﻿The ants of the genus Rhopalothrix Mayr, 1870 (Hymenoptera, Formicidae, Myrmicinae) in Colombia

﻿Abstract The ants of the genus Rhopalothrix are diverse in the Neotropical region, with 14 of the 16 described species. Based on museum material and recent fieldwork, Rhopalothrix ants in Colombia were reviewed. Morphological analysis of the workers allowed delimitation of six species, including two new species, Rhopalothrixmandibularis Guerrero & Grajales, sp. nov. and Rhopalothrixmariaemirae Tocora, Fiorentino & Fernández, sp. nov. A new combination Rhopalothrixamaticomb. nov. is proposed for Eurhopalothrixamati. A worker-based taxonomic key, high-definition images of the workers, and a distribution map of all Rhopalothrix species present in Colombia are provided.


Introduction
Ants are a dominant and ecologically key component of the highly diverse fauna of mostly exceedingly small arthropods that live in the litter layer that accumulates on the forest floor.Habitat type, as well as leaf-litter quality and heterogeneity, can influence the ant community (Silva et al. 2011), allowing some genera of ants to become more conspicuous (e.g., Pheidole Westwood, 1839 or Strumigenys Smith, 1860), while others are cryptic and poorly represented, such as Rhopalothrix Mayr, 1870.
The Basiceros genus group contains the genera Basiceros Schulz, 1906, Eurhopalothrix Brown & Kempf, 1961, Octostruma Forel, 1912, Protalaridris Brown, 1980, Rhopalothrix, and Talaridris Weber, 1941.The ants of the genus Rhopalothrix are small and with a distinctive combination of features.The worker mandible is an arched shaft with an apical fork; most other members of the Basiceros genus group have triangular mandibles.The genus Protalaridris has elongate mandibles, similar to Rhopalothrix, but can be distinguished by their antennae with 9 segments, instead of 7 in Rhopalothrix.Rhopalothrix workers also have squamiform setae varying in number and size on the head, mesosoma or gaster.
Currently, sixteen species are recognized within Rhopalothrix (Bolton 2023), but at least a dozen species await description (AntWeb 2023).The genus Rhopalothrix is distributed in Australia (1 species), New Guinea (1 species) and the Neotropical region (14 species) (Longino and Boudinot 2013).Longino and Boudinot (2013) studied Rhopalothrix, mainly the Mesoamerican forest fauna.These authors proposed the Rhopalothrix isthmica clade, defined by two synapomorphies: absence of squamiform setae on the face and development of shallow arcuate grooves and ridges on the face.This clade contains 13 of the 14 described Neotropical species.Several additional Neotropical species of uncertain phylogenetic position are now known, with characters that place them outside the isthmica clade (see images on AntWeb 2023).
We describe two new species, one that fits in the isthmica clade and another with scale-like setae on the face, similar to those present in Rhopalothrix ciliata Mayr, 1870.We also propose a new combination for one species previously described in the genus Eurhopalothrix.We provide a key to the six Colombian species, new occurrence records, and results on the distribution of the species in the country.

Specimen processing
We used the worker-based key to species of Rhopalothrix proposed by Longino and Boudinot (2013) for identification of the studied specimens.To integrate our data into a review of the genus Rhopalothrix from the Brazilian Atlantic and Amazonian rainforest (J.Chaul, personal communication) and better characterize the species described here, we implemented several complementary measurements (for definitions see below; Fig. 1) to the one used by Longino and Boudinot (2013).The latter only used the maximum width of the head capsule in full-face view (HW) as a surrogate measure of ant size.Although HW is useful for the separation of known species of Rhopalothrix, the morphological diversity within the genus requires the exploration of other measures to support the delimitation of new taxa.
Specimens were observed using a Nikon SMZ 745 stereomicroscope.Measurements were made with a dual-axis micrometer stage with output in increments of 0.001 mm.However, variation in specimen orientation, alignment of crosshairs with edges of structures, and interpretation of structure boundaries resulted in measurement accuracy to the nearest 0.01 mm.All measurements (Fig. 1) are presented in mm:

ClyL
in full-face view, maximum width of the clypeal plate including the lateral expansions above the insertion of the mandibles.

ClyW
in full-face view, maximum length of the clypeal plate from the most anteroclypeal projection to the most posterior clypeal margin.

GL
in lateral view, the straight-line length of the gaster measured from the most anterior margin of the first tergite to the posterior margin of the fourth tergite.The taxonomic key provided here includes the relationship between labral width (LabW) and labral length (LabL) (see also couplet 4 in Longino and Boudinot (2013)).In full-face view, we measured the width (LabW) and the length of the labrum (LabL) in those specimens corresponding to Rhopalothrix isthmica (Weber, 1941), R. mandibularis sp.nov.and R. weberi Brown & Kempf, 1960.The values are expressed as a percentage, (LabW/LabL)*100.When preparing specimens of both species it is recommended to carefully open the mandibles so that the labrum is completely exposed.
High-resolution images of Rhopalothrix ciliata Mayr, 1870 lectotype (CASENT0915695) and Rhopalothrix mariaemirae sp.nov.(= Rhopalothrix jtl021: ANTWEB1038216, UFV-LABECOL-001953 and USNMENT01127994) workers were downloaded from http://www.antweb.org.For each image, we record all the measurements indicated above using Image J software (NIH, Bethesda, MD, USA).For the R. ciliata lectotype, only those measurements of the head, mandible, and mesosomal and petiolar/postpetiolar dorsum were recorded; those measurements taken in lateral view could not be recorded due to the position of the specimen on the pin.
The global distribution of Rhopalothrix was obtained from AntMaps (Janicki et al. 2016).The distribution map for all species of Rhopalothrix in Colombia was made with R software (R Core Team 2020), using information from specimen collection labels.The shapefiles were extracted from the rnaturalearth library World Map Data from Natural Earth v. 0.3.3(Massicotte 2024).The digital elevation model was developed using the geodata library.The final map presented here was obtained using the graphical tool "ggplot2".
For comparative purposes, type, and non-type specimens of different Rhopalothrix species were studied from high-quality images downloaded from www.antweb.org(AntWeb 2023); unique specimen numbers are provided in all cases.

Specimen drawing, imaging, Micro-CT scanning and 3D-reconstruction
Drawings of the general habitus of Rhopalothrix with measurements, and the mandibular apical fork of each species recorded here were created using Adobe Sketchbook v. 9.0.
Color montage images of the species were created using an Auto-Montage Leica M205A and the images were combined using the program LAS v. 4.6.The images were edited (Corel Photo-Paint X3 v. 13.0) to enhance brightness and contrast details.Finally, all figures were arranged using CorelDRAW Graphics Suite X3.
Micro-CT scans of a specimen of Rhopalothrix mariaemirae sp.nov.were generated with a Zeiss Xradia 510 Versa 3D X-ray microscope operated with the Zeiss Scout-and-Scan Control System software (v. 14.0.14829.38124).The scan was carried out at the Okinawa Institute of Science and Technology Graduate University, Japan.Scans were conducted with a 40 kV (75 μA) / 3 W beam using the 4x magnification objective.The scan was performed at an exposure time of 25 s and a voxel size of 0.645545 μm.
Mandible elongated (MdL > 0.5) with five teeth on masticatory margin (Fig. 7A).Labrum about as long as broad (LabW/LabL < 100%), medial notch deep.Petiole with well-developed peduncle (Fig. 7A Head broader than long, with slightly rounded cephalic lateral margins at the level of the crest on the face (Fig. 6A).Posterior cephalic margin strongly concave.Mandible with three teeth on masticatory margin, middle tooth largest (Fig. 6A).Subapical tooth longer than width of mandible at base, about twice as long as apical tooth ................R. isthmica (Weber) -Head as broad as it is long, with cephalic lateral margins projecting at an angle at the level of the crest on the face (Fig. 10A).Posterior cephalic margin slightly concave.Mandible with only two small teeth at the base of the masticatory margin (Fig. 10A).Subapical tooth shorter than width of mandible at base, only slightly longer than apical tooth .. Comments.Holotype and paratype workers (IAvH-55017 and IAvH-55018) and three non-type specimens were analyzed and measured (HW 0.39-0.43)showing a mandibular dentition different from either of the two states described for Eurhopalothrix (Longino 2013).Fiorentino et al. (2022) indicate that the workers of this species have "masticatory margin with a single row of ~13 long needle shaped teeth…", but this dentition does not match the simple row of 11 similar, low, triangular teeth mentioned for Eurhopalothrix by Longino (2013).Reanalyzing the mandible dentition of the workers, they present a row of between seven (IAvH55005 in AntWeb 2023) to ten teeth (holotype).The shape of the mandible of the workers of this species also does not match those of Eurhopalothrix, being more like the man- dible of some undescribed Rhopalothrix (e.g., CASENT0639185 Rhopalothrix jtl014 or CASENT0646264 Rhopalothrix jtl023).It is possible that the triangular shape of the mandible of Rhopalothrix amati has generated the misclassification of this species in the genus Eurhopalothrix.All the workers studied, however, have the subapical tooth larger than the apical one (Fig. 3C), the latter being below the subapical tooth, a combination of traits present in Rhopalothrix.Based on all morphological evidence, we transfer this species to the genus Rhopalothrix generating the following new combination Rhopalothrix amati (Fiorentino, Tocora & Fernández, 2022).Worker measurements (N = 13).MdL 0.3-0.36,MdbW 0.05-0.09,ClyL 0.15-0.22,ClyW 0.32-0.47,HL 0.43-0.81,HW 0.49-0.72,WL 0.49-0.8,PrnW 0.29-0.45,PetL 0.28-0.35,PpetL 0.1-0.16,PetW 0.14-0.21,PpetW 0.23-0.37,T4L 0.43-0.68,GL 0.52-0.86.

Rhopalothrix ciliata Mayr, 1870
Geographic range.Colombia, Ecuador, Venezuela.In Colombia, this species is known from Antioquia, Cundinamarca, Huila, Magdalena (Sierra Nevada de Santa Marta), Quindío and Valle del Cauca (Fernández and Serna 2019).New records in Colombia come from Caldas, Risaralda, and Santander.Natural history.In Colombia, this species is known from forests at altitudes above 1500 m, with populations in the Sierra Nevada de Santa Marta and in regions of the central and western cordilleras.It is a very abundant species in modified environments and in agroecosystems such as coffee crops that include native trees.
Geographic range.Colombia, Honduras, Guatemala, Panama.In Colombia, this species is known from Antioquia, Bolívar, Santander and Sucre.
Examined material.In the latter it is a relatively abundant species, being found in 8 of 20 MiniWin-kler litter samples.Rhopalothrix isthmica populations have an elevational distribution from near sea level to 1000 m.
Comments.The morphology of the workers matches the diagnostic characters of R. isthmica, including HW 0.57-0.61(N = 6) recorded by Longino and Boudinot (2013).In the workers of populations from La Guajira (northern Colombia), the portion of the lamella ventral to the propodeal tooth (infradental lamella) is relatively straighter on the outer margin than described by Longino and Boudinot (2013).Description.Worker.Head in full-face view broader than long, diamond-shaped, with straight cephalic lateral margins strongly diverging posteriorly, extending below the level of the dorsal crest of the head, at the level of the latter a rounded widening that continues on lateral margins converging towards the rounded posterolateral cephalic corners; wide and concave posterior cephalic margin; front visibly protruding in dorsal view, with an arched transverse carina (= crest), and depression impressed behind the crest.In lateral view, mandible dorsally inclined in relation to head plane (Fig. 7B); mandible with four to five teeth on masticatory margin as follow: three large equidistant teeth located medially on masticatory margin, basalmost (first) large tooth with a small tooth (sometimes undeveloped) above its base, a middle tooth almost half as long as the previous one, third tooth as long as first, a small fourth tooth as long as 1/3 of first; subapical tooth about twice as long as apical tooth, with denticles at base of both subapical and apical tooth.Trapezoidal labrum as long as broad, with slightly concave sides, subparallel anteriorly, and straight-sided base, labrum with two long blunt subrectangular lobes, with parallel inner faces and deep notch between, length of lobe equal to about 1 /3 of the distance from the base of the notch to the transverse carina at the base of the labrum, Clypeus almost twice as wide as long, with anteroclypeal lobes projecting anterad.Scape just reaching maximum width of head; pedicel and second flagellomere conical towards the base as long as wide, third and fourth flagellomere rectangular wider than long, fifth flagellomere subsquare, last flagellomere finger-shaped tapering apically, as long as the previous four funiculus.

Rhopalothrix mandibularis
In lateral view, pronotum and mesonotum at the same level, divided by arcuate promesonotal groove and metanotal groove moderately impressed; propodeal dorsum sloping in lateral view.In dorsal view, pronotum with slightly concave anterior margins, narrowing anteriorly to form a distinguishable neck, pronotum with rounded corners at maximum width; in dorsal view, mesonotum wider than long, narrowing posteriorly.Petiole with well-developed peduncle; in lateral view, with straight dorsal face and posteriorly convex ventral face, the latter with a small subpetiolar process projecting anterad as a blunt tooth; petiolar scale rounded in lateral view; postpetiole twice as wide as long (Fig. 7C).
Short decumbent hairs on surface of head and mesosoma; dorsum of clypeal plate and above posteroclypeal margin with small squamiform hairs.External margin of scape with about 7-9 squamiform hairs similar in size; apex of scape with erect hairs shorter than squamiform ones; funicles with simple subdecumbent hairs.Legs with coxa and femur with few short decumbent hairs; tibia with abundant long, thick semi-erect hairs, and a pair of long flattened hairs located apically on the external surface of each one.About 4-8 squamiform setae on posterior half of first gastral tergite, unspecialized curved hairs scattered over the disc of the first gastral tergite.
Head, mesosoma, petiole and postpetiole shagreened, legs shiny with granular surface, except all tibiae with smooth surface; surface of first gastral tergite finely shagreened.Color reddish brown to ferruginous brown, with yellowish brown distal antennal flagellomeres.
Natural history.This species inhabits humid forests between 1400 and 1700 m above sea level.The holotype and several paratypes were collected in fragments of humid gallery forest and Guadua (bamboo) forest in the city of Armenia.All known specimens are from Winkler samples of sifted leaf litter.
Etymology.The name refers to the long mandibles of the worker, a trait not found in any other species in the isthmica clade.
Comments.This species is placed in the diverse isthmica clade because it shares the two synapomorphies proposed by Longino and Boudinot (2013).This new species, however, has been widely confused in some Colombian collections with R. ciliata due to its similarity in mandibular shape, the shape of the two lobes of the labrum and the depth of the sinus.Rhopalothrix mandibularis can be differentiated from R. ciliata by the absence of squamiform setae on the rostrum, the latter with specialized setae and a flattened surface on the rostrum.Also, mandibles are longer and thinner along their length in R. mandibularis while in R. ciliata they are short and broad; in R. ciliata the tip of the labral lobes can reach up to half the length of the mandible, but in R. mandibularis the anterior margin of the labrum barely reaches the first tooth of the mandible.
Habitus of the worker of Rhopalothrix mandibularis is similar to that of R. stannardi Brown & Kempf, 1960, but the mandible length is remarkably different between the two, as well as the mandible dentition; R. mandibularis has three teeth located towards the middle of the masticatory margin (the most basal tooth is far from the base), while in R. stannardi the three teeth are equidistant, with the most basal tooth starting at the base of the masticatory margin.Another different feature is the infradental lamella, which is very poorly developed in R. mandibularis, while in R. stannardi the lamella is broad and descends almost perpendicularly from the tooth.Description.Worker.Head in full-face view wider than long, with cephalic lateral margins subparallel to each other, profile interrupted by a deeply impressed notch at the level of the antennal insertions and a triangular notch shallower than the previous one below the level of the diadem of circular/squamiform hairs, at the level of the latter, widened profile projecting rounded angulations continuing on slightly convergent lateral margins towards the angled posterolateral cephalic corners; wide and strongly concave posterior cephalic margin.In lateral view, mandible in the same plane of the head; subapical tooth with prominent recurved acute tooth, directed posteriorly, subapical tooth shorter than width of mandible at base, about twice as long as apical tooth.In fullface view, anterior margin of labrum with shallow median notch, posteromedial portion of labrum translucid.Clypeus about 2.5 times wider than long, with rounded anteroclypeal lobes projecting anterad.Scape just beyond the most posterior notch of the lateral cephalic margin; Pedicel subsquare, second to fourth flagellomere conical towards the base, fifth flagellomere rectangular longer than wide, last flagellomere finger-shaped tapering towards the apex, almost as long as almost as long as the five flagellomeres combined.

Rhopalothrix mariaemirae
In lateral view, promesonotum convex continuing with the profile of the dorsum of the propodeum, promesonotal depression and metanotal groove slightly impressed; propodeal dorsum falling on a slight slope in lateral view; propodeal tooth developed, distinctly in top half of declivitous face of propodeum in lateral view; infradental lamella very narrow.In dorsal view, pronotum with straight lateral and convex anterior margin, pronotum with angled corners at their maximum width that continue towards slightly convex lateral margins; mesonotum trapezoid-shaped, wider than long, narrowing posteriorly.Petiole with poor-developed peduncle; in lateral view, with the dorsal face short, strongly inclined to connect with the anterior face of the rounded petiole scale, ventral surface straight with a small elongated subpetiolar process projecting anterad; in dorsal view, postpetiole 1.5 times wider than width of petiole (Fig. 8C).Head with short decumbent squamiform hairs, notably dispersed and arranged transversely; anterodorsal portion of clypeus with small squamiform hairs broadened apicad.External margin of the scape with about 6-7 squamiform hairs similar in size; apex of scape with few shorter and thicker erect hairs, widely scattered; flagellomeres with simple subdecumbent hairs.Coxa and femur with few very short hairs; tibiae with long, thick semi-erect squamiform hairs on the inner surface, external face of the tibiae devoid of erect hairs, with only a few long, flattened hairs located apically.First gastral tergite largely devoid of setae, with 2-3 squamiform setae at posterolateral margins.
Queen and male.Unknown.
Etymology.This species is named after María Emir Sánchez (1953-2023), as a tribute to María C. Tocora's beloved and inspiring abuela, who recently passed away.
Comments.This species is easily recognized by the anterior labral convexity condition, the two small blunt lobes of the labrum, and the large squamate hairs, 8 in total, on the frons like those of R. diadema (Longino and Boudinot 2013).
The workers of Rhopalothrix jtl021 (ANTWEB1038216, UFV-LABECOL-001953, and USNMENT01127994) match to R. mariaemirae.Those specimens coincide in the strongly convex labrum, distal margin of labrum slightly notched, and the two poorly-developed lobes.In Rhopalothrix jtl021 the posteromedial portion of labrum is translucid.Also, all specimens share eight strongly convex rounded scales located below the maximum width of the head.
Natural history.The only specimen studied here was extracted from the low-density litter of a riparian forest with shrubby vegetation.
Comments.This species is recorded by Achury and Suarez (2018) from the Colombian inter-Andean valley, but we were not able to study those specimens to corroborate the identity.Ants recently collected in Puerto Wilches (Santander) in eastern Colombia included one specimen that matches the taxonomic definition of R. weberi, thus corroborating the presence of this species in Colombia.

General comments
The ant genus Rhopalothrix is reviewed for the first time for Colombia.Previously only three species were known: R. ciliata, R. isthmica and R. weberi (Janicki et al. 2016); the last with an uncertain record for Antioquia (Achury and Suarez 2018).Our study increases the number of species to six, with the description of two new species, R. mandibularis and R. mariaemirae.
Rhopalotrhix ciliata and R. isthmica are the most widely distributed species in Colombia, the first species with populations mainly in the Andean region and the Sierra Nevada de Santa Marta (northern Colombia), while R. isthmica has populations in dry forests of the Colombian Caribbean, in cleared open areas and in remnants of riparian forest in Santander in the valley of the eastern Cordillera of Colombia.Interestingly, Rhopalotrhix ciliata overlaps its distribution with both R. amati and R. mandibularis in the coffee-producing region of central Colombia and in Valle del Cauca; in the latter, however, a disjunct altitudinal distribution is evident, as R. ciliata can be found above 2000 m while R. mandibularis is at 1700 m.Another example of sympatric distribution is recorded for R. isthmica and R. weberi in riparian forests in northeastern Colombia, where both species were found coexisting in leaf litter.cus images and the map, and Brandon E. Arredondo for all the drawings included in this manuscript.As well, thanks to Mayron E. Escárraga for generating all the measurements included in this work.Júlio Chaul, Jack Longino, and John Lattke thoroughly reviewed this manuscript, providing comments and suggestions that improved its scientific quality.Julio Chaul kindly shared information and results from the review of the genus Rhopalothrix from the Brazilian Atlantic and Amazonian rainforest that is in progress.We would like to thank Evan P. Economo along with the Okinawa Institute of Science and Technology Imaging Section for access to and maintenance of the micro-CT scanner and Julian Katzke for the segmentation of the scan.This contribution is part of the research project "Patrones de diversidad histórica y ecológica de las hormigas en el socioecosistema bosque seco tropical de Colombia y sus implicaciones para la conservación" supported by Minciencias with contract 2021-1029 (Universidad del Magdalena and Icetex).

HL
in full-face view, maximum length of the head measured from the most anterior projection of the clypeus to the most posterior projection of the cephalic capsule.MdL in full-face view, the straight-line length of the mandible from the basalmost mandibular external margin to the apex of the subapical tooth.MdbW in full-face view, shortest diagonal line connecting the most basal point of the masticatory margin with the mandibular external margin.PetL in profile view, the distance from the inflection point marking the juncture of the cylindrical posterior portion of the segment to the anterior inflection point where the petiole is obscured by the posteroventral lobes of the propodeum.PpetL in lateral view, the distance from anterior to posterior inflections of postpetiole node.PetW maximum width of the petiolar node in dorsal view.PpetW maximum width of the postpetiolar node in dorsal view.PrnW maximum width of the pronotum in dorsal view.T4L in lateral view, length of the fourth abdominal tergite (= first gastral tergite) measured with the anterior and posterior margins in the same plane.WL the diagonal length of the mesosoma in profile from the point at which the pronotum meets the cervical shield to the posterior basal angle of the metapleuron.

Figure 1 .
Figure 1.Measurements recorded in the habitus of Rhopalothrix worker.Definitions of the acronyms are described in Material and methods.

Figure 2 .
Figure 2. Distribution map of Rhopalothrix species in Colombia.

Figure 3 .
Figure 3. Rhopalothrix amati paratype worker (IAvH-55018) A full-face view B lateral view C dorsal view D portion of the head viewed obliquely showing the mandibles and the apical fork of the left mandible; the black arrow points to the subapical tooth while the white one points to the apical tooth of the apical fork of the mandible.Scale bars: 0.2 mm.

Figure 4 .
Figure 4. Rhopalothrix ciliata worker (CBUMAG:ENT:35948) A full-face view B lateral view C mandible distinguishing the teeth of the apical fork; drawing inserted showing the arrangement of the teeth of the apical fork of the mandible D dorsal view.Scale bars: 0.3 mm (A, D); 0.5 mm (B).

Figure 6 .
Figure 6.Rhopalothrix isthmica worker (IAvH-E-172166) A full-face view B lateral view C mandible distinguishing the teeth of the apical fork; drawing inserted showing the arrangement of the teeth of the apical fork of the mandible D dorsal view.Scale bars: 0.3 mm.

Figure 7 .
Figure 7. Rhopalothrix mandibularis sp.nov.Holotype worker (CBUMAG:ENT:35947) A full-face view B lateral view C mandible distinguishing the teeth of the apical fork; drawing inserted showing the arrangement of the teeth of the apical fork of the mandible D dorsal view.Scale bars: 0.5 mm.

Figure 10 .
Figure 10.Rhopalothrix weberi worker (IAvH-E-233235).A full-face view B lateral view C mandible distinguishing the teeth of the apical fork; drawing inserted showing the arrangement of the teeth of the apical fork of the mandible D dorsal view.Scale bars: 0.3 mm.

Tocora, Fiorentino & Fernández, sp. nov.
Jaci mental surface, in perpendicular view they look like eight large, rounded white scales on head.