Taxonomic revision of the cryptic ant genus Probolomyrmex Mayr ( Hymenoptera , Formicidae , Proceratiinae ) in Madagascar

The alpha taxonomy of the ant genus Probolomyrmex in Madagascar is revised on the basis of the worker caste. Two new species are described: P. curculiformis sp. n. and P. zahamena sp. n. and the previously known P. tani is re-described. All three species are members of the P. greavesi species group. The species descriptions include diagnoses, taxonomic discussions, high quality montage images, and distribution maps. In addition, we provide an illustrated species level identification key.


Introduction
The rare ant genus Probolomyrmex is distributed throughout most of the world's tropics and subtropics, and contains 24 valid species at present (Bolton 2014). Based on current knowledge, Probolomyrmex are cryptic ants that live in small (around 20 workers), subterranean colonies in a variety of forest habitats (Taylor 1965, Shattuck et al. 2012). If encountered alive in the field they move very fast in straight movements with stretched out antennae (Agosti 1994). Due to their cryptic lifestyle they are seldom collected and only very little information about the biology of these peculiar ants exists (Taylor 1965, Agosti 1994, Shattuck et al. 2012. The Oriental species P. dammermani Wheeler turned out to be a specialised predator of polixenid millipeds (Ito 1998), but whether this prey

Material and methods
The material examined in this study is based on ant inventories carried out on Madagascar from 1992 to 2011 which included more than 6,000 leaf litter samples, 4,000 pitfall traps, and 9,000 additional hand collecting events (see Fisher 2005 for additional details). Despite such an intensive sampling effort throughout the whole island, Probolomyrmex ants were only rarely collected. This is reflected in the just 40 specimens available for this study.
All new type material and all imaged specimens can be uniquely identified with specimen-level codes affixed to each pin (e.g. CASENT0078328). In the presented descriptions we list all of the available specimen-level codes for the whole type series. It should be noted, however, that the number of stated paratype workers does not necessarily match the number of listed specimen-level codes because pins can hold more than one specimen. Digital colour images were created using a JVC KY-F75 digital camera and Syncroscopy Auto-Montage software (version 5.0), or a Leica DFC 425 camera in combination with the Leica Application Suite software (version 3.8). All images presented are available online and can be seen on AntWeb (http://www.antweb.org). The distribution maps provided at the end of the study (Fig. 6) were generated with the software R (R Core Team 2014). The measurements were taken with a Leica MZ 12.5 equipped with an orthogonal pair of micrometers at a magnification of 100×. Measurements and indices are presented as minimum and maximum values with arithmetic means in parentheses. In addition, all measurements are expressed in mm to two decimal places. The measurements and indices used in this study are mostly based on Taylor (1965), Eguchi et al. (2006), Fisher (2007), and Shattuck et al. (2012):

HL
Head length: in full-face view maximum longitudinal length of head from anterior-most portion of projecting clypeus to midpoint of line across back of head HW  Taylor 1965: 346] Notes. Detailed diagnoses were given by Taylor (1965), Bolton (2003), Eguchi et al. (2006), and Keller (2011). The material from Madagascar treated herein matches them almost perfectly with one exception. The lack of any sutures or grooves on the mesosoma of Probolomyrmex is a widely accepted genus-specific character, but we observed the presence of a small but noticeable metanotal groove in two of the three Malagasy species. Probolomyrmex ants are always easily recognisable from other ants on the basis of their long and slender bodies, almost complete lack of pilosity, the long sting, and especially the frontoclypeal shelf bearing the antennal insertions (Taylor 1965, Agosti 1994. Taylor (1965) pointed out that the structural reduction in Probolomyrmex is extreme, which leaves only a few, useful diagnostic characters, such as dimensions and proportions of head, antennae, petiole, as well as surface sculpture. Based on the material from Madagascar, however, we do not consider surface sculpture to be too important for species diagnostics. We observed some noticeable variation within species. Consequently, we tried to avoid using surface sculpture as primary diagnostic character, and used it only as supporting character.
All three species treated in this study are placed in the P. greavesi species group sensu Eguchi et al. (2006), mostly on the basis of the well-developed ventral process. The two species groups hypothesised by Eguchi et al. (2006) work very well for the Oriental and Indo-Australian regions, and there is no reason to create a new group for the three species from Madagascar. As already pointed out by Fisher (2007) for P. tani, the Afrotropical species, which can also be placed in the P. greavesi species group, appear morphologically close to the three species from Madagascar suggesting a close relationship. At the moment however, it is not possible to assess the phylogenetic relationships of the Malagasy species with the species from other regions in a comprehensive way due to the high morphological uniformity and lack of diagnostic characters. A highly desirable multi-gene molecular phylogenetic analysis might provide insights into the subgeneric relationships within Probolomyrmex.
Worker measurements ( Worker description. In full-face view head between 1.5 to 1.6 times longer than broad (CI 62-65), posterior head margin more or less flat; lateral margins of head convex, broadest medially, posterolateral corners rounded; clypeus and anterior part of frons strongly protruding anteriorly as narrow frontoclypeal, subrectangular shelf or socket; antennal sockets exposed and closely approximated, separated by a thin, vertical lamella formed by fused frontal carinae; mandibles small, triangular to elongate-triangular, masticatory margin armed with one larger apical tooth and a series of six smaller denticles, in full-face view mandibles obscured by frontoclypeal shelf; palp formula 4,2; eyes absent; antennae 12-segmented, funicular antenommeres growing in size and width towards apex without forming well defined antennal club, apical antennomere much larger than remaining funicular antenommeres, antennal scape short (SI 91-94), far from reaching posterior head margin. Mesosoma slender, long, and relatively low (LMI 33-35), in profile mesosomal outline flat; propleurae enlarged and projecting ventrally; promesonotal suture and metanotal groove absent; declivitous face of propodeum margined by low, obtuse, and concave lamella on each side, propodeal lamella posterodorsally and posteroventrally with small, blunt tooth or rounded lobe; posterior declivity of propodeum weakly concave in dorsal view. Legs long and slender; all tibiae with single, pectinate spur; pretarsal claws simple without median tooth; hind tibia around 1.1 to 1.2 times shorter than head width . In profile petiole with subpetiolar process around 1.2 to 1.3 times higher than long (LPeI 76-86), petiole without subpetiolar process around 1.1 to 1.2 times longer than high (LPNeI 107-116), petiolar dorsum strongly arched, much higher posteriorly, anterior face curving smoothly onto dorsum without well developed anterodorsal margin, posterior face vertical and concave, enclosed laterally and dorsally by low, thick carina; in dorsal view petiole around 1.3 to 1.3 times longer than broad (DPeI 76-82); pronotum between 1.5 to 1.7 times longer than petiolar width (PeNI 60-67); subpetiolar process well developed and lamelliform, ventral face weakly concave, anteroventral portion rounded to moderately angled, posteroventral portion sharper and stronger angled, projecting backwards, usually as small acute tooth. Abdominal segment III in profile narrowed anteriorly, broadest posteriorly. Sting well developed and very long. Surface sculpture generally weakly to moderately foveolate overlaying conspicuous very fine, more or less dense, coriaceous microsculpture, usually foveolate sculpture better developed and more conspicuous on cephalic dorsum, lateral mesosoma, and lateral petiole than remainder of body. Pilosity strongly reduced throughout and virtually absent, except for few short hairs below frontoclypeal shelf, some longer hairs on mandibles, and some short, fine hairs around metapleural gland orifice. Pubescence whitish, extremely fine, very short, and appressed, present over most of body, funicular antenommeres with such pubescence overlaid by much scattered, much longer, appressed hairs. Colour dark reddish brown, appendages light brown. Pilosity strongly reduced throughout and virtually absent, except for few short hairs below frontoclypeal shelf, some longer hairs on mandibles, and some short, fine hairs around metapleural gland orifice. Pubescence whitish, extremely fine, very short, and appressed, present over most of body, funicular antenommeres with such pubescence overlaid by much scattered, much longer, appressed hairs. Colour orange to light brown, appendages yellowish.
Etymology. The name of the new species is a combination of the Latin noun "curculio", which means weevil, and the suffix "formis", which means alike. The long and narrow head with its anteriorly projecting frontoclypeal shelf resembles the elongated head shape of a weevil.
Distribution and biology. Probolomyrmex curculiformis is widely but patchily distributed in western Madagascar (Fig. 6). Its known distribution ranges from the southernmost localities Tsimanampetsotsa and Amboasary to Anabohazo in the northwest. The localities are all tropical dry forest or spiny forest habitats situated at very low elevations of 20 to 130 m. Even though the new species was entirely collected by sifting litter, we suspect it is not a genuine leaf litter inhabitant. Instead, P. curculiformis is likely to be a hypogaeic species and the available material was sampled accidentally through the collection of soil for leaf litter sifting. A hypogaeic lifestyle would also explain the patchy distribution pattern. If true, intensive soil sampling in western Madagascar will likely yield more material of this species. The natural history of P. curculiformis is unknown.
Discussion. Probolomyrmex curculiformis is unlikely to be confused with the other two Malagasy Probolomyrmex species. The shape of the petiole is fairly distinct and separates the western P. curculiformis from the northern P. tani since the latter species has a much lower and longer petiole than the first. The third species, P. zahamena, from eastern Madagascar shares a higher and stouter petiole with P. curculiformis. However, P. zahamena possesses a small, but distinct metanotal groove, which is absent in P. curculiformis. In addition, the two species also differ in head shape, which is slightly broader in P. zahamena (CI 67-70) than in P. curculiformis (CI 62-65). Nevertheless, the last difference is sometimes hard to observe and requires measuring. Variation. Despite a very broad distribution pattern in western Madagascar, we could not observe any significant intraspecific variation except for surface sculpture. There is some moderate variation in density and depth of foveolate sculpture throughout the material examined here. Some specimens display very little sculpture while sculpture is very well developed in others. Diagnosis. The following character set distinguishes P. tani from its congeners in Madagascar: head in fullface view between 1.5 to 1.6 times longer than broad (CI 64-66); SI 92-103; in profile mesosomal outline flat to very weakly convex, metanotal groove usually absent, but rarely weakly developed; hind tibia around 1.0 to 1.1 times longer than head width (HTLI 100-111); petiole relatively longer, lower, and less arched, in profile (without ventral process) around 1.3 to 1.5 times longer than high (LPNeI 127-150), and in dorsal view around 1.4 to 1.6 times longer than broad (DPeI 63-69).

Probolomyrmex tani
Worker measurements ( Worker description. In full-face view head between 1.5 to 1.6 times longer than broad (CI 64-66), posterior head margin flat or weakly concave; lateral margins of head convex, broadest medially, posterolateral corners rounded; clypeus and anterior part of frons strongly protruding anteriorly as narrow frontoclypeal, subrectangular shelf or socket; antennal sockets exposed and closely approximated, separated by a thin, vertical lamella formed by fused frontal carinae; mandibles small, triangular to elongate-triangular, masticatory margin armed with one larger apical tooth and a series of six smaller denticles, in full-face view mandibles obscured by frontoclypeal shelf; palp formula 4,2; eyes absent; antennae 12-segmented, funicular antenommeres growing in size and width towards apex without forming well defined antennal club, apical antennomere much larger than remaining funicular antenommeres, antennal scape short (SI 92-103), far from reaching posterior head margin. Mesosoma slender, long, and relatively low (LMI 33-37), in profile mesosomal outline flat to very weakly convex; propleurae enlarged and projecting ventrally; promesonotal suture absent; metanotal groove usually absent, rarely present but very weak; declivitous face of propodeum margined by low, obtuse, and concave lamella on each side, propodeal lamella posterodorsally with small, blunt tooth, posteroventrally with rounded lobe or very blunt tooth; posterior declivity of propodeum weakly concave in dorsal view. Legs long and slender; all tibiae with single, pectinate spur; pretarsal claws simple without median tooth; hind tibia around 1.0 to 1.1 times longer than head width (HTLI 100-111). In profile petiole with subpetiolar process around 1.0 to 1.1 times longer than high (LPeI 97-110), petiole without subpetiolar process around 1.3 to 1.5 times longer than high (LPNeI 127-150), petiolar dorsum strongly arched, much higher posteriorly, anterior face curving smoothly onto dorsum without well developed anterodorsal margin, posterior face vertical and concave, enclosed laterally and dorsally by low, thick carina; in dorsal view petiole around 1.4 to 1.6 times longer than broad (DPeI 63-69); pronotum between 1.5 to 1.7 times longer than petiolar width (PeNI 60-66); subpetiolar process well developed and lamelliform, ventral face weakly concave, anteroventral portion rounded to moderately angled, posteroventral portion sharper and stronger angled, projecting backwards, variably developed, ranging from right angle to a elongate-triangular tooth. Abdominal segment III in profile narrowed anteriorly, broadest posteriorly. Sting well developed and very long. Surface sculpture generally weakly to moderately foveolate overlaying conspicuous very fine, more or less dense, coriaceous microsculpture, foveolate sculpture better developed and more conspicuous on cephalic dorsum and lateral mesosoma than remainder of body. Pilosity strongly reduced throughout and virtually absent, except for few short hairs below frontoclypeal shelf, some longer hairs on mandibles, and some short, fine hairs around metapleural gland orifice. Pubescence whitish, extremely fine, very short, and appressed, present over most of body, funicular an- tenommeres with such pubescence overlaid by much scattered, much longer, appressed hairs. Colour light reddish brown to darker brown, appendages lighter, yellowish to light brown.
Distribution and biology. It has to be pointed out that P. tani is much less broadly distributed as previously thought. Indeed, its distribution is restricted to a narrow strip in the northeast of Madagascar ranging from Makirovana and Ambondrobe north to Montagne des Français. Most of the remaining locality data listed under P. tani in the original description (Fisher 2007) are actually records of the new species P. curculiformis, except for Manongarivo. The only available specimen from the latter locality is damaged, and it cannot be assigned to any species. Therefore, we do not list this locality for P. tani nor any of the other two species treated here. Probolomyrmex tani is found in a variety of forest habitats, such as littoral rainforest, tropical dry forest, lowland rainforest, and montane rainforest, and has an altitudinal range of 10 to 1100 m. Despite that most of the material was collected from leaf litter, P. tani is more likely subterranean in lifestyle, very much like P. curculiformis.
Discussion. Probolomyrmex tani is the most distinctive species of the three treated herein. The shape of the petiole alone separates it very well from P. curculiformis and P. zahamena. In these two the petiole is shorter, higher and stronger arched, in profile (without ventral process) around 1.3 to 1.5 times longer than high (LPNeI 127-150), and in dorsal view around 1.4 to 1.6 times longer than broad (DPeI 63-69). By contrast, the petiole of P. tani is relatively longer, lower, and less arched, in profile (without ventral process) around 1.3 to 1.5 times longer than high (LPNeI 127-150), and in dorsal view around 1.4 to 1.6 times longer than broad (DPeI 63-69).
Variation. Despite being less variable than previously thought (Fisher 2007), P. tani still displays some noticeable intraspecific variation within a relatively small area in northern Madagascar. The specimens from Makirovana possess a much better developed and conspicuous foveolate surface sculpture than the rest of the material of P. tani. Most of the material from Makirovana also has a small but distinct metanotal groove, which is absent in the material from other localities, and longer antennal scapes (SI 99-102). These dissimilarities could be used to separate this series as different species. However, there are several good arguments against it. The length of the antennal scapes is always stable within localities, but shows some noticeable geographical variation. The specimens from the type locality possess the shortest scapes (SI 92-94), whereas the ones from Makirovana and Ambondrobe have the longest scapes (SI 99-102). However, the material from Binara and Montagne des Français shows intermediate values . Furthermore, the metanotal groove is extremely weak in one specimen from Makirovana, almost absent. Also, as mentioned above, surface sculpturing is relatively variable in all three Probolomyrmex species. So, consequently, we prefer to keep all the material listed as P. tani as one somewhat variable species. Type material. Holotype, pinned worker, Toamasina,Parc National de Zahamena,Onibe River,17.75908°S,48.85468°E,780 m,rainforest,sifted litter (leaf mold,rotten wood), collection code BLF22214, 21.-23.II.2009 (B.L. Fisher et al.) (CASC: CASENT0914279). Paratypes, ten paratypes with same data as holotype (BMNH: CASENT0247390; CASC: CASENT0150894; CASENT0150896; CASENT0150897; CASENT0150898; CASENT0150899; CASENT0150900; CASENT0247389; MCZ: CASENT0247391).
Diagnosis. The following character combination distinguishes P. zahamena from the other two Malagasy species: in full-face view head around 1.4 to 1.5 times longer than broad (CI 67-70); SI 99-102; hind tibia around 1.1 to 1.2 times shorter than head width (HTLI 82-89); petiole relatively shorter, higher and stronger arched, in profile (without ventral process) between 1.0 to 1.2 times longer than high (LPNeI 103-116), in dorsal view petiole around 1.2 to 1.3 times longer than broad (DPeI 76-86).
Worker measurements ( Worker description. In full-face view head between 1.4 to 1.5 times longer than broad (CI 67-70), posterior head margin weakly concave; lateral margins of head convex, broadest medially, posterolateral corners rounded; clypeus and anterior part of frons strongly protruding anteriorly as narrow frontoclypeal, subrectangular shelf or socket; antennal sockets exposed and closely approximated, separated by a thin, vertical lamella formed by fused frontal carinae; mandibles small, triangular to elongate-triangular, masticatory margin armed with one larger apical tooth and a series of six smaller denticles, in full-face view mandibles obscured by frontoclypeal shelf; palp formula 4,2; eyes absent; antennae 12-segmented, funicular antenommeres growing in size and width towards apex without forming well defined antennal club, apical antennomere much larger than remaining funicular antenommeres, antennal scape short (SI 99-102), far from reaching posterior head margin. Mesosoma slender, long, and relatively low (LMI 34-38), in profile mesosomal outline relatively flat; propleurae enlarged and projecting ventrally; promesonotal suture absent; metanotal groove present but weak; declivitous face of propodeum margined by low, obtuse, and concave lamella on each side, propodeal lamella posterodorsally with small, blunt tooth, posteroventrally with rounded lobe; posterior declivity of propodeum weakly concave in dorsal view. Legs long and slender; all tibiae  with single, pectinate spur; pretarsal claws simple without median tooth; hind tibia around 1.1 to 1.2 times shorter than head width . In profile petiole with subpetiolar process around 1.2 times longer than high (LPeI 79-86), petiole without subpetiolar process between 1.0 to 1.2 times longer than high (LPNeI 103 -116), petiolar dorsum strongly arched, much higher posteriorly, anterior face curving smoothly onto dorsum without well developed anterodorsal margin, posterior face vertical and concave, enclosed laterally and dorsally by low, thick carina; in dorsal view petiole around 1.2 to 1.3 times longer than broad (DPeI 76-87); pronotum between 1.5 to 1.7 times longer than petiolar width (PeNI 60-67); subpetiolar process well developed and lamelliform, ventral face weakly concave, anteroventral and posteroventral corners well angled, posteroventral portion slightly sharper but not projecting backwards or dentate. Abdominal segment III in profile narrowed anteriorly, broadest posteriorly. Sting well developed and very long. Surface sculpture very conspicuous, throughout whole body densely foveolate overlaying conspicuous very fine, dense, coriaceous microsculpture. Pilosity strongly reduced throughout and virtually absent, except for few short hairs below frontoclypeal shelf, some longer hairs on mandibles, and some short, fine hairs around metapleural gland orifice. Pubescence whitish, extremely fine, very short, and appressed, present over most of body, funicular antenommeres with such pubescence overlaid by much scattered, much longer, appressed hairs. Colour dark reddish brown, appendages light brown.

Etymology.
The new species is named after the type locality, the Zahamena National Park in eastern Madagascar. Zahamena is part of the UNESCO World Heritage Site "Rainforests of the Atsinanana", and considered as one of the WWF's Global 200 priority eco-regions for conservation priority. By naming the new species after this locality we want to draw attention to this very important locality with its high conservation value. The species epithet is treated as a noun in apposition, and thus invariant.
Distribution and biology. At present, P. zahamena is only known from the type locality, which is a tropical rainforest situated at an elevation of 780 m. All the available material is from a single leaf litter collection. It is surprising that P. zahamena is the only known species found in eastern Madagascar, especially considering the very high leaf litter sampling effort performed by the Malagasy ant project from 1992 to the present. This suggests that the species is either comparatively rare or predominantly hypogaeic. As for the other two species, the use of soil sampling methods might yield additional material.
Discussion. Probolomyrmex zahamena is fairly distinct and its identification straightforward. The shape of the petiole, which is relatively short, high and stout, distinguishes it clearly from P. tani, while the presence of a metanotal groove separates it from P. curculiformis. In addition, P. zahamena has a slightly broader head (CI 67-70) than the other two (CI 62-66).
Variation. Since P. zahamena is only known from one collection event, the observable variation is insignificant.