Tetramorium polymorphum

This is a first case of worker polymorphism among the Tetramorium although geographical variation in worker size has been reported in the European species Tetramorium moravicum (Schlick-Steiner et al., 2005). Tetramorium polymorphum is known only from the type locality in Laos, and Chacheongsao Provo and Nakornratchasima Prov., Thailand.

Identification
Yamane and Jaitrong (2011) - Puncto-reticulation on dorsum of head restricted to posterior 3/4 of its surface; anterior portion of frons with longitudinal rugulae only. Antennal scrobe strongly developed. Clypeus with a median and a few lateral longitudinal carinae; anterior margin medially emarginate. Propodeal declivity smooth and shiny; propodeal spine acute, longer than propodeal lobe. Petiolar node in dorsal view broader than long (much broader in larger specimens); with petiole in profile tergal portion of the node higher than long. Gastral tergite 1 entirely smooth and shining. Dorsum of body densely covered with branched hairs; hairs on gastral tergite 1 universally trifid, forming a dense pelt over surface in the minor worker, but standing hairs on dorsum of body partly or entirely simple in the media and major workers.

The workers from the type series are roughly grouped into 3 size classes, which are distinguished in head width as follows: minor workers, 0.58-0.75 mm; media workers, 0.80-0.90 mm; major workers, 1.03-1.10 mm. The minor and media workers are somewhat continuous in all the measurements, while the major workers are clearly separable from the others.

The queen is very large, has a similar set of characteristics to that of the worker. However, all the body hairs are simple, and the petiole and postpetiole are relatively shorter and higher compared with the worker.

The workers of Tetramorium polymorphum from the single colony are roughly sorted into three size classes. Although these classes are not very distinctive in structural characters, the standing hairs on the body are mostly simple in the major worker. The cephalic index and the relative length of the antennal scape also change with body size. Also in the queen all the standing hairs are simple. The queen of T. hasinae has abundant branched hairs over body surface as in the worker. The body size of the T. polymorphum queen is also exceptionally large among the Asian Tetramorium species.

This species is similar to two other species and the three can be separated using the following key:

1

 * Anterior clypeal margin weakly concave. Clypeus with one median and a few lateral carinae running longitudinally. Anterior portion of frons just behind clypeus not reticulate, with longitudinal rugulae only . . . . . Tetramorium polymorphum


 * Anterior clypeal margin straight or feebly convex. Clypeus with one longitudinal median carina and a few shorter and obliquely running carinae, or entirely reticulate. Dorsum of head almost entirely puncto-reticulate . . . . . 2

2

 * Clypeus with carinae, not reticulate. Propodeal declivity extensively smooth and shining, with a strong transverse carina between propodeal spine, and a weaker carina below it . . . . . Tetramorium kheperra


 * Clypeus reticulate. Upper half of propodeal declivity with several fine transverse carinae and mat . . . . . Tetramorium hasinae

Distribution based on Regional Taxon Lists
Oriental Region: Laos, Thailand. Palaearctic Region: China.

Biology
Yamane and Jaitrong (2011) - The information from the type material, a collection from a colony, suggests that this species inhabits the evergreen forest and prefers more natural conditions than Tetramorium kheperra. The Tetramorium polymorphum colony was found in June 2010 nesting under a relatively large stone in a dry evergreen forest located at ca. 300 malt., in Phang Dang Village, Pak-Gnam District, Vientiane Province, Laos. Most of the ants were found in chambers and galleries dug just below the stone but the nest was spread in shallow soil. Nothing is known of the function of the major workers in Tetramorium polymorphum. Judging from the aggressive behaviour of this species as shown during the collecting, the majors possibly playa defensive role.

Nomenclature

 *  polymorphum. Tetramorium polymorphum Yamane & Jaitrong, 2011: 68, figs. 2D-F, 4A-F (w.) LAOS.

Worker
Minor, Holotype. HL 0.67, HW 0.66, CI 99, SL 0.44, SI 67, ML 0.36, ED 0.14, PW 0.48, MSL 0.66, PL 0.26, PH 0.25, DPW 0.24.

Minor Head almost as long as broad, in full-face view with weakly convex sides, broadly rounded posterolateral comers and almost straight posterior margin. Disc of clypeus in profile weakly convex with a steep slope anteriorly; with head in full-face view clypeus with an anteromedian indentation; anterior margin of clypeus with a narrow transverse plate-like fringe that is situated below the level of the disk and weakly concave medially. Frontal lobe weakly developed; frontal carinae strongly divergent, extending well beyond the level of posterior margin of eye; antennal scrobe distinct, strongly margined dorsally by the frontal carina but ill defined ventrally. Eye moderate in size, composed of ca. 30 ommatidia, distinctly narrowed anteriorly, located anterior to the mid-length of head; distance between mandibular base and anterior margin of eye slightly longer than maximum eye length. Mandible subtriangular, with a protruding apical tooth and a smaller preapical tooth; remaining portion of masticatory margin with a few indistinct denticles or almost edentate. Antenna 12-segmented; scape relatively short, not reaching posterolateral corner of head, slightly broadened in distal two-thirds; second segment slightly longer than broad; third to eighth segments each much broader than long and shorter than the second; antennal club 3-segmented, longer than the remaining portion of funiculus; apical segment slightly longer than tenth and eleventh combined. Mesosoma short and robust, seen from above distinctly broader anteriorly than posteriorly, with broadly rounded anterolateral corners, without any suture separating pronotum, mesonotum and propodeum from one another; in profile Mesosoma with evenly convex dorsal outline. Propodeal spines as long as tenth antennal segment; propodeal declivity short, concave, separated from dorsum by a distinct transverse carina and laterally walled by rather developed propodeal lobes, which is roundly convex posteriad. Petiole with a short peduncle; its node seen from above distinctly broader than long, seen in profile higher than long and with anterior and posterior slopes steep and almost parallel; postpetiole without peduncle; its node similar to petiolar node in size and shape, but slightly shorter and lower, with indistinct anterior and posterior slopes. First gastral tergite slightly longer than wide, occupying 4/5 of the whole dorsum of gaster, with anterolateral comer rounded.

Head dorsally longitudinally rugulose but not very regularly; dorsum of head in anterior portion, particularly its central portion, without cross meshes, but posterior 3/4 with cross-meshes in addition to rugulae to show irregular reticulation; lateral face of head more regularly puncto-reticulate. Clypeus with a median longitudinal carina as well as a few lateral ones. Antennal scrobe superficially sculptured, often with a distinct median carina and irregular shorter carinae. Mandible densely striate over the surface. Dorsum of mesosoma, lateral face of pronotum and upper mesopleuron densely and coarsely punctate; lower mesopleuron much more minutely punctate, in addition to them sometimes with weak irregular carinae; metapleuron and lateral face of propodeum coarsely reticulate; propodeal declivity extensively smooth and shiny. Nodes of petiole and postpetiole almost entirely punctate; dorsum of petiolar peduncle smooth to superficially sculptured; side of the peduncle anteriorly densely sculptured and somewhat mat. Gastral tergites smooth and shining. Fore coxa with dense punctation rather than striation. Standing hairs on dorsum of head simple or branched; frequency of branched hairs higher near posterior margin; on the venter of head most of hairs decumbent. Hairs on dorsum of mesosoma longer than those on head, rather uniform in length, and mostly branched (trifid); lateral face of mesosoma with much shorter and indistinct hairs. Dorsa of petiole and postpetiole, and gastral tergites and sternites with dense trifid standing hairs; peduncle and venter of petiole without standing hairs; venter of postpetiole with a few standing hairs. All legs covered with numerous standing or suberect hairs. Body brown to reddish brown; antenna slightly darker; gaster dark reddish brown. Apical segment of antenna often and legs light brown to yellowish.

Variation. In the media and major workers head broader than long; antennal scape relatively shorter than in the minor worker; petiolar node seen from above much broader than long. In the major, clypeus often with obliquely running carinae, and eye composed of more than 50 ommatidia. In the media many of the standing hairs on the dorsum of body simple (shorter hairs tending to be branched), and in the major almost all the standing hairs on the dorsum of body simple (a few short hairs on head and mesosoma may be branched).

Queen
(paratype). HL 1.05, HW 1.10, CI 105, SL 0.68, SI 62, ML 0.55, ED 0.32, PW 1.25, AL 1.77, PL 0.66, PH 0.50, DPW 0.53.

Very distinctive in its large body. In structure, sculpture and pilosity similar to the major worker, with the following differences: Eye large, distinctly longer than and broader than apical antennal segment; distance between eye and mandibular base only slightly longer than half the length of eye; pro-mesonotal suture distinct; mesothoracic paraptera broadly convex anteriorly and posteriorly, separated from scutellum by a shallow but wide suture and from mesoscutum by a narrower suture; an episternum separated from katepisternum by a wide furrow provided with a series of carinae on bottom; metanotum very short, separated from meso scutellum and propodeum by deep sutures; propodeal dorsum sloping, not separable from declivity; petiole and postpetiole much broader than long; their nodes seen in profile distinctly higher than long. Median furrow on the dorsum of head indistinct, incomplete, not reaching the posterior margin of head; anterior portion of frons medially with rugae only, but reticulate laterally; mesonotum in posterior 2/3 rugulose rather than puncto-reticulate; anepisternum irregularly sculptured; katepisternum extensively smooth to superficially sculptured, shining; propodeal dorsum weakly and longitudinally rugulose; declivity with several transverse carinae and weakly shining. All the standing hairs on the body simple. Body uniformly dark reddish brown; coxae and femora slightly paler.

Type Material
Holotype: Worker, Phang Dang Village, Pak-Gnam District, Vientiane Province, Laos, Sk. Yamane leg., deposited in. Paratypes: one dealated queen and 60 workers (40 minors, 7 medias and 13 majors) collected by Sk. Yamane and W. Jaitrong from the same colony to which the holotype belongs (LAIO-SKY-IOO & WJTlO-LAO-1l4), deposited in, , SKYC, , Museum Zoologicum Bogoriense (Cibinong, Indonesia), and Bomeensis at Malaysia Sabah University.

References based on Global Ant Biodiversity Informatics

 * Jaitrong W., B. Guenard, E. P. Economo, N. Buddhakala, and S. Yamane. 2016. A checklist of known ant species of Laos (Hymenoptera: Formicidae). Asian Myrmecology 8: 1-32. DOI: 10.20362/am.008019
 * Liu C, B. Guénard, F Hita Garcia, S. Yamane, B. Blanchard, and E. Economo. New records of ant species from Yunnan, China. Submitted to Zookeys
 * Yamane S.; Jaitrong W. 2011. A first species of Tetramorium (Hymenoptera, Formicidae, Myrmicinae) with a polymorphic worker caste. Insecta Matsumurana (n.s.)67:61-74.