Strumigenys is one of the most diverse ant genera in the world and arguably the most morphologically diverse, exhibiting an exceptional range of mandible shape and function. A new species, Strumigenys ayersthey sp. nov., discovered in the Chocó region of Ecuador is described. With two morphological characters, this species is shown to be a morphologically unique outlier among Strumigenys globally, having predominately smooth and shining cuticle surface sculpturing and long trap-jaw mandibles. Using µCT scans, we produced 3D images of the worker ant and static images to examine and compare mandible articular morphologies with most morphologically similar members of the mandibularis species group. Cuticular, pilosity, and articular mandible morphological differences supports placing the new species in its own new species group.

3D scan, µCT, LaMSA (latch-mediated spring-actuation), Myrmicinae, Northwest Ecuador, power amplified, Strumigenys ayersthey, taxonomy, tropical forest

Ecuador has one of the highest animal and plant species richness of any country, both in terms in of species per area and total species richness (Sierra et al. 2002). This unusually high diversity is due to the three very distinct bioregions within Ecuador: the Amazon basin in eastern Ecuador, the Chocó-Darién bioregion in the northwest, and the Tumbesian drylands in the southern portion of the country (Sierra et al. 2002). Of these, the areas west of the Andes have been the least studied, and particularly the Chocó-Darién is a hotspot for new, previously unknown ant species (Donoso and Ramón 2009; Donoso et al. 2009; Salazar and Donoso 2013; Salazar et al. 2015; Donoso 2017; Hoenle et al. 2020). The Strumigenys fauna of Ecuador currently includes 51 species (Salazar et al. 2015), several of which are endemic (e.g., Strumigenys madrigalae Lattke and Aguirre 2015). Here, we report the finding of another likely endemic Strumigenys species from the Ecuadorian Chocó, contributing to a better understanding of this hyperdiverse region.

Strumigenys is one of the most diverse ant genera known with currently 852 extant and four fossil species, and is present on all continents except Antarctica (Guénard et al. 2017; Bolton 2020). Over the past two decades this genus received much taxonomic attention, but given the number of recent species descriptions, it is certain that many species are still waiting to be discovered (e.g., Booher et al. 2019; Sarnat et al. 2019; Dong and Kim 2020). Strumigenys are comparatively small ants (most < 4 mm) and are primarily litter dwelling although there are a few arboreal species (Bolton 2000). Most species assessed for diet are specialist predators of entomobryomorph Collembola (springtails), which may have led them to evolve a range of peculiar mandible forms to facilitate predation of fast-moving prey (Wesson and Wesson 1939; Wilson 1953; Masuko 1984; Dejean 1985; Brown and Wilson 1995; Masuko 2009; Lattke et al. 2018; Gray et al. 2019; Booher et al. 2021). Most spectacular, many Strumigenys possess trap-jaws, fast-snapping mandibles that function via a power amplified latch-mediated spring-actuation (LaMSA) (Booher et al. 2021; Ilton et al. 2018; Longo et al. 2019) akin to a biological mousetrap (Gronenberg 1996; Larabee and Suarez 2014). Performance and evolution of the trigger and latch system has been studied in detail, however there has been little attention given to additional undefined mandibular morphology that may contribute to the stability of trap-jaw movement in Strumigenys and other trap-jaw ants (Gronenberg 1996; Larabee et al. 2018; Booher et al. 2021). Within Strumigenys, the LaMSA mechanism has evolved independently multiple times, with each evolution convergent in morphology, function, and performance (Booher et al. 2021). However, the morphological variation in articular surfaces and articular processes involved in mandible movement across Strumigenys with or without LaMSA is morphologically variable and not well understood (Booher, unpublished data, Silva and Feitosa 2019). Here, we construct and define the single species ayersthey species group, describe the mandible articular morphology in detail within the description of the previously unknown S. ayersthey sp. nov., and compare it to that of morphologically similar members of the S. mandibularis species group to support species group separation.

As morphological convergence is rampant among Strumigenys morphotypes (short or long mandible species) it is difficult to determine by morphology alone how species are related (Ward et al. 2014). However, within biogeographic regions, species groups of morphologically similar Strumigenys species are often phylogenetically most closely related (Booher 2021). As such, morphological species groups are relevant and useful for identification as well as evolutionary research (Booher et al. 2021). In the construction of Strumigenys morphological species groups, differences in the position, presence, and shape of pilosity are of major importance. For example, the direction and shape of hairs along the clypeal margin and along the leading edge of the scape separates several Nearctic species groups, e.g., pulchella, ornata, and talpa groups (Bolton 2000). Similarly, slight differences in sculpturing help to identify similar species, but major differences in sculpture (i.e., having sculpture present across most cuticular surfaces compared to no sculpture) do not occur among species within any Strumigenys species group. We further justify the formation of a new single species group with differences in basal mandibular morphology from most morphologically similar mandibularis-group members.

The general mandibular morphology of LaMSA Strumigenys has been well described with the base of the mandible having three articular processes; the dorsal and ventral articulatory processes are responsible for holding mandibles in place during movement and a third lateral process is attached via apodemes to opening muscles (Fig. 6) (Silva and Feitosa 2019), alternatively termed the abductor swelling of the mandible or ‘atala’ (Richter et al. 2019; Richter et al. 2020). Although articular morphology of mandibles has been described in LaMSA Strumigenys and more generally in the more typical ant genera Wasmannia (Richter et al. 2019), Formica, and Brachyponera (Richter et al. 2020), there has not yet been a comparison in morphological features between them and there are a few important differences associated with the derived morphology of trap-jaws in Strumigenys (Table 1). Most morphological features in S. ayersthey have homologous features shared with other ants, however there are a few features that are not shared or have not been previously reported and are worth discussing. A most apparent difference is the dorsal articulation of the mandible and head. In what is described in Wasmannia and other ants, mandibles are tightly connected to the head capsule with primary and secondary joints, with the “secondary joint (dmah-dmap) formed by a ventrolateral longitudinal smooth elongation of the clypeus… which articulates with a smooth dorsolateral area on the mandibular base” (Richter 2019). In S. ayersthey sp. nov., this secondary joint is not connected to the head and the dmap moves freely within the dfc. We hypothesize that in contrast to typical ants, the clypeal articular process present in S. ayersthey helps to stabilize dorsal mandible articular movement. A second morphological feature important to note, is the derived labral hood (lh) present in S. ayersthey. This dorsal expansion of the basal area of the labrum is highly sclerotized, hypothesized to reduce damage from self-piercing and over-rotation, and is common to trap-jaw Strumigenys (Booher et al. 2021). We provide a table of mandible terminology (Table 1), however a more extensive comparative study across ants is needed to truly understand homology of mandible morphology.

Less prominent morphological features differ between trap-jaw Strumigenys and, for instance, mandible dentition has been used as focal distinguishing character between species groups. Strumigenys ayersthey, although most similar to members of the mandibularis-group, the dorsal articular process of the mandible differs in shape with mandibularis-group species. In members of mandibularis species group the dorsal articular process arises from a laterally extending dorsal surface forming a shelf like lamellate ridge at the basal portion of the mandible. In dorsal view, this lamellate process overhangs the lateral articular process obscuring most of it from view. In S. ayersthey sp. nov., the lateral corner of the dorsal articular surface is gradually rounded and does not form a lamellate margin. Additionally, in the only species with a detached mandible that could be visually inspected by us (S. planeti) the posteriormost articular surface of the dorsal process contained three small bulbous points connected by indented lamellae, wherein S. ayersthey sp. nov. there is a single bulbous articular point. Therefore, S. ayersthey sp. nov. is an exceptional morphological outlier and a rare addition to the hyperdiverse genus Strumigenys. It does not fit cleanly into any of Bolton’s species groups, nor can existing species-group definitions envelope this species with minor changes – hence, we placed it as the only member of a new species group. We find morphological articular structure of mandibles are important taxonomic characters and should be investigated in future taxonomic works in this genus.

Our species description includes a µCT 3D render of the holotype worker, and its surface model is freely available for download (Suppl. material 1). This offers any reader virtual morphological details of the new species and the ability to view morphological features at all angles. 3D imaging techniques, and in particular micro-computed X-ray tomography (µCT), are being frequently used in taxonomy and functional morphology, particular in ants (Faulwetter et al. 2013; Akkari et al. 2015; Garcia et al. 2017; Sarnat et al. 2017; Staab et al. 2018). For Strumigenys, they already lead to detailed morphological analysis, and µCT scans of Fijian Strumigenys have even been suggested as a tool for teaching with augmented reality (Sarnat et al. 2019). In our case, the µCT scan facilitated additional descriptions of mandibular morphology and function of S. ayersthey sp. nov.

The discovery of Strumigenys ayersthey sp. nov. advanced our understanding of the global morphology of this genus: It’s unique combination of almost no surface sculpturing and long trap-jaw mandibles make it stand out among nearly a thousand other Strumigenys species. Because of S. ayersthey sp. nov. unusual morphology, information about its general biology could prove to be valuable. However, subsequent attempts in obtaining more specimens at the previous location with Winkler traps in 2019 have failed, and a large ecological ant study in the Canandé reserve did not reveal any more specimens. Strumigenys ayersthey sp. nov. can therefore be considered as rare. The discovery of such an unusual rare ant highlights the importance of scientific exploration and conservation of the Chocó region in Ecuador, which is at the same time one of the most biodiverse and threatened areas on our planet (Dinerstein et al. 1995; Olson and Dinerstein 1998; Myers et al. 2000).

We thank the Fundacíon Jocotoco and the associated Tesoro Escondido for logistic support and their permission to do research on their forest properties. First, we like to thank Adriana Argoti and Adrian Brückner for help during field collection. We are grateful for the local support from the park staff in the Canandé and Tesoro Escondido reserve, that made the field collection easier and made two great field stays possible: Bryan Amayo, Alcides Zombrano, Roberto de la Cruz, Jorge Zambrano, Amado de la Cruz, Yadria Giler, Patricio Encarnación and Vanessa Moreira.

We thank musician and artist Michael Stipe for contributing to the etymology in honor of a hero to many and dear friend to those that knew him, Jeremy Ayers. We further thank the support from the LGBTQIA+ and gender dynamic community, for which we hope this species furthers pride. Additional thanks to Adrienne Truscott for input and suggestions.

Further, we like to thank Sebastian Schmelzle for providing and editing the µCT scans. We thank Roberto Keller for comments on the functional aspects of the trap-jaw, and James Trager for his Latin expertise. We thank Christoph von Beeren for the use of his photostacking equipment. David Donoso, Martin Schäfer and Nico Blüthgen provided supervision and logistic support for the studies in Ecuador, and without them this discovery would not have been possible.

The SRµCT data of the single Strumigenys ayersthey sp. nov. specimen was acquired within the project NOVA (Network for Online Visualization and synergistic Analysis of tomographic data) funded by the German Federal Ministry of Education and Research (05K16VKB) within the call for proposals “Erforschung kondensierter Materie an Großgeräten” 20162019. We thank the proposers of the NOVA project Michael Heethoff (Technische Universität Darmstadt), Vincent Heuveline (Heidelberg University), and Jürgen Becker (Karlsruhe Institute of Technology). We thank the associated partners of the NOVA project Felix Beckmann from the HelmholtzZentrum Geesthacht, and Andreas Kopmann and Wolfgang Mexner from the Karlsruhe Institute of Technology. We acknowledge DESY (Hamburg, Germany), a member of the Helmholtz Association HGF, for the provision of experimental facilities. Parts of this research were carried out at PETRA III and we would like to thank Jörg Hammel and Fabian Wilde for assistance in using the experimental station at P05 and reconstruction of the data.

Finally, we are grateful for the suggestions from two anonymous reviewers and Brian L. Fisher, which substantially improved the manuscript.

We acknowledge support by the Open Access Publishing Fund of Technische Universität Darmstadt. PH was supported by a scholarship from the German National Academic Foundation.