Research Article |
Corresponding author: Wendy Y. Wang ( wywang24@gmail.com ) Academic editor: Brian Lee Fisher
© 2020 Wendy Y. Wang, Aiki Yamada, Seiki Yamane.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Wang WY, Yamada A, Yamane S (2020) Maritime trap-jaw ants (Hymenoptera, Formicidae, Ponerinae) of the Indo-Australian region – redescription of Odontomachus malignus Smith and description of a related new species from Singapore, including first descriptions of males. ZooKeys 915: 137-174. https://doi.org/10.3897/zookeys.915.38968
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The maritime trap-jaw ant Odontomachus malignus Smith, 1859 is thought to be widespread throughout islands in the Indo-Pacific and parts of the Oriental realm. Because of its unique nesting preference for harsh littoral habitat and distinct morphology, O. malignus has usually been assumed to consist of only one species. We, however, describe a new species similar to O. malignus found in the mangroves of Singapore, Southeast Asia – Odontomachus litoralis sp. nov. We find strong evidence of both species existing in (near) sympatry, and also distinct morphological differences between O. malignus and the new species. Additional complementary DNA evidence in the form of COI barcodes (313 bp) supporting putative species identification and delimitation is provided. Defining morphological characteristics for the O. malignus species group (nested within the larger O. infandus clade) are given in detail for the first time. The worker and queen castes of the new species are described; a redescription of the worker caste of O. malignus, based on specimens from Singapore and the Philippines in addition to the holotype, is also given. The males of both species are also described for the first time, including male genitalia. A preliminary key to most known species of the O. infandus group based on the worker caste is provided.
inter-tidal ants, littoral habitat, mangroves, systematics, sympatric species
The trap-jaw ant genus Odontomachus Latreille, 1804 (Hymenoptera: Formicidae: Ponerinae) comprises 72 valid extant and three fossil species to date (
The taxonomic history of O. malignus follows a rather convoluted trajectory. The species is thought to be widespread and not rare across intertidal areas throughout islands in the Indo-Pacific (
Similar controversy surrounded the form Odontomachus malignus var. retrolatior Viehmeyer, 1914, which is now considered a junior synonym of O. malignus. The variety was described by
In addition,
In this study, we describe a new species from Singapore (Southeast Asia) belonging to the O. malignus species group, and provide morphological evidence, supported by the existence of workers and males of both species in (near) sympatry and partial molecular evidence (i.e., COI barcodes), showing that the new species is different from O. malignus. Based on the limited DNA evidence, we further hypothesise that the new species may confer with the aforementioned unidentified species from Sarawak in
Type images of the following species and subspecies (synonymised) available at AntWeb v.7.27.2 (Available from https://www.antweb.org. Accessed 14 August 2019) were examined:
“Odontomachus tuberculatus Roger, 1861” (
“Odontomachus malignus var. retrolatior Viehmeyer, 1914” (
Morphological observations of specimens were made using Olympus SZX16 and Nikon SMZ18 stereomicroscopes, while measurements were made using micrometres on the Olympus SZX16. Measurements of the type images provided in AntWeb v.7.27.2 (Available from https://www.antweb.org. Accessed 14 August 2019) were made using ImageJ 1.52c (
Male genitalia of one male from each species (ZRC_ENT00007636.1, ZRC_BDP0014515), preserved in 80% ethanol, were slide-mounted by following the preparation steps described in Yamada & Eguchi (2016), and examined with a Nikon Eclipse E600 microscope.
Morphological terminology follows mainly
EL Compound eye (hereafter simply termed ‘eye’) length measured along its maximum longitudinal diameter with head in lateral view.
EW Maximum eye width perpendicular to EL.
FWL Maximum forewing length (alate queen and male only).
HL Maximum length of head in full-face view, measured from anteriormost point of clypeus to midpoint of a line drawn across posterior margin of head (including ocelli for male).
HW Maximum width of head in full-face view. For males, measurement includes compound eyes.
IFLW Inter-frontal lobe width, measured as maximum distance between outermost margins of frontal lobe (worker and queen only). The frontal lobe is taken as the median arch of the torulus, as in
MDL Maximum length of mandible measured from mandibular insertion to apex of mandible (worker and queen).
OL Ocellus length, measured as maximum diameter of major axis of median ocellus (queen and male).
OED Ocello-ocular distance, measured as maximum distance between lateral ocellus and eye (male only).
PTH Petiole height, measured as maximum height of petiole in lateral view, perpendicular to petiole length, from an imaginary line tangential to petiolar apex to ventral surface of postpetiolar helcium, where the latter inserts into the petiole. Note: PTH here is measured only to the ventral surface of the postpetiolar helcium, instead of the ventral-most point of the subpetiolar process sensu
PTL Petiole length, measured as maximum length from anteriormost to posteriormost inflections of petiolar node in lateral view.
SL Maximum length of antennal scape excluding basal constriction.
WL Weber’s length, maximum diagonal distance of mesosoma in lateral view, measured from base of anterior slope of pronotum to posterior-most point of propodeal lobe.
CI Cephalic index: HW/HL × 100
MDI Mandible index: MDL/HL × 100
PTHI Petiole height index: PTH/PTL × 100
SI Scape index: SL/HW × 100
SKYC Seiki Yamane Collection, Kagoshima, Japan
Source images for focus stacking were taken using a Canon EOS Kiss X9 digital camera, attached to a Nikon AZ100 stereomicroscope (for O. litoralis worker, queen, and male bodies of all species, excluding male genitalia), and a Nikon Eclipse E600 microscope (for male genitalia). Focus-stacked images were produced using Helicon Focus Pro 7.0.2 (Helicon Soft Ltd., http://www.heliconsoft.com/), and improved with the retouching function of the same software. Colour balance and contrast were adjusted using GIMP 2.8 (The GIMP Development Team, http://www.gimp.org).
Workers of O. malignus were imaged with a Dun Inc. Passport II macrophotography imaging system, using a Canon MP-E 65 mm lens; focus-stacked images were produced using Zerene Stacker v1.04 (Zerene Systems LLC, https://zerenesystems.com/cms/stacker). All final images were further adjusted, annotated, and scale bars added using Adobe Photoshop CS6.
DNA barcoding and subsequent objective clustering were conducted to roughly sort specimens into putative molecular taxonomic units for further morphological review. DNA extraction was performed on six individuals (4 workers, 1 queen, 1 male) from the type series of the new species (catalogue numbers ZRC_ENT00000917.01–06) collected in Singapore, and 10 unidentified males collected in (near) sympatry from mangroves in Pulau Semakau (Singapore) using malaise traps (catalogue numbers each with prefix ‘ZRC_BDP’). One leg per individual was used for DNA extraction with QuickExtract DNA extraction solution (
Barcodes were aligned using MAFFT v7 (
Summary of specimen data associated with COI sequences used in objective clustering.
Species | Specimen identifier/catalogue no. | GenBank accession no. | Geographic origin of specimen | Caste/sex |
---|---|---|---|---|
O. litoralis | ZRC_ENT00000917.1 | MK910364 | Singapore | Worker |
ZRC_ENT00000917.2 | MK910365 | Singapore | Worker | |
ZRC_ENT00000917.3 | MK910366 | Singapore | Worker | |
ZRC_ENT00000917.4 | MK910367 | Singapore | Worker | |
ZRC_ENT00000917.5 | MK910368 | Singapore | Queen | |
ZRC_ENT00000917.6 | MK910369 | Singapore | Male | |
Sp. BOR002, MJ19771 | KU146009 | Sarawak, Borneo | Worker | |
O. malignus | ZRC_BDP0014432 | MK910354 | Singapore | Male |
ZRC_BDP0014442 | MK910355 | Singapore | Male | |
ZRC_BDP0014515 | MK910356 | Singapore | Male | |
ZRC_BDP0014516 | MK910357 | Singapore | Male | |
ZRC_BDP0014535 | MK910358 | Singapore | Male | |
ZRC_BDP0014676 | MK910359 | Singapore | Male | |
ZRC_BDP0014677 | MK910360 | Singapore | Male | |
ZRC_BDP0014712 | MK910361 | Singapore | Male | |
ZRC_BDP0014733 | MK910362 | Singapore | Male | |
ZRC_BDP0016086 | MK910363 | Singapore | Male | |
MJ13287 | KU146082.1 | Palau | Worker | |
USNMENT01124387 | KU504894.1 | Philippines | Worker | |
O. rixosus | ZRC_BDP0016035 | nil | Singapore | Worker |
O. pararixosus | ZRC_BDP0012747 | nil | Singapore | Worker |
ZRC_BDP0012737 | nil | Singapore | Male | |
O. simillimus | ZRC_BDP0016524 | nil | Singapore | Male |
ZRC_BDP0016500 | nil | Singapore | Male |
We found a number of reliable morphological differences between O. malignus and the new species O. litoralis sp. nov. as fully accounted below in detailed species descriptions. Comparisons of images with directly examined physical specimens revealed noticeable morphological differences between the holotype of O. malignus (OMH), O. malignus specimens from Singapore (OMSG-w) and the Philippines (OMPH-w), against workers of O. malignus var. retrolatior (OR-w). The type specimen of O. tuberculatus was too badly damaged and its image was thus excluded from morphological comparisons. Firstly, OMH/ OMSG-w/OMPH-w appear to have a relatively shorter scape with respect to head width, compared to OR-w (OMH SI 121, OMSG-w/OMPH-w SI 120–123; OR-w SI 127–130). Next, the gaster of OMH/OMSG-w/OMPH-w is generally uniformly dark brown in colour, but in OR-w the base of gastral tergite I is paler brown relative to the rest of the gaster. Thirdly, the petiolar spine of OR-w appears shorter and stouter relative to those of OMH/OMSG-w/OMPH-w. In view of the lack of more striking morphological differences or sympatry of the alternate forms, we conservatively infer these specimens as one and the same species with some observed variation between allopatric populations. Considering the broad distribution of O. malignus, more comprehensive DNA or other forms of molecular evidence from different geographic populations will be required to verify if these actually comprise of multiple cryptic species.
The COI sequences of the new species from Singapore (GenBank accession numbers: MK910364–MK910369) were nearly identical (BLAST: 100% query cover, 99% identity) to that of specimens obtained from Sarawak and identified by
In a rough sorting to putative species using objective clustering (
Cluster dendrogram of COI (313 bp) barcodes of Odontomachus litoralis and O. malignus from (near) sympatric populations in Singapore, specimens collected from Borneo (KU146009.1), Palau (KU146082.1), and the Philippines (KU504894.1), and additional reference barcodes of O. rixosus, O. pararixosus, and O. simillimus. Nodes are annotated with numbers indicating percentage (%) uncorrected p-distance thresholds at which sequences diverge.
The following account compounds and adds on to the characters as already mentioned in
Head rather elongate, with CI 79–82. Temporal prominences and ocular furrow distinct. Vertex posteriorly with a protuberance on each side of median furrow. Mandible with a series of teeth throughout the length of its masticatory margin; apical, intercalary and subapical teeth elongate, none apically truncate, all somewhat acute; palp formula 4, 4. Pronotal dorsum anteriorly with two or more long erect setae that are clearly discernible from short appressed or suberect hairs around them (this condition is occasionally seen in some members of the O. infandus group). Mesopleuron with anteroventral margin strongly developed, looking like a tubercle when seen from above. Petiolar spine rather short, not very sharply pointed apically. First gastral tergite not flattened dorsally, without anteromedian pit. Valviceps of male genitalia lacking dorsolateral carina.
Holotype. Worker, SINGAPORE, Sungei Buloh Wetland Reserve (1.44676N, 103.73018E), back mangrove, nest in Thalassina mound, 28 Mar 2018, W. Wang & M.S. Foo leg. ZRC_ENT00013883 (
Paratypes.
Twenty workers, 3 queens, 1 male (
Singapore: 3 workers, Sungei Buloh Wetland Reserve, near Coastal Path, mangrove, nest in soil mound among roots of uprooted mangrove tree, 5 Dec 2018, W. Wang leg., colony no. WW-SG18-Odonto2, ZRC_ENT00007634 (
Worker. With features mentioned for the O. malignus species group. Large-sized, with moderate intranidal size variation. Head in full-face view with posterior margin weakly concave, median furrow deep and rather broad; almost entire head extensively striate to rugose, only faintly shining. Pronotum entirely with dense, fine sculpture and matte; mesonotum strigate and weakly shiny while propodeal dorsum generally more finely strigate and matte; metapleuron distinguished from propodeal dorsum by shallow longitudinal furrow; propodeal junction distinctly angular with strong transverse carina separating propodeal dorsum and declivity; posterior face of propodeum strongly marginate laterally. Basal portion of anterior face of petiolar node with strongly strigate triangular area that is rather distinctly defined; lateral face of node smooth in upper area including spine and striate in lower area. Head and petiole orange-brown; antenna, mesosoma and gaster dark reddish brown; coxae light dull-yellowish brown, femora in apical portion and tibiae orange-brown, tarsi dark brown. Legs covered with dense yellowish pubescent hairs.
Male. Body relatively smaller than that of worker and queen; body sculpture well-marked. Mandible subrectangular or quadrate. Furrow separating metapleural and propodeal bulges broad and deep, lined with transverse carinae along its length; petiolar node in anterior view broadly rounded apically and nearly truncate, largely microsculptured and matte, with only area around apex smooth. Maximum diameter of lateral ocellus slightly shorter than minimum distance between lateral ocellus and eye. Basal disc of abdominal sternite IX subpentagonal with broadly rounded posterolateral corners; posterior lobe of sternite IX almost as long as basal disc and slightly tapering apicad, with apical corner rounded but angulate; anterodorsal margin of valviceps strongly produced; dorsolateral carina of valviceps absent; ventral margin of valviceps broadly concave with ca. 23 denticles. Body almost entirely light dull-yellowish brown; areas around ocelli and pronotum somewhat darker; tibiae and tarsi darker than coxae and femora.
Worker measurements. Holotype: EL 0.65; EW 0.50; HL 3.35; HW 2.78; IFLW 0.78; MDL 2.25; PTH 1.55; PTL 0.95; SL 3.35; WL 4.80; CI 82; MDI 67; PTHI 163; SI 122. Three paratypes, 7 non-types (N = 10): EL 0.55–0.65; EW 0.40–0.50; HL 2.75–3.35; HW 2.18–2.75; IFLW 0.65–0.75; MDL 1.80–2.25; PTH 1.20–1.55; PTL 0.75–0.95; SL 2.80–3.40; WL 4.10–4.90; CI 79–82; MDI 65–70; PTHI 156–168; SI 122–130.
Queen measurements. Paratypes (N = 2): EL 0.70–0.75; EW 0.58–0.60; FWL 9.30–9.90; HL 3.35; HW 2.85–2.90; IFLW 0.85–0.90; MDL 2.20–2.30; OL 0.18–0.20; PTH 1.70–1.75; PTL 1.00–1.05; SL 3.25; WL 4.95–5.00; CI 85–87; MDI 66; PTHI 162–175; SI 112–114.
Male measurements. 1 paratype, 2 non-types (N = 3): EL 0.75–0.80; EW 0.45–0.50; FWL 6.20–6.89; HL 1.10–1.15; HW 1.45–1.50; OL 0.20–0.25; PTH 0.70–0.75; PTL 0.75–0.80; SL 0.28–0.30; WL 3.50–3.75; CI 132–135; PTHI 93–94; SI 19.
Worker. Relatively large compared to male, with moderate variation in size (HL 2.75–3.35; WL 4.10–4.90). Head in full face view with posterior margin broadly and shallowly concave; occipital carina well-developed and dark-pigmented; median furrow deep and rather broad, with a fine longitudinal carina that is stronger anteriorly; area along each side of furrow slightly swollen; vertex area in front of occipital carina with a pair of conspicuous protuberances, each located at same distance from occipital carina and median furrow (Fig.
Paratype worker of O. litoralis sp. nov. A Head in full face view B mesosoma in dorsal view C head in lateral view D mesosoma in lateral view E closeup of petiole in lateral view F gaster in lateral view. Red arrows on A, C indicate protuberances on head dorsum. White arrow on B indicates tubercle-like projection of the strongly developed anteroventral margin of mesopleuron.
Head in full face view extensively striate and only faintly shiny; frons and frontal lobes with longitudinal to weakly diverging striae, with interspaces microsculptured; ocular ridge, extraocular furrow and area anterior to vertex protuberances with weaker striae; vertex lobe distinctly strigate, temple and gena very finely microsculptured and more shiny; anterolateral portion of antennal fossa with very fine striation; entire ventral face of head covered with superficial microsculpture; median disc of clypeus superficially sculptured and shiny, with feeble striation on border with frontal lobe; mandible superficially microsculptured on dorsal and ventral faces, smooth and shiny on outer face. Pronotum in dorsal view with very fine and dense striae arranged roughly concentrically, and stronger transverse striae in anterior portion including anteromedian lobe; lateral face irregularly rugulose in anterior and posterior sections, densely microcolliculate in median portion. Mesonotum densely and finely strigate, with interspaces micropunctate; mesopleuron extensively smooth to superficially sculptured and shiny, with anterior and posterior areas transversely striate; metapleuron coarsely striate with microsculptured interspaces and weakly shining. Propodeum in dorsal view mostly with dense strigae that are weaker than those on mesonotum and metapleuron; posterior declivitous face with a few coarse transverse carinae. Anterior face of petiolar node strigate in its basal triangular area; lateral face with median section striate; remainder of anterior and lateral faces superficially microsculptured and weakly shiny; posterior face smooth and shiny. Gaster largely smooth and shining, sometimes mildly pruinose but still shiny. Legs entirely covered with microsculpture and faintly shiny.
Entire dorsal, lateral and ventral faces of head covered with numerous but scattered minute standing or decumbent hairs; frons posteriorly with pair of long erect setae, which may sometimes be lost during specimen processing; mandible covered with scattered short whitish pubescent hairs, ventral face lined with multiple long yellowish setae along masticatory margin. Dorsum of mesosoma with sparse short suberect, decumbent or appressed hairs; pronotal disc with a few sparse and long erect setae. Anterior face of petiolar node covered with short appressed or decumbent hairs; posterior face without hairs. Entire gaster with sparse fine appressed pubescence and sparser long erect setae. Legs covered with fine but dense yellowish pubescent hairs; anterior and posterior faces of procoxa and ventral faces of pro- and meso-femora with sparse erect hairs; ventral face of basal segment of protarsus with dense yellowish and stiff bristle-like hairs.
Head and petiole orange-brown, mandible somewhat darker and more reddish brown; antenna, mesosoma and gaster uniformly dark reddish brown, with apical portion of gaster yellowish; legs including coxae lighter dull-yellowish brown to orange brown; tarsus darker brown than rest of leg, but colour obscured by thick yellowish pubescence.
Queen. Similar to worker in general appearance, except for characters of reproductive caste. Area around ocelli not swollen; distance between lateral ocelli slightly longer than that between lateral and median ocelli, and longer than major axis of median ocellus; with head in profile median ocellus protruding dorsad; with head seen in posterodorsal view, lateral ocelli directed laterad. Mandible slightly broader relative to head than in worker; masticatory margin strongly dentate with 12–14 denticles; dentition not equal between left and right mandibles. Mesosoma in dorsal view (Fig.
Head in full-face view extensively striate and similarly sculptured as in the worker, but with interspaces generally more weakly microsculptured than in the latter. Pronotum dorsally with dense transverse striae with shiny interspaces; lateral face densely and more irregularly striate, with interspaces punctate and weakly shiny; mesonotum longitudinally and rather regularly striate with interspaces punctate; “anepisternum” largely finely striate with posteroventral area smooth; oblique median furrow separating “anepisternum” from “katepisternum” weakly scrobiculate; “katepisternum” extensively minutely punctate and weakly shiny, posteriorly with coarse transverse rugae; scutoscutellar sulcus strongly scrobiculate; mesoscutellar disc largely smooth and shiny. Propodeum except for declivity transversely and regularly striate with interspaces punctate and weakly shiny; declivitous face with several widely spaced and strong transverse carinae. Anterior basal triangular area of petiolar node transversely striate with interspaces minutely punctate and weakly shiny; lateral face of node smooth and shiny except for median section with longitudinal rugae; entire petiolar spine and posterior face of node smooth to very superficially sculptured and shiny. Gaster largely smooth and shiny.
Head excluding mouthparts almost lacking erect setae; only ocellar region with pair of long erect setae; upper frons with pair of long erect hairs; entire dorsal, lateral and ventral faces of head with scattered minute standing hairs. Mandible covered with scattered fine appressed hairs; masticatory margin ventrally lined with multiple long yellowish setae. Dorsum of mesosoma with sparse short erect hairs; pronotum posteriorly bearing at least two long erect setae; propodeum with fine standing hairs mainly on dorsolateral and posterior margins but dorsum almost hairless. Petiolar spine, anterior and lateral faces of petiolar node with fine short decumbent or appressed hairs; anterior face slightly more pilose than lateral; posterior face of node without hairs. Entire gaster with scattered fine appressed pubescent hairs, and numerous but scattered long yellowish setae. Legs covered with fine but dense yellowish pubescence; ventral and lateral faces of procoxa with sparse suberect hairs; coxae and femora of mid- and hind-legs with fewer and shorter suberect hairs; ventral face of basal segment of protarsus with dense bristle-like and yellowish hairs.
Overall body colouration much as in worker. Head and petiole dark orange-brown, head sometimes more blackish in tone; mandibles slightly darker and more reddish brown; mesosoma and gaster dark reddish brown; coxae and femora yellowish to orange brown; rest of legs more reddish brown, increasingly darker towards tarsi.
Tegula distinct, elongate-ovate, much longer than broad. Wings slightly infuscate, entirely and finely setose. Forewing (Fig.
Male. Body smaller than workers and queens (HL 1.10–1.15, WL 3.5–3.75), rather robust. Head in full-face view (Fig.
Non-type male of O. litoralis (ZRC_ENT00007636.1, Singapore: Sungei Buloh Wetland Reserve). A Head in full face view B head in lateral view C head in dorsal view D mesosoma in lateral view E mesosoma in dorsal view F closeup of petiole in lateral view G gaster in lateral view H closeup of gastral apex in lateral view I forewing J hindwing.
Head extensively weakly and superficially sculptured, generally shiny; clypeus densely microsculptured and matte; mandible, antennal scape and pedicel superficially sculptured and shiny; entire flagellum densely microsculptured and matte. Mesosoma extensively weakly sculptured and shiny; mesoscutellum and narrow median section of metapleuron smoother and shiny; scutoscutellar sulcus longitudinally striate; shallow furrow separating metapleuron from propodeum with many strong transverse carinae across its length. Propodeum extensively very densely microsculptured, with anterior portion of dorsum strigate. Petiolar node extensively weakly sculptured, but smooth and shiny around apex. Gaster smooth to very superficially sculptured and shiny. Coxae nearly entirely smooth and shiny; femora weakly sculptured and shiny; tibiae slightly more strongly sculptured and less shiny; tarsi with dense microsculpture and matte.
Dorsum of head densely covered with short suberect hairs; longer hairs sparsely present around ocelli; ventral face of head covered with much shorter near-appressed hairs; mandible basally with some suberect hairs and apically a few longer hairs; scape with appressed hairs; pedicel and flagellum densely covered with short pubescence. Mesosoma and petiole almost entirely covered with short appressed to suberect or erect hairs; legs entirely covered with appressed pubescence. Gastral tergites and sternites with appressed to decumbent short hairs and very sparse longer hairs that are slightly denser near the posterior margin of each tergite.
Almost entire body light dull-yellowish brown; areas around ocelli and pronotum somewhat darker; areas where notauli occur present as pale-pigmented bands’; tibiae and tarsi darker than coxae and femora.
Tegula distinct, suboval, longer than broad. Wings (Fig.
Posterior spine of abdominal tergite VIII rather short and robust in lateral view (Fig.
Genitalia of O. litoralis male, non-type (ZRC_ENT00007636.1, Singapore: Sungei Buloh Wetland Reserve). A Pygostyle in dorsal view B abdominal sternite IX in ventral view C paramere and volsella, right side, inner view D penisvalva, left side, outer view. Abbreviations: (for B, C) Cu – cuspis; Di – digitus; Tm – telomere; Bm – basimere; BmC – carina of lower basimere; Sp – spiculum; Vo – volsella. (for D) ADL – apicodorsal lobe; AP – apicoventral process; AVP – anteroventral process; DS – diagonal sclerotisation; SAL – subapical lamina; Vc – valviceps; Vu – valvura.
Singapore, Borneo (Sarawak).
This species inhabits the mangroves, and nests have been found in abandoned mud lobster (Thalassina) mounds located near back forest. It has also been found foraging near mudflats and in mangrove backforest.
The species name alludes to the littoral intertidal zone where this species is exclusively found.
The worker of the new species O. litoralis (OL-w) is similar to O. malignus workers from Singapore (OMSG-w) and the Philippines (OMPH-w), but has consistently stronger body sculpture and generally darker body colour than the latter. Some important morphological differences are as follows: 1) Propodeal junction in OL-w is strongly angulate, with the dorsum separated from the declivity by a strong transverse carina, whereas in OMSG-w and OMPH-w the propodeal junction is round with a much weaker carinate edge. 2) Propodeal declivity in OL-w is distinctly marginate laterally with raised ridges and clearly differentiated from the lateral faces of propodeum, while in OMSG-w and OMPH-w it is not distinctly margined laterally, instead rounding into the lateral faces. 3) Metapleuron in OL-w is delineated from propodeum by a shallow but broad longitudinal furrow spanning between the basalar lobe and propodeal spiracle, while in OMSG-w and OMPH-w the furrow is very weak or not recognised, thus the metapleuron is not clearly distinguished from propodeum. 4) Sculpture on the lateral face of pronotum in OL-w is always relatively coarser than that on the dorsal face and less shiny, while in OMSG-w and OMPH-w, sculpture on the lateral face of pronotum is either similar to or weaker than that on the dorsal face, with interspaces mostly smooth and shining. 5) Mandible in OL-w is longer (albeit slightly) relative to head length as compared to that of OMSG-w and OMPH-w (i.e., OL-w MDI 65–70, OMSG-/OMPH-w MDI 63–64). 6) Mesosoma is uniformly dark reddish brown in OL-w, but the pronotum has a lighter shade of brown compared to the rest of the mesosoma in OMSG-w and OMPH-w. 7) In OL-w the anterior and lateral faces of petiolar node are generally more matte with stronger superficial sculpture, while in OMSG-w and OMPH-w these are mostly smooth and shining, with only few rugae in the median section of the lateral face. Condition of sculpture on the lateral petiolar face, however, may vary between different geographic populations of the same species, for example OL-w from Sarawak has a smoother and shinier lateral petiolar face compared to OL-w from Singapore.
Males of the two species (i.e., OL-m, OMSG-m) are also morphologically similar to each other, but OL-m has consistently stronger body sculpture than OMSG-m. Males of the two species can be distinguished based on the following characters: 1) Mandible is subrectangular or quadrate in OL-m, while it is falcate and apically bluntly pointed in OMSG-m. 2) In OL-m, furrow separating the metapleuron from propodeum is broad and deep, lined with transverse carinae along its length, whereas in OMSG-m the furrow is shallow, though also lined with transverse carinae along its length. 3) Petiolar node in anterior view is apically broadly rounded in OL-m, but sharply or bluntly pointed in OMSG-m. 4) Petiolar node in OL-m is largely microsculptured and matte, with only area around the apex smooth, while in OMSG-m, the petiolar node is almost entirely smooth to superficially sculptured and shiny. 5) Maximum diameter of lateral ocellus in OL-m is slightly shorter than the minimum distance between lateral ocellus and eye, whereas it is equal to or slightly longer than the same distance for OMSG-m.
In addition, male genitalia of the two species may be differentiated using the following traits: 1) Basal disc of abdominal sternite IX is subpentagonal with broadly rounded posterolateral corners in OL-m, but with angular posterolateral corners in OMSG-m. 2) Posterior lobe of abdominal sternite IX is almost as long as basal disc, slightly tapering apicad with almost truncated apex, apical corners rounded but angulate in OL-m, but a little longer than disc, with almost parallel sides, and apex broadly and weakly convex in OMSG-m. 3) Anterodorsal margin of valviceps is strongly produced in OL-m, but weakly produced in OMSG-m. 4) Ventral margin of valviceps is broadly concave with around 23 denticles in OL-m, but undulate with ca. 33 denticles in OMSG-m.
Odontomachus malignus
Smith, 1859: 144.
Odontomachus tuberculatus
Roger, 1861: 28 (syn.
Odontomachus malignus var. retrolatior
Viehmeyer, 1914: 113 (syn.
SINGAPORE: 1 worker, Lim Chu Kang mangrove, 23 Sep (23/9-1), D.H. Murphy leg., ZRC_HYM0000902 (
Worker. With features mentioned for the O. malignus species group. Body sculpture much weaker and coloration somewhat paler than in O. litoralis. Head in full-face view with posterior margin weakly concave; median furrow deep and rather broad; head extensively and finely (often indistinctly) striate, frontal lobes and frons with stronger striae; frontal carinae short and only slightly divergent posterad. Sculpture on pronotum much weaker than in O. litoralis; separation of metapleuron from propodeum indistinct; propodeal junction not strongly angulate, showing rather round transition from dorsum to declivity, declivity laterally only weakly marginate. Petiolar node extensively smooth and shiny. Head and petiole largely dark orange-brown; mandible, antenna and gaster slightly darker brown; mesosoma disc dark reddish brown.
Male. Body relatively smaller than that of worker, body sculpture not consistently strong. Mandible falcate, apically bluntly pointed. Furrow separating metapleuron from propodeum shallow; petiolar node in anterior view with sharply or bluntly pointed apex, almost entirely smooth to superficially sculptured and shiny. Maximum diameter of lateral ocellus equal to or slightly longer than minimum distance between lateral ocellus and eye. Body almost entirely light yellowish brown with darker greyish blotches present on head and mesosomal dorsum. Basal disc of abdominal sternite IX subpentagonal with angular posterolateral corners; anterodorsal margin of valviceps weakly produced; dorsolateral carina of valviceps absent; ventral margin of valviceps undulate with ca. 33 denticles.
Worker measurements. Holotype (CASENT0901334): EL 0.55; EW 0.45; HL 2.80; HW 2.30; IFLW 0.71; MDL 1.71; PTH 1.15; PTL 0.70; SL 2.86; WL 4.09; CI 82; MDI 59; PTHI 164; SI 119. Non-types (N = 6; values of PTH, PTL, SL, WL, PTHI, SI obtained from 5 out of 6 specimens – previous syntype of O. tuberculatus excluded from measurement due to extensive damage): EL 0.46–0.60; EW 0.29–0.40; HL 2.68–3.31; HW 2.15–2.73; IFLW 0.65–0.81; MDL 1.57–1.78; PTH 1.05–1.25; PTL 0.64–0.80; SL 2.80–3.08; WL 3.99–4.33; CI 80–82; MDI 54–64; PTHI 156–164; SI 127–130.
Male measurements. 3 non-types (N = 3): EL 0.95–0.96; EW 0.55–0.58; FWL 7.20–7.60; HL 1.30–1.36; HW 1.68–1.75; OL 0.27–0.28; PTH 0.88–0.96; PTL 0.70–0.75; SL 0.25 ; WL 3.51–3.65; CI 128–129; PTHI 123–128; SI 14–15.
(based on holotype, Singapore and Philippine specimens). Relatively large compared to male (HL 2.70–2.80; WL 3.95–4.00). Head in full face view (Figs
Head in full face view densely striate in most parts; frontal lobes and frons with strong striae that are longitudinal to diverging, interspaces mostly smooth and shining; anterolateral area of antennal fossa smooth and shiny; vertex with much finer striae that are often indistinct; vertex lobe, temple and lower gena with faint striation, partly micropunctate, slightly shiny; median disc of clypeus largely sculptured superficially and shiny. Dorsum and venter of mandible finely and superficially sculptured and rather shiny, lateral face smooth and shiny. Pronotum in dorsal view densely micropunctate to microreticulate with weak lustre, in lateral view irregularly striate in its anterior and posterior portions; anteromedian lobe (neck) with coarse strigae; mesonotum in dorsal view coarsely strigate with interspaces weakly microsculptured; mesopleuron densely sculptured and matte in its anterodorsal portion (upper 1/5), finely punctate but rather shiny in remaining portions, with anteriormost and posteriormost parts striate; metapleuron with sculpture similar to that of mesonotum, but with interspaces distinctly punctate. Propodeal dorsum with strigae similar to those on mesonotum; propodeal declivity with strong transverse striae that are more widely separated from each other than on propodeal dorsum. Anterior face of petiolar node mostly smooth and shiny, basally superficially sculptured with weak strigae, lateral face largely smooth and shining with median section bearing short striae, posterior face smooth and shining. Gaster mostly smooth and shining.
Head largely covered with sparse short pubescent hairs; frons posteriorly with a pair of long erect setae; mandible with scattered appressed pubescence, masticatory margin ventrally lined with multiple long yellowish setae. Dorsum of mesosoma with sparse appressed or decumbent pubescent hairs, pronotal disc with a few sparse long erect hairs. Anterior face of petiolar node with short appressed or decumbent pubescent hairs, relatively more pilose than mesosoma; posterior face of node without hairs. Entire gaster with sparse fine appressed pubescence and scattered long erect setae. Legs including coxae largely with fine but dense pubescence; ventral faces of coxae with a few sparse long erect hairs; ventral face of basal segment of protarsus with dense yellowish and stiff bristle-like hairs.
Head and petiole dark orange-brown; mandible, antenna and gaster slightly darker brown, mesosoma generally darker reddish brown; legs including coxae lighter brown and more yellowish; tarsus darker brown than rest of leg.
Male (based on Singapore specimens). Body smaller than worker (HL 1.30–1.36; WL 3.51–3.65), relatively robust compared to worker. Head in full-face view including eyes much broader than long, with roundly convex posterior margin (Fig.
Non-type male of O. malignus (ZRC_BDP0014535, Singapore: Pulau Semakau, old mangrove forest). A Head in full face view B head in lateral view C head in dorsal view D mesosoma in lateral view E mesosoma in dorsal view F closeup of petiole in lateral view G gaster in lateral view H closeup of gastral apex in lateral view I forewing J hindwing.
Head extensively weakly and superficially sculptured, generally shiny; clypeus largely densely microsculptured and matte, supraclypeal area with weaker sculpture and more shiny; mandible, antennal scape and pedicel superficially sculptured and shiny; entire flagellum densely microsculptured and matte. Mesosoma extensively weakly sculptured and shiny; “katepisternum” smoother and shiny in its anterior half; mesoscutellum largely smooth and shiny; median section of metapleuron striate, or relatively smoother than other parts of metapleuron and shiny (variable among individuals); scutoscutellar sulcus longitudinally striate and shiny; shallow furrow delineating metapleuron from propodeum with many strong transverse carinae across its length. Propodeum extensively densely microsculptured, with anterior portion of dorsum strigate, dorsolateral area lined with coarse rugae. Petiolar node largely smooth and shiny. Gaster smooth to very superficially sculptured and shiny. Coxae and femora nearly entirely smooth and shiny; tibiae weakly sculptured and less shiny; tarsi densely microsculptured and weakly shining.
Dorsum of head densely covered with short suberect and erect hairs; longer standing hairs present around ocelli; clypeus and lower frons with longer hairs; ventral face of head densely covered with much shorter standing hairs; ventral edge of mandible with some long erect hairs; scape covered with short decumbent hairs, with slightly longer hairs apically; pedicel and flagellum covered with dense and short pubescence. Mesosoma and petiole almost entirely covered with short appressed to suberect or erect hairs, with a few sparse long erect hairs on pronotum; legs mostly covered with dense appressed pubescence. Gastral tergites and sternites with appressed to decumbent short hairs, and very sparse longer hairs that are largely patchily distributed but more dense nearer posterior margin of each tergite; hair covering sternites generally more dense than that on tergites.
Body almost entirely light yellowish brown; darker greyish blotches present on head and mesosomal dorsum; areas where notauli occur present as pale-pigmented bands; coxae and femora uniformly pale yellowish brown, tibiae less pale, tarsus darker brown than rest of leg, though actual colour often obscured by dense yellowish pubescence.
Tegula distinct, suboval, longer than broad. Wings not infuscate, entirely and finely setose. Forewing (Fig.
Posterior spine of abdominal tergite VIII rather long and slender in lateral view (Fig.
Genitalia of O. malignus male, non-type (ZRC_BDP0014515, Singapore: Pulau Semakau, old mangrove forest). A Pygostyle in dorsal view B abdominal sternite IX in ventral view C paramere and volsella, right side, inner view D penisvalva, left side, outer view. Abbreviations: (for B, C) Cu – cuspis; Di – digitus; Tm – telomere; Bm – basimere; BmC – carina of lower basimere; Sp – spiculum; Vo – volsella. (for D) ADL – apicodorsal lobe; AP – apicoventral process; AVP – anteroventral process; DS – diagonal sclerotisation; SAL – subapical lamina; Vc – valviceps; Vu – valvura.
Asia – Borneo (Sarawak), Indonesia (Maluku, Sulawesi (Celebes)), Philippines, Singapore; Oceania – Palau, Papua New Guinea, Solomon Islands.
Intertidal littoral areas; workers of this species have been observed foraging in and around or emerging from exposed nest entrances in coral rubble (
The workers of O. malignus from Singapore (OMSG-w) and the Philippines (OMPH-w) are similar to that of O. litoralis (OL-w), but may be distinguished from the latter by the characters listed in ‘Remarks’ under O. litoralis. Minor but non-negligible morphological differences between the O. malignus holotype (OMH), synonymised forms (e.g., O. retrolatior (OR-w)), OMSG-w and OMPH-w are discussed in the ‘Results’ section.
1 | Vertex posteriorly with protuberance on each side of median furrow. Mesopleuron with anteroventral margin that is strongly developed and looks like a tubercle when seen in dorsal view O. malignus group | 2 |
– | Vertex without such protuberance. Anteroventral margin of mesopleuron less developed, not like tubercle when seen in dorsal view | 3 |
2 | Propodeal junction strongly angulate, with dorsum separated from declivity by strong transverse carina. Mesosoma uniformly dark reddish brown. Body sculpture stronger; lateral face of pronotum coarser than that on dorsal face and less shiny. [Singapore, Borneo] | O. litoralis sp. nov. |
– | Propodeal junction rounded, with more weakly carinate edge. Pronotum paler than rest of mesosoma. Body sculpture weaker; sculpture on lateral face of pronotum similar to or weaker than that on dorsal face, and interspaces mostly smooth and shiny. [Singapore, Borneo, Lesser Sundas to Oceania] | O. malignus Smith, F., 1859 |
3 | First gastral tergite entirely micropunctate, covered with dense, short appressed hairs, without flattened area and pit. Dorsum of head and almost entire mesosoma with dense erect or suberect hairs that are very short and of almost same length. Pronotal dorsum without long erect hairs. [Vietnam] | O. silvestrii Wheeler, 1927 |
– | First gastral tergite essentially smooth and shiny (punctation, if any, very faint and sparse), anteriorly extensively flattened and often with pit corresponding to petiolar spine. Condition of short hairs on head and mesosoma variable, but hairs may be longer, appressed, curved, or much sparser depending on species. Pronotal dorsum occasionally with one or two pairs of long hairs. O. infandus group (s. str.) | 4 |
4 | Head orange-yellow to yellowish brown contrasting with dark brown to blackish mesosoma. | 5 |
– | Body generally concolourous, but sometimes head, especially lateral faces, lighter than rest of body | 6 |
5 | Gaster dark brown. Mesosoma with dense decumbent to appressed hairs that are relatively long (generally much longer than space between hairs). Anterior face of petiole with similar hairs that are often suberect. Dorsum of head posteriorly with superficial sculpture and somewhat shiny. Lateral face and basal part of anterior face of petiole with distinct striae | O. banksi Forel, 1910 |
– | Gaster light brown, similar to head in coloration. Hairs on mesosoma shorter (generally as long as or shorter than space between hairs). Hairs on anterior face of petiole very fine, appressed and less conspicuous. Dorsum of head posteriorly distinctly striate and matte. Petiole almost entirely smooth. | O. alius Sorger et Zettel, 2011 |
6 | Dorsum of head behind ocular prominence entirely sculptured and matte. Mesopleuron entirely striate | 7 |
– | Dorsum of head behind ocular prominence essentially smooth or very faintly sculptured and shiny. Mesopleuron extensively smooth and shiny | 8 |
7 | Striae on pronotal dorsum essentially longitudinal, rarely weakly concentric. Petiolar spine curved backward in lateral view | O. infandus Smith, F., 1858 |
– | Striae on pronotal dorsum more distinctly transverse. Petiolar spine almost straight in lateral view | O. schoedli Sorger & Zettel, 2011 |
8 | Body orangish light brown. Pronotal dorsum predominantly with regular transverse striae | O. ferminae General, 2018 |
– | Body reddish brown to dark brown. Pronotal dorsum in posterior 2/3 largely with longitudinal striae | 9 |
9 | With petiole in dorsal view spiracle distinctly protruding laterad. Entire extraocular furrow essentially smooth with very sparse minute punctures. With mesosoma in profile, dorsal outline weakly and broadly concave | O. scifictus Sorger & Zettel, 2011 |
– | With petiole in dorsal view spiracle not protruding laterad. Extraocular furrow predominantly smooth but with superficial striae at least partly. With mesosoma in profile, dorsal outline more straight | O. philippinus Emery, 1893 |
It has long been assumed that only one species of Odontomachus trap-jaw ants, O. malignus, dominated tropical littoral habitats throughout Oceania and Southeast Asia. The discovery and validation of the new species, O. litoralis, existing in sympatry with O. malignus not only overrides this assumption, but also begs the more general question of whether we should re-assess the common notion of harsh intertidal habitats being poor in terms of diversity of terrestrial species.
For morphology, we found both species of the littoral trap-jaw ants to be similar to each other but with evident differences in structure and sculpture, especially for male bodies and genitalia between the two species. It is important to note that examined male specimens of the two species were collected from (near) sympatric populations in the small island city-state of Singapore, which has a total land area no more than 725 square kilometres (
A notable shared feature in male genitalia of the two littoral Odontomachus species is the absence of a dorsolateral carina curving ventrally near the apex of the valviceps and producing the subapical lamina along the lateral apodeme; this carina is present in and deemed to be apparently unique to Nearctic and Oriental Odontomachus males (
The exact nesting habits of O. malignus remain largely unknown; past and present records indicative of its habitat type have been based on individual foraging workers and males caught in malaise traps. Limited anecdotal accounts and observations of O. malignus workers emerging from apparent nest entrances exposed during low tide suggest that the species possibly nests in or around coral rubble (
A Distribution of Odontomachus malignus and O. litoralis in Singapore B map of intertidal and terrestrial habitat types in Pulau Semakau (by Feng Yikang, for RMBR [Raffles Museum of Biodiversity Research]) and locations where O. malignus males were collected (indicated by red-filled circles).
In Singapore, O. litoralis nest series were sampled from northern and northeastern mangrove areas (Fig.
Species of the O. malignus species group are known to be distributed throughout the Oriental and Oceanian realms sensu
In contrast, O. litoralis populations have only been found in Singapore and East Malaysia (Sarawak, Borneo) (Fig.
We thank James Hogan (Oxford University Museum of Natural History) for the O. malignus holotype loan, and Herbert Zettel (Naturhistorisches Museum Wien) for kindly providing additional important specimens of the O. infandus group. We are also grateful to Milan Janda (Universidad Nacional Autónoma de México) and Magdalena Sorger (North Carolina State University) for providing additional specimens collected from outside Singapore, and also collection information on relevant specimens from which DNA sequences were obtained. We thank Pavel Matos-Maraví (Institute of Entomology ASCR) for initial discussion and kind assistance with tracking down the specimens. We also thank Rudolf Meier (National University of Singapore) for allowing use of lab resources for DNA barcoding and Foo Maosheng (Lee Kong Chian Natural History Museum) for help with fieldwork. Finally, we thank the reviewers for their comments and suggestions that helped improve the manuscript. Specimens required for this study were collected with permissions granted by the National Parks Board (Singapore), permit numbers: NP/RP12-022, NP/RP18-023.