﻿Two new species and a new record of the ant genus Meranoplus Smith, 1853 (Hymenoptera, Formicidae, Myrmicinae) from Thailand

﻿Abstract Meranoplus Smith, 1853 is distributed in the Old World tropics, from Africa, Asia, New Guinea to Australia. There are four species Meranoplusbicolor (Guérin-Méneville, 1844), M.castaneus Smith, 1857, M.laeviventris Emery, 1889, and M.mucronatus Smith, 1857 previously recorded from Thailand. In the present paper, two new species of the genus, M.siamensis Yodprasit & Jaitrong, sp. nov. and M.tanomtongi Yodprasit & Jaitrong, sp. nov., are described based on the worker caste. Additionally, M.malaysianus Schödl, 1998 is recorded for the first time for Thailand. A key to the Oriental and Indo-Australian species, based on the worker caste, is provided. The new species and the new record were found to nest in soil.


Introduction
Meranoplus Smith, 1853 is distributed in the Old World tropics from Africa, Asia, and New Guinea to Australia (Bolton 2024).Members of this genus nest in the soil, in rotten wood, or under stones (Bolton 1981;Jaitrong et al. 2020) and are known to be active both day and night (Gross et al. 1991).The genus has been revised and reviewed across its entire distribution over the past few decades (Australasia and New Guinea: Donisthorpe 1947;Andersen 2006;Taylor 2006;Schödl 2007;Africa: Bolton 1981;Madagascar: Boudinot and Fisher 2013;and Asia: Chapman and Capco 1951;Wu and Wang 1995;Schödl 1998Schödl , 1999;;Terayama 2009;Guénard and Dunn 2012;Bharti and Akbar 2014;Bharti et al. 2016;Jaitrong et al. 2016;Dias et al. 2020).Currently, 91 valid species of the genus are known (Bolton 2024).Among them, 60 species have been recorded from the Australasian region, 13 from the Oriental region, eight from the Afrotropical region, and four from the Malagasy region (Boudinot and Fisher 2013;AntWiki 2024;Bolton 2024).Until now, only four species, Meranoplus bicolor (Guérin-Méneville, 1844), M. castaneus Smith, 1857, M. laeviventris Emery, 1889, and M. mucronatus Smith, 1857, were known from Thailand (Jaitrong and Nabhitabhata 2005;Jaitrong et al. 2020;Khachonpisitsak et al. 2020) We examined Meranoplus specimens from Thailand and recognized seven species.Meranoplus malaysianus Schödl, 1998 is newly recorded from the country, and M. siamensis Yodprasit & Jaitrong, sp. nov. and M. tanomtongi Yodprasit & Jaitrong, sp. nov.are new to science and described here based on the worker caste.A key to the Oriental and Indo-Australian species of Meranoplus, based on the worker caste, is presented.

Materials and methods
This study was mainly based on the specimens deposited in the Natural History Museum of the National Science Museum, Thailand.Almost 500 specimens of the genus Meranoplus were examined.Specimens of the new species and the new record were compared with the images available on Antweb (2024) of holotypes and paratypes of small species distributed in Asia: M. borneensis Schödl, 1998;M. loebli Schödl, 1998;and M. malaysianus Schödl, 1998.A paratype of M. malaysianus deposited in the Seiki Yamane Collection, Kagoshima, Japan was also examined.
Most morphological observations were made with a Zeiss Discovery V12 stereoscope.Multi-focused montage images were produced using NIS-Elements-D from a series of source images taken with a Nikon Digital Sight-Ri1 camera attached to a Nikon AZ100M stereoscope.Specimens were measured for the following parts using a micrometer on a Zeiss Discovery V12 stereoscope.All measurements are given in millimeters and recorded to the second decimal place.
The abbreviations for the measurements and indices used are as follows: (edited from Bolton 1981;Hölldobler and Wilson 1990;Schödl 1998):

HL
Head length, straight-line length of head in full-face view, measured from the mid-point of the anterior clypeal margin to the midpoint of the posterior margin.In species where one or both of these margins are concave, the measurement is taken from the mid-point of a transverse line that spans the apices of the projecting portions.HW Head width, maximum width of head in full-face view, excluding the compound eyes.ML Mesosomal length, 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.PML Length of promesonotal shield, measured from anterior mid-point of pronotum behind collar that is the mid-point of a virtual line, where the anterior pronotal margins meet, to mid-point of behind margin of mesonotum above propodeum.PW Pronotal width, measured right behind base of anterolateral pronotal projection (angle) in dorsal view.

SL
Scape length, straight-line length of the antennal scape, excluding the basal constriction or neck.

TL
Total length, total outstretched length of the individual, from the mandibular apex to the gastral apex.
The Thai specimens agreed well with the holotype (CASENT0902029) in structure, sculpturing, and pilosity.However, body color of the specimens collected from Thailand are reddish brown, while the holotype is paler, yellow (Fig. 1).Reticulations on dorsum of head rather denser and smaller than that on the holotype (see Fig. 1A, B for comparison).
Habitat.Two specimens from Narathiwat Province were collected from the ground in a disturbed area near a lowland evergreen forest, near the Thai-Malay border.A specimen (THNHM-I-00028943, THNHM) from Songkhla Province was collected in a primary evergreen forest.  .

Meranoplus siamensis
Description of worker.Head in full-face view subquadrate, slightly shorter than broad, with sides broadly convex, posterior margin distinctly convex, posterolateral corner bluntly angulate.Antennal scapes short, reaching level of posterior margin of compound eyes, apical half incrassate; antennal segment II slender, longer than each of segments III-VI, and almost as long as III+IV+V; segment VI broader than each of segments II-V.Clypeus roughly subrectangular, shorter than broad, its anterior margin feebly concave medially, while posterior clypeal margin almost straight.Mandibles subtriangular, masticatory margin with four teeth.Compound eyes large and convex when seen in full-face view, located laterally and well behind mid-length of head, with 8 or 9 ommatidia along longest axis, each facet hexagonal (Fig. 6A).Frontal lobes broad, its anterior corners right angled and lateral margin almost straight (Fig. 5H).Frontal carinae long reaching posterolateral corners of head.
Mesosoma in dorsal view, promesonotal shield distinctly shorter than broad, its lateral margin convex, serrate, margined and slightly overhanging mesosoma; lateral and posterior portions of promesonotal shield with translucent fins; posterior margin of promesonotal shield sinuate and distinctly concave; anterolateral corners of promesonotal shield bluntly angulate and posterolateral corners of promesonotal shield roundly angulate; promesonotal shield with two pairs of fenestrae laterally; metanotal groove absent.Declivity of propodeum almost invisible from above, mostly overhung by posterior margin of promesonotal shield (propodeal spines are visible in profile).Mesosoma in profile subquadrate, dorsal outline weakly convex, lateral face of mesosoma relatively flat; lateral portion of pronotum subtriangular; metapleuron not clearly demarcated from mesopleuron and lateral face of propodeum.Propodeal spines long and acute, longer than wide at its base, located at middle of propodeal declivity length.
Petiole in profile subtriangular.Subpetiolar process low, its ventral outline weakly convex, with small anterior denticle.Postpetiole in profile subquadrate, shorter than high; in dorsal view, distinctly shorter than broad, anterior margin weakly convex, posterior margin distinctly convex; dorsum of postpetiole somewhat flat, marginated with distinct ridge, posterior face convex.Gaster about as large as head and mesosoma combined; first gastral tergite largest, in dorsal view, its anterior margin distinctly concave.
Sculpture.Mandibles striate, shiny.Antennal scapes superficially striate.Head dorsally sparsely reticulate-rugulose laterally, while median portion weakly sculptured; half posterior portion of antennal scrobes shagreened mixed with few transverse ridges.Promesonotal shield more weakly sculptured than dorsum of head, with median portion smooth, shiny, and lacking any rugae; in profile, upper one-third portion of pronotum shagreened, while lower two-third portion with sparse irregular ridges; upper one-third portion of mesopleuron shagreened, lower two-thirds longitudinal weakly striate; metapleuron, and lateral faces of propodeum somewhat smooth and shiny.Propodeum declivity shagreened.Petiole smooth and shiny, postpetiole somewhat smooth but posterior face of postpetiole scabrous.First gastral tergite superficially shagreened with smooth and shiny interspaces.
Pilosity and coloration.Dorsa of head and mesosoma with dense erect hairs mixed with sparse longer hairs; antennae with dense suberect hairs; in profile, lower two-thirds of pronotum with sparse suberect hairs; lower one-third of mesopleuron and metapleuron with sparse suberect hairs; area around propodeal spiracle with sparse suberect hairs; femora and tibiae with numerous long outstanding hairs as well; petiole with weakly sparse erect hairs on its anterior face and dorsum; postpetiole with dense long erect hairs, except anterior face without hairs; gaster with dense long erect hairs.Body mainly reddish brown; mandibles, antennae, legs, and tip of gaster yellowish brown.
Etymology.The specific name is after Thailand where the type locality is located; Thailand was called "Siam" in the past.
Habitat.This species can be found in dry evergreen and dry dipterocarp forests.The specimens collected from northeastern Thailand (colony code TH22-WJT-264) nested in the soil.Workers moved slowly on the ground.
Differential diagnosis.Meranoplus siamensis sp.nov. is a small species that is most similar to Meranoplus tanomtongi sp.nov. in general appearance, having a pair of fenestrae along each lateral margin of the promesonotal shield, and having a subrectangular postpetiole when seen in profile.However, M. siamensis can be distinguished from M. tanomtongi by: 1) anterior corners of frontal lobes right angled and lateral margin almost straight (round and lateral margin weakly convex in M. tanomtongi, see Figs 5E, H for comparison); 2) compound eyes with 8 or 9 ommatidia along longest axis, each facet hexagon (each facet round or elliptical in M. tanomtongi, see Figs 5G, 6A for comparison); 3) dorsum of head weakly sculptured (dorsum of head entirely and distinctly reticulate in M. tanomtongi, see Fig. 5F, I for comparison); 4) dorsum of postpetiole somewhat flat, marginated with distinct ridge (shallowly concave, marginated with distinct ridge in M. tanomtongi); 5) entire head with dense short hairs mixed with sparse longer hairs (hairs along head margin clearly longer than hairs on middle of head in M. tanomtongi, see Fig. 5F, I for comparison).
The type series of M. siamensis sp.nov. is very similar to the non-type specimens from Central Thailand (TH02-WJT-039).However, the two colonies have some variations: 1) compound eyes with 9 ommatidia along longest axis in the type series (8 ommatidia in colony no.TH02-WJT-039); 2) promesonotal shield shorter than broad in the type series (almost as long as broad in colony no.TH02-WJT-039); 3) posterior half of head with sparse and weak reticulation in the type series (dense distinct reticulations in colony no.TH02-WJT-039); 4) propodeal declivity somewhat shagreened in the type series (smooth and shiny in colony no.TH02-WJT-039); 5) first gastral tergite superficially shagreened with smooth and shiny interspaces in the type series (distinctly shagreened in colony no.TH02-WJT-039).These characters are not clear enough to distinguish the two populations.eyes large, strongly convex in full-face view, located laterally behind mid-length of head, with eight ommatidia along longest axis, each facet round or elliptical (Fig. 5G).Frontal lobes broad, its anterior corners round, its lateral margin weakly convex (Fig. 5E).Frontal carinae long, reaching posterolateral corners of head.

Meranoplus tanomtongi
Mesosoma in dorsal view promesonotal shield distinctly shorter than broad, laterally convex, sinuate, margined and slightly overhanging mesosoma; lateral and posterior portions of promesonotal shield with translucent fins; posterior margin of promesonotal shield sinuate and distinctly concave; anterior corners of pronotum and posterior corners of mesonotum bluntly angulate; promesonotal shield with two pairs of fenestrae laterally; metanotal groove absent.Declivity of propodeum almost invisible from above, overhung by posterior margin of promesonotal shield (propodeal spines are visible in profile).Mesosoma in profile subquadrate, weakly convex dorsal outline, lateral face of mesosoma flat; lateral face of pronotum subtriangular; metapleuron not clearly demarcated from mesopleuron and lateral face of propodeum.Propodeal spines long and acute, located at middle of propodeal length, in profile.
Petiole in profile subtriangular, both anterior and posterior faces weakly convex; when viewed from behind, dorsal margin transverse and smoothly convex.Subpetiolar process low, its ventral outline weakly convex, with small anterior denticle.Postpetiole in profile subquadrate, shorter than high; in dorsal view, distinctly shorter than broad, anterior margin almost straight, while posterior margin distinctly convex; dorsum of postpetiole shallowly concave marginated with sinuate ridge, posterior face convex.Gaster larger than head and mesosoma combined; first gastral tergite largest, in dorsal view, its anterior margin distinctly concave.
Sculpture.Mandibles striate but shiny.Antennal scapes superficially striate.Dorsum of head in full-face view entirely reticulate; posterior half of antennal scrobes shagreened mixed with a few transverse ridges.Dorsum of promesonotal shield distinctly reticulate but median region with weaker reticulation than elsewhere; in profile, upper half portion of lateral faces of pronotum shagreened, while lower half portion with sparse irregular ridges; upper one-third portion of mesopleuron shagreened, lower two-third portion weakly longitudinally striate; metapleuron and lateral face of propodeum smooth and shiny.Propodeum declivity superficially shagreened.Petiole smooth and shiny.Postpetiole somewhat smooth but upper portion of posterior face with wrinkles.First gastral tergite superficially shagreened with smooth and shiny interspaces.
Pilosity and coloration.Dorsum of head with dense erect hairs (usually a closed cell with a hair), hairs along head margin clearly longer than hairs on middle of head; antennae with dense, suberect hairs; promesonotal shield with dense, erect hairs; legs with dense suberect hairs; in profile, lower two-thirds of pronotum with sparse suberect hairs; lower one-third of mesopleuron and metapleuron with sparse suberect hairs; area around propodeal spiracle with sparse, suberect hairs; femora and tibiae with numerous long, outstanding hairs as well; petiole with sparse, erect hairs on its dorsum; postpetiole with dense, long, erect hairs, except anterior face without hairs; femora and tibiae with numerous long, outstanding hairs as well; gaster with dense, long, erect hairs.Dorsum of body (head, mesosoma, and gaster) and waist yellowish brown; mandibles, antennae, legs, and tip of gaster yellow.
Etymology.The specific name is dedicated to Professor Alongklod Tanomtong of Khon Kaen University, who is an excellent specialist in biological sciences in Thailand, who helped and inspired many young biologists.
Habitat.This species can be found in lowland primary forest (300-600 m a.s.l.).The type series was collected from a dry dipterocarp forest.Lao specimens (colony code WJT10-LAO111) were collected from a dry evergreen forest.Specimens from Mukdahan Province, northeastern Thailand were collected in a mixed deciduous forest.
Differential diagnosis.Meranoplus tanomtongi sp.nov. is a small species that is most similar to M. siamensis sp.nov., see differential diagnosis under M. siamensis.This species is also similar in general appearance to M. malaysianus and M. borneensis from Sundaland, in having two pairs of fenestrae along each lateral margin of the promesonotal shield and having a concave anterior margin of first gastral tergite.However, M. tanomtongi can be distinguished from M. malaysianus and M. borneensis by 1) anterior corners of frontal lobes round and lateral margin weakly convex (right angled and lateral margin almost straight in M. malaysianus and M. borneensis); 2) petiole in profile subquadrate, almost flat dorsally (round, usually convex dorsal outline in M. malaysianus and M. borneensis); 3) entire lateral margin of the promesonotal shield is serrate and convex (parallel sides in M. malaysianus and M. borneensis); 4) in profile, the tip of petiole acute (truncate in M. malaysianus and M. borneensis); 5) head in full-face view entirely reticulate (densely reticulaterugulose in M. malaysianus and M. borneensis); 6) the petiole and postpetiole are smooth and shiny (sculptured in M. malaysianus and M. borneensis).

Discussion
Until now, 19 species (including the two new species) of the genus Meranoplus have been known from the Oriental and Indo-Australian regions.Among them, seven species are found in Thailand (Meranoplus bicolor, M. castaneus, M. laeviventris, M. malaysianus, M. mucronatus, M. tanomtongi sp.nov., and M. siamensis sp.nov.).The presence of a pair of spines or teeth upon the petiolar dorsum, shape of propodeal spines, shapes of petiole and postpetiole were used by Bolton (1981) to distinguish the three species groups (M.magrettii, M. nanus, and M. spininodis groups) of the genus Meranoplus in the Ethiopian zoogeographical region.For Thai species, M. castaneus share a pair of spines or teeth on the petiolar dorsum with the members of M. spininodis group, but the other characters, such as shapes of promesonotal shield and postpetiole are different from the species group.For the moment, we do not place M. castaneus in the M. spininodis group.The other Thai species do not He did not mention the species groups.We followed his morphological characters to distinguish the Thai species of the genus.The body size, the length of posterior corners of mesonotum and propodeal spines, the shape of promesonotal shield, and the sculpturing on the first gastral tergite were used to distinguish the species of Meranoplus in previous papers (Bolton 1981;Schödl 1998Schödl , 2007;;Boudinot and Fisher 2013).We also use these characters to separate the Thai species.The shape of frontal lobes is an important characteristic that can be used to distinguish the two new species (see couplet 2 in the key; see also Fig. 5E, H for comparison).This character was not used in the previous works.The two new species are small, and they have the subrectangular postpetiole when seen in profile and its dorsum is almost flat or shallowly concave.These characters are unique within Meranoplus, and thus the new species are placed in a distinct group (Meranoplus siamensis species group).
Members of the ant genus Meranoplus can be found throughout Thailand from lowland to highland (Fig. 4).Meranoplus castaneus, M. malaysianus, and M. mucronatus were mainly found in Sundaland (Borneo, Indonesia, and Malaysia) (Schödl 1998).Recently, only M. castaneus and M. mucronatus were recorded in Thailand.Meranoplus malaysianus is recorded for the first time in the country.In Thailand, these three species are restricted to the south.The northernmost limit distribution range of M. malaysianus is in Songkhla Province (ca 530 km south of the Isthmus of Kra).
All species of the ant genus Meranoplus in Thailand nest in soil and are usually found walking on the ground except M. castaneus, which nests in dead branches in the canopy (ca 35 m above the ground in evergreen and swamp forests).Itino and Yamane (1995) collected M. castaneus in the canopy (25-35 m above ground) in mixed dipterocarp forest in Malaysia.Jantarit et al. (2009) and Watanasit et al. (2007) also found M. castaneus high on trees in evergreen forests.Meranoplus castaneus can be identified as an arboreal ant.
The two new species were found to nest in soil and walk on the ground.Meranoplus siamensis sp.nov.was found in the dry evergreen forest in eastern Thailand and in the dry dipterocarp forest in the western, northeastern, and central parts of the country.Meranoplus tanomtongi sp.nov.was collected from dry dipterocarp and mixed deciduous forests in northeastern Thailand.This species was also found in a dry evergreen forest in Laos (colony No. WJT10-LAO111 and WJT10-LAO111), but the body size of Lao population is slightly larger than the type series.

Figure 1 .
Figure 1.Meranoplus malaysianus A, C, E holotype (CASENT0902029) B, D, F non-type worker (THNHM-I-00028943) A, B head in full-face view C, D promesonotal shield in dorsal view E, F body in profile.

Figure 2 .
Figure 2. Meranoplus siamensis sp.nov.(holotype, THNHM-I-00027303) A head in full-face view B body in profile view C promesonotal shield in dorsal view D body in dorsal view.

Figure 3 .
Figure 3. Meranoplus tanomtongi sp.nov.(holotype, THNHM-I-00028903) A head in full-face view B body in profile view C promesonotal shield in dorsal view D body in dorsal view.

Figure 4 .
Figure 4. Distribution of Meranoplus Thai species in Thailand.

Figure 9 .
Figure 9. Characters used in key A, B, E Meranoplus montanus (in Bharti and Akbar 2014) C, G M. boltoni (holotype, CASENT0902031) D M. periyarensis (in Bharti and Akbar 2014) F, H M. nepalensis (paratype, CASENT0902026) A, F promesonotal shield in dorsal view B, G, H clypeus in full-face view C petiole and postpetiole in profile D, E body in profile.