 1977; Cokendolpher et al. 2009), neither is unambiguously troglomorphic in phenotype. Compelling arguments for troglomorphism among described ants have heretofore only been made for Leptogenys khammouanensis and Aphaenogaster gamagumayaa. When compared to their respective closest relatives, both display a gracile habitus, pale colouration and reduced compound eyes (Roncin and Deharveng 2003; Naka and Maruyama 2018). Additionally, L. khammouanensis was collected in two large calcareous caves in Laos, ranging from 500 m to several kilometres from the cave entrance (Roncin and Deharveng 2003), whereas A. gamagumayaa was collected ~20 m within a calcareous cave on Okinawa, apparently nesting in the floor of an aphotic guano hall (Naka and Maruyama 2018, pp. 138–139).

Generic classification

Since Y. laventa is the sole species of Yavnella for which the worker has been identified, the range of morphological variation in the worker caste of Yavnella is unknown, as is the prevalence of troglomorphism in Yavnella. If all that is required for the evolution of troglomorphism in the Leptanillinae is the presence of the MSS, troglomorphism could be prevalent across the worker caste in Yavnella, since the geographical extent of the MSS is unknown (Juberthie and Decu 2006; Mammola et al. 2016).

It must be cautioned that without troglomorphic elongation of the soma and extremities, and putative trigger hairs, workers of Y. laventa cannot be discriminated from those of Leptanilla. Disregarding these apomorphies, the phenotype of Y. laventa shows close affinity to Leptanilla escheri (Kutter, 1948) and Leptanilla judaica Kugler, 1987. The 2,1 palp formula of Y. laventa (Fig. 5c) resembles that in Leptanilla havilandi Forel, 1901, L. escheri, L. judaica, and Leptanilla ujjalai Saroj et al., 2022 (this study; Kugler 1986; Saroj et al. 2022), whereas the anteromedian frontoclypeal process of Y. laventa resembles that observed in these and other Leptanilla spp. (Kugler 1986; Wong and Guénard 2016, fig. 1A, B; Leong et al. 2018, fig. 14B, C, 15A; Saroj et al. 2022, fig. 3B).

It is possible that L. escheri and L. judaica are non-troglomorphic representatives of Yavnella. In the absence of molecular data for L. escheri, L. judaica or their close relatives Leptanilla lamellata Bharti & Kumar, 2012 and L. ujjalai, we refrain from transferring any Leptanilla spp. to Yavnella. The hypothesis that these Leptanilla spp. represent Yavnella has biogeographical plausibility, because these species are known from the Indian subcontinent and Israel, with Iran intervening between these regions (Fig. 16). Under this hypothesis, it is plausible that Yavnella indica Kugler, 1987 and Y. argamani respectively represent the males of L. escheri and L. judaica, a prediction that could be tested with molecular data, as in this study and others (e.g. Ward and Brady 2009; Griebenow 2020).

Fig. 16. 

Distribution of known specimens attributed to Yavnella, according to Collingwood and Agosti (1996) and AntWeb (see  https://www.antweb.org), with generic attribution of undescribed morphospecies following Griebenow (2021). Generated using SimpleMappr ( https://www.simplemappr.net).

Identification of L. escheri or any of its relatives as representatives of Yavnella would erase the distinction between that genus and Leptanilla in the worker caste. The subapical mandibular setae have not been comprehensively surveyed across the known diversity of the Leptanillini and therefore are not of monothetic use. Yavnella and Leptanilla s. l. are uniformly discriminated based upon male morphology (Griebenow 2021) and robustly recovered as reciprocally monophyletic by ML and Bayesian phylogenomic inference (Griebenow 2020, 2021; this study). Resolving the taxonomic status of the major subclades of the Leptanillini, including Yavnella, will require sequencing of further worker material across this tribe, including the as-yet-unknown worker caste of Scyphodon, Noonilla, and the undescribed Bornean morphospecies-group (Griebenow 2020, 2021).