Odontomachus rixosus

A widely distributed forest species.

Identification
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, Indonesia, Malaysia, Philippines, Singapore. Oriental Region: Bangladesh, Cambodia, India, Myanmar, Thailand. Palaearctic Region: China.

Biology
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. (Sorger & Zettel 2011)

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.

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.