Odontomachus rixosus

Odontomachus rixosus inhabits plantations, secondary and primary forests in lowlands, and nests under leaf litter, in the soil near the base of living trees, and under rotten logs and stumps. It is a A widely distributed forest species. This species has been collected nesting together with Pheidole tandjongensis. Details concerning the nature of the relationship between O. rixosus and Pheidole spp. are unknown. (Satria et al. 2015, Sorger & Zettel 2011)

Identification
A member of the Odontomachus rixosus group.

Satria et al. (2015) - In the syntypes of O. rixosus and O. rixosus conifer the pronotal disc is finely striate transversely, but finely concentric in O. rixosus obscurior.

Odontomachus rixosus is very similar in general appearance to Odontomachus pararixosus, but is distinguishable from the latter by the weakly notched posterior head margin (almost straight in the latter); the apical spine of the petiolar node is needle-shaped (acutely triangular in the latter); and the first gastral tergite lacks long suberect setae (present in the latter). The delimitation between O. rixosus and Odontomachus minangkabau is discussed under O. minangkabau.

Sorger & Zettel (2011) - There is considerable variation in O. rixosus, especially in size and colour, but it seems unconnected with certain geographical populations. This includes the var. obscurior, described from Myanmar and Thailand (Forel 1900), and later recorded from Palawan by Stitz (1925). In the Philippines, O. rixosus can be recognized by group characters, especially by the elongate and truncate subapical tooth of the mandible.

Distribution
Sorger & Zettel (2011) - Southeast Asian mainland (Myanmar, Thailand, Peninsular Malaysia, Singapore), Sumatra, Mentawai Islands, Java, Borneo, Philippines.

Distribution based on Regional Taxon Lists
Indo-Australian Region: Borneo, Brunei Darussalam, Indonesia, Malaysia, Philippines, Singapore. Oriental Region: Bangladesh, Cambodia, India, Laos, Myanmar, Thailand, Vietnam. Palaearctic Region: China.

Biology
Polygynous (Ito et al. 1996) with multiple dealate queens.

Sorger & Zettel (2011) - The only encounter in the Philippines by the senior author was at a waterfall area in a slightly degraded dipterocarp forest. Observations from Borneo confirm that O. rixosus is a forest species.

This ant is parasitized by the wasp Schizaspidia nasua (Walker).

Nomenclature

 *  rixosus. Odontomachus rixosus Smith, F. 1857a: 64 (w.) SINGAPORE. Crawley, 1924: 388 (q.); Wheeler, G.C. & Wheeler, J. 1952c: 651 (l.); Imai, Brown, et al 1984: 67 (k.). Senior synonym of conifera, obscurior: Brown, 1976a: 105. See also: Brown, 1976a: 163; Sorger & Zettel, 2011: 157.
 * obscurior. Odontomachus rixosus var. obscurior Forel, 1900c: 58 (w.) MYANMAR. Viehmeyer, 1916a: 116 (q.). Junior synonym of rixosus: Brown, 1976a: 105.
 * conifera. Odontomachus rixosus var. conifera Forel, 1913k: 19 (w.q.) INDONESIA (Java). Junior synonym of rixosus: Brown, 1976a: 105.

Worker
Sorger & Zettel (2011) - Worker with smallest HW: CI 71, HL 2.38, HW 1.68, MdI 57, MdL 1.37, MsL 3.33, PnW 1.05, PtH 0.78, PtL 0.77, PtW 0.42, SI 145, SL 2.43, TL 10.31; worker with largest HW: CI 76, HL 2.57, HW 1.95, MdI 56, MdL 1.43, MsL 3.62, PnW 1.17, PtH 0.84, PtL 0.92, PtW 0.48, SI 131, SL 2.63, TL 12.94.

Structures: Mandibles long, with ca. 6 basal denticles (widely separated from each) and three apical teeth: proximate tooth truncated, intercalary tooth only slightly shorter than apical. Head rectangular, broadest at level of eyes. Striation on head until ocular ridge, some more striation between ocular and temporal ridge, rest of head smooth and shiny. Microsculpture on head with fine isodiametric reticulum. Mesosoma elongate, slender and low, broadest at level of pronotum. Pronotum rounded, metanotal groove in lateral view present. Coarse rounded sculpture on pronotum (closed circles visible in dorsal view), metanotum and propodeum with coarse transverse sculpture. Petiole short, smooth and shiny, almost conical, with very short petiolar spine, which is rarely absent in small specimens (for example, O. rixosus var. conifera Forel, 1913).

Pilosity: Fine loose semi-appressed white pubescence on head, mesosoma and petiole; distance between hairs approximately their length. Head with two standing setae, tergite 1 without setae, tergite 2 with a few setae, number of setae and length increasing towards apex of abdomen.

Colour: Medium brown, mesosoma slightly darker than head, petiole and gaster.

Satria et al. (2015) - Syntype (n=1, gaster missing): HW 2.10 mm, HL 2.98 mm, SL 2.93 mm, IFLW 0.64 mm, EL 0.36 mm, MDL 1.64 mm, WL 4.00 mm, PTL 0.72 mm, PTH 1.21 mm, CI 70, SI 139, MDI 55, PTHI 168.

Non-types (n=10): HW 1.77–2.19 mm, HL 2.56–3.03 mm, SL 2.56–3.03 mm, IFLW 0.45–0.56 mm, EL 0.31–0.42 mm, MDL 1.45–1.67 mm, WL 3.35–4.00 mm, PTL 0.57–0.69 mm, PTH 1.09–1.26 mm, CI 64–73, SI 131–150, MDI 55–57, PTHI 171–196.

Relatively small (HL 2.56–3.03 mm, WL 3.35–4.00 mm). Head in full-face view much longer than broad, with posterior margin weakly concave; median furrow on vertex present as dark line; each side of line not swollen; frontal lobes followed by strong frontal carinae which are nearly parallel; minimum distance between margin of ocular ridge and margin of compound eye less than half major axis of compound eye; mandible relatively slender; masticatory margin with 6–10 denticles (7 in a syntype of O. rixosus examined; 7 in a syntype of O. rixosus conifer [ANTWEB CASENT 0907432]; 6 in a syntype of O. rixosus obscurior [ANTWEB CASENT 0907431]); subapical tooth ca. 2.5 times as long as broad, with truncate apex; palp formula 4, 4. Mesosoma in lateral view relatively slender; pronotum including its anteromedian lobe long, in lateral view with anterodorsal slope gentle; mesopleuron without anteroventral ridge, with anterodorsal margin weakly carinate, clearly separated by distinct dorsal carina from mesonotum and metapleuron; propodeum in lateral view with dorsum almost straight and gradually sloping posteriad, and with posterior face steeply sloping; propodeal dorsum without median longitudinal depression. Petiolar node conical, with sharply pointed apical spine; node in lateral view, excluding apical spine with anterior and posterior faces weakly convex; apical spine needle-shaped, less than 1/4 as long as petiolar height, sometimes weakly curved posteriad (but shape variable within species); subpetiolar process anteroposteriorly as long as dorsoventrally high, lobate, directed ventrally. First gastral tergum in lateral view long, with anterior face relatively short and vertical.

Head in full-face view largely smooth and shiny, but with striate area bordered by frontal carinae and ocular ridges; frontal lobe with almost smooth and shiny; extraocular furrow faintly striate; median part of vertex along median furrow smooth and shiny or sometimes with rough texture; lateral face and venter of head smooth and shiny; median disc of clypeus smooth and shiny. Pronotum in dorsal view finely concentrically striate (but sculpture variable within species); mesonotum finely striate transversely; mesopleuron largely smooth and shiny, with anterior 1/3 and posteriormost parts finely striate; metapleuron and propodeum with transverse striation which is a little sparser and stronger than on pronotum and mesonotum. Petiolar node largely smooth and shiny, but with its basal area faintly striate.

Vertex with a pair of long erect setae; frontal lobe with erect seta; pronotal disc without long erect setae; 1st gastral tergum without erect setae. Head, mesosoma, petiole and gaster with fine appressed pubescence which is sparse and very short on head and petiole and is sparse and extremely short on gaster.

Body orange-brown, with darker mesosoma and gaster; leg yellowish brown to orange-brown, with coxae and femora paler.

Queen
Satria et al. (2015) - (n=10): HW 1.87–2.17 mm, HL 2.53–2.96 mm, SL 2.49–2.76 mm, IFLW 0.42–0.57 mm, EL 0.39–0.50 mm, OL 0.12–0.18 mm, MDL 1.45–1.63 mm, WL 3.50–4.05 mm, FWL 6.46–8.33 mm, PTL 0.65–0.70 mm, PTH 1.40–1.56 mm, CI 72–76, SI 124–136, MDI 52–58, PTHI 214–229.

In general appearance queen is similar to worker. Vertex near ocelli not swollen; ocular ridge clearly developed; distance between lateral ocelli shorter than or as long as distance between lateral and median ocelli, and as long as major axis of median ocellus; ocelli in lateral view protruded dorsad. Mesosoma with main sclerites associated with wing function, in dorsal view long and slender; anterodorsal slope of pronotum in lateral view gentle; anterodorsal outline of mesoscutum in lateral view relatively gentle; mesoscutum without posteromedian depression; parapsidal furrow very weak, and slightly curved; anterior third of mesopleuron with fine, oblique furrow (sometimes without any furrow); propodeum in lateral view long, with dorsum almost straight and gradually sloping posteriad. Wing venation as in Figs. 12E and 12F. Petiolar node in lateral view, excluding apical spine with anterior and posterior faces weakly convex; apical spine short and stout, and sometimes weakly curved posteriad; subpetiolar process anteroposteriorly as long as dorsoventrally high. First gastral tergum in lateral view long, with anterior face relatively short and vertical.

Head in full-face view largely smooth and shiny, but with a striate area bordered by frontal carinae and ocular ridges; frontal lobe almost smooth and shiny; extraocular furrow faintly striate; median part of vertex along median furrow smooth and shiny or sometimes with rough texture; lateral face and venter of head smooth and shiny; median disc of clypeus smooth and shiny. Pronotum densely and weakly striate transversely (often dorsolateral part smooth and shiny); mesoscutum faintly longitudinally striate (but sometime largely smooth and shiny, and faintly striate along posterior margin of mesoscutum); mesopleuron largely smooth and shiny, with anterior third and posteriormost part faintly striate; mesoscutellum smooth and shiny; propodeum strongly and sparsely striate transversely. Petiolar node largely smooth and shiny, with its basal area faintly striate.

Pair of long erect setae present on vertex near lateral ocelli; frontal lobe with a erect seta; pronotum and 1st gastral tergum without erect setae. Head, mesosoma, and petiole with sparse subdecumbent to decumbent pubescence which is extremely short in head and gaster; mesopleuron very sparsely pubescent.

Male
Satria et al. (2015) - (n=10): HW 1.19–1.35 mm, HL 1.04–1.14 mm, SL 0.20–0.26 mm, EL 0.70–0.84 mm, EW 0.37–0.46 mm, OL 0.16–0.22 mm, OES 0.16–0.18 mm, WL 2.63–2.93 mm, FWL 4.52–5.58 mm, PTL 0.56–0.64 mm, PTH 0.78–0.90 mm, CI 107–120, SI 15–20, PTHI 135–152.

Size small (HL 1.04–1.14 mm, WL 2.63–2.93 mm). Major axis of median ocellus as long as minimum distance between lateral ocelli; antenna 13-merous; scape very short; 1/3 as long as 3rd antennomere; 2nd antennomere 1/2 as long as scape; 3rd to 13th antennomeres each extremely long; palp formula 6, 4; dorsal outline of clypeus in lateral view weakly convex. Mesosoma in lateral view relatively slender and long; dorsal outline of pronotum in lateral view almost straight; anterodorsal outline of mesoscutum in lateral view gently sloping; mesoscutum without median depression; parapsidal furrow very weak and almost straight; oblique mesopleural furrow relatively shallow and wide; ventrolateral part of katepisternum with very weak longitudinal furrow; propodeum in lateral view with its dorsal outline roundly convex; metapleuron separated from propodeum indistinctly by suture; wing venation similar to queen (see Figs. 12E and 12F for queen wings). Petiolar node in lateral view tapering gently to bluntly pointed apex; its anterior slopes weakly convex, and posterior slope almost straight; subpetiolar process in lateral view anteroposteriorly shorter than dorsoventrally high, lobate and slender; petiolar sternum without posteroventral process. First gastral tergum in lateral view short; posterior spine of 8th abdominal tergum long and slender, very weakly curved (Fig. 18B); pygostyle with long setae in its apical third; disc of 9th abdominal sternum much broader than long, with posterolateral corner expanding laterad; apical lobe much longer than disc and gently tapering to almost truncate apex; telomeral apex in lateral view longer than high; distiventral apex of valviceps strongly produced; basiventral corner of valviceps distinctly produced; ventral margin of valviceps with 21−22 denticles.

Head including area between lateral ocelli entirely smooth and shiny. Pronotum in dorsal view smooth and shiny; mesoscutum smooth and shiny; scuto-scutellar suture with sparse, strong, longitudinal rugae; mesopleural anepisternum and katepisternum smooth and shiny; metapleuron smooth and shiny; propodeum in dorsal view with rough texture and shiny. Petiole smooth and shiny.

Head, mesosoma, legs, petiole and gaster with fine sparse subdecumbent to decumbent pubescence; mandible, vertex near ocelli, and gaster except 1st gastral tergum with long erect setae.

Males from Thailand are always darker than those from the other localities.

Type Material
Satria et al. (2015) - Syntype worker of Odontomachus rixosus: syntype (1 worker in ), Singapore [head detached and missing gaster]. Singapore.

Smith Types
Odontomachus rixosus

The specimen at labelled as “type” by Donisthorpe may not be so. One cannot be positive as there is no datum number on the disc of this specimen, which merely states “SING” (= Singapore). However, a specimen in, labelled “SING. 55/9” has the following information in Acc. Reg.: “1859 no. 9. Singapore (Malacca), purchased of Stevens. Collected by Mr A.R. Wallace.”

I suspect that this specimen may be the true holotype but have no way of conclusively proving it. The only evidence in support of this view is that in the original description of rixosus, Smith states that the thorax is “tranversely striated.” The specimen more or less fits this description, whereas the  specimen has weakly whorled, not transverse, striation on the pronotum.