The genus Leptanilla Emery, 1870 of the family Formicidae, subfamily Leptanillinae, comprises miniscule, pale, blind ants that are rarely collected and poorly understood due to their hypogaeic (i.e. underground) lifestyles. Here we describe a new Leptanilla species from two workers collected via subterranean scraping in the arid Pilbara region of Western Australia. Leptanilla voldemort sp. nov. is the second leptanilline species documented in Australia after the elusive Leptanilla swani Wheeler, 1932. Workers of L. voldemort sp. nov. display a remarkably gracile morphology characterised by elongated legs, antennae, and mandibles, and they are easily differentiated from other Leptanilla species. We also provide new measurements for L. swani from two workers found proximally to the type locality of L. voldemort sp. nov. A key to the worker caste of Leptanilla species of the Australian continent is presented.

Australia, hypogaeic, Leptanilla, Milieu Souterrain Superficiel, subterranean

Although lacking the impressive colours, armoury and colony sizes seen in many of the world’s ~14,000 ant species (Bolton 2024), the diminutive members of the genus Leptanilla Emery, 1870 (Formicidae, Leptanillinae) are darlings of myrmecologists, for they include some of the most elusive and bizarre ants on Earth. All known Leptanilla species are hypogaeic, living in small colonies that nest and forage exclusively underground (Griebenow 2024; Qian et al. 2024). As such, individuals of Leptanilla are rarely collected by conventional methods for sampling ants (e.g. pitfall traps), which tend to target surface habitats (Wong and Guénard 2017). In adaption to subterranean life, Leptanilla ants characteristically display cryptobiotic morphological traits; their workers are very small (often <2 mm), depigmented and blind (Bolton 1990; Wong and Guénard 2016). The few observations of live Leptanilla colonies have also revealed intriguing, specialised behaviours, such as specialised predation on geophilomorph centipedes (Masuko 1990), and adult feeding on haemolymph (i.e. blood) of the larvae from a unique duct-like organ in the larval integument—the “larval haemolymph tap” (Masuko 1989). Moreover, recent phylogenomic analyses show that together with the monotypic Martialinae, the Leptanillinae constitute the sister group to all other extant ant species, making them especially significant for our understanding of ant evolution (Borowiec et al. 2019; Romiguier et al. 2022; but see Cai 2024).

The 61 described species of Leptanilla are found in tropical and temperate regions of the Old World (Bolton 1990; Griebenow 2024; Qian et al. 2024). The apparently patchy distribution of the genus may be an artefact of geographic biases in ant research in general (Kass et al. 2022) and the use of subterranean sampling techniques in particular (Wong and Guénard 2017). While multiple Leptanilla species have been documented in Africa, Europe, and Asia, only one species, Leptanilla swani Wheeler, 1932 has been described from Australia thus far. Moreover, few if any workers of L. swani have been found in the 90 years since it was first described (Wheeler 1932; Heterick 2022; AntWeb 2024). Although winged Leptanilla males have been collected in flight-interception traps throughout the continent, the identities of these are questionable, as none occur in sympatry with definitive workers of L. swani (AntWeb 2024; Zachary Griebenow pers. comm.).

Here, we describe a new Leptanilla species, Leptanilla voldemort sp. nov., from two workers collected in the arid Pilbara region of Western Australia. Leptanilla voldemort sp. nov. represents the second species of Leptanilla and second member of the subfamily Leptanillinae from the Australian continent. Notably, workers of L. voldemort sp. nov. display a distinctly gracile morphology, characterised by elongated legs, antennae, and mandibles. This remarkable phenotype is not seen in most other Leptanilla species – including the other Australian species, L. swani, for which new morphological measurements are hereby reported from two workers collected proximally to the type locality of L. voldemort sp. nov. A key to the worker caste of Leptanilla from Australia is also presented.

Photographs of specimens were obtained with an incorporated digital camera mounted on a Leica M205C dissecting microscope through the Leica Application Suite V4 software. A total of 33–72 images were taken and stacked together. Morphological measurements and indices were calculated following Griebenow et al. (2022). These are detailed below.

HW (head width): 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.

CI (cephalic index) = HW ÷ HL × 100.

SI (scape index) = SL ÷ HW × 100.

MI (mandibular index) = MaL ÷ HW × 100.

PI (petiolar index) = PTW ÷ PTL × 100.

PPI (postpetiolar index) = PPW ÷ PPL × 100.

PPHI (postpetiolar height index) = PPH ÷ PPL × 100.

Abbreviations of the type depositories are as follows:

ANIC Australian National Insect Collection;

WAM Western Australian Museum.

Despite Australia being a global hotspot for ant diversity (Kass et al. 2022), Leptanilla voldemort sp. nov. is only the second species of Leptanilla and second member of the subfamily Leptanillinae known from the continent since the discovery of L. swani in 1932. To this end, it is envisaged that the targeted employment of currently underutilised techniques for collecting hypogaeic ant species such as subterranean scraping, subterranean pitfall traps, and soil extractions (Wong and Guénard 2017) may facilitate the discovery of additional Australian leptanilline species. The Pilbara region of north-western Australia, in particular, is one of the oldest land surfaces on Earth (Pepper et al. 2008). Due to multiple vicariant isolation and divergence events that have occurred with surface aridification, this region harbours globally significant levels of endemic diversity in subterranean invertebrate species (Mokany et al. 2019). The presence of both L. swani and L. voldemort sp. nov. in the Pilbara highlights the importance of this region for Australian leptanilline species.

Although we collected individuals of both L. voldemort sp. nov. and L. swani within the same general locality (sites <15 km apart) in the Pilbara, the two sympatric species clearly exhibit contrasting morphologies. Whereas L. swani is stout and compact (Fig. 5), L. voldemort sp. nov. is distinctly gracile (Fig. 2). We posit that these morphological differences may relate to the species’ use of dissimilar microhabitats within the subterranean environment. Given that a colony of L. swani was collected from under a large stone (Wheeler 1932), it is reasonable to assume that L. swani inhabits the shallower layers of soil. Moreover, the generally compact body plan of L. swani resembles that of most other Leptanilla species that have been collected via the sifting of topsoil (10–20 cm depth) or via the installation of subterranean pitfall traps (10–50 cm depth) in the soil layer (Wong and Guénard 2016, 2017). In contrast, the distinctively gracile morphology of L. voldemort sp. nov. is unlike that of any other Leptanilla species except L. laventa from Iran. Interestingly, L. laventa was not collected from soil, but via subterranean pitfall traps buried 0.6–1 m belowground in the Milieu Souterrain Superficiel, an underground network of empty air-filled voids and cracks developing within multiple layers of rock fragments, located below the soil layer (Mammola et al. 2016; Griebenow et al. 2022). We can only presently speculate whether L. voldemort sp. nov. occupies a similar microhabitat; systematic sampling of the ants from different vertical sections of the subterranean column (via subterranean pitfall traps or excavations) coupled with measurements of climatic and structural parameters of the substrate should provide clarification on the matter. Finally, the elongated mandibles and large, sharp basal teeth of L. voldemort sp. nov. suggest that the ants are likely specialised predators; however, it remains to be seen whether they prey on geophilomorph centipedes, as in other species of Leptanilla (Masuko, 1990), or other hypogaeic taxa, such as centipedes of the family Cryptopidae (Chilopoda, Scolopendromorpha), hypogaeic cockroaches, and pauropods, all of which were collected from the same drill hole as L. voldemort sp. nov.

We are grateful to Stuart Halse, Melanie McGellin, and Melita Pennifold from Bennelongia Environmental Consultants for their assistance with the specimens and comments on the manuscript, and Nikolai Tatarnic from the Western Australian Museum for his assistance with deposition of the specimens. We also thank Zachary Griebenow and Francisco Hita Garcia for their comments on a previous version of the manuscript.