Tetramorium isectum

Tetramoirum isectum does not seem to be as rare as might be expected from the original description. We were able to examine material from a number of localities, although the species is admittedly never very common or abundant. Tetramorium isectum is found in a strip in eastern Madagascar, which ranges from central-eastern to northeastern Madagascar. The southernmost localities are Ambatovy, Andasibe-Mantadia, and Sahafina, and the northernmost are Montagne d'Anjanaharibe and Amparihibe. The species prefers rainforests and montane rainforests at elevations of 125 to 1040 m, and was mostly collected from leaf litter.

Identification
A member of the Tetramorium tortuosum-species group / Key to Tetramorium tortuosum-group species

Hita Garcia and Fisher (2012) - Within the T. andrei complex T. isectum is distinguishable by the following characters: very small eyes (OI 15 - 16); long propodeal spines (PSLI 28 - 33); petiolar node with the anterodorsal margin slightly situated higher and more angular than the posterodorsal margin; posterodorsal corner of petiole not strongly protruding posteriorly; body of uniform bright orange colour.

Tetramorium isectum is morphologically fairly close to Tetramorium andrei, and during this revision we considered the synonymisation of the first under the latter. Both differ mainly in eye size, which is very small in T. isectum (OI 15 - 16) versus small to moderately large in T. andrei (OI 19 - 25), but there a few more supporting characters found in both species, although not consistently. All specimens of T. isectum have, in addition to very small eyes, a petiolar node shape with the anterodorsal margin higher and sharper than the posterodorsal with a dorsum that tapers weakly posteriorly, as well as a bright orange body colour. However, within the vast T. andrei material available, there are specimens with a node shape like the one of T. isectum, although these are mainly restricted to northwestern Madagascar where T. isectum does not occur. Many specimens of T. andrei are also bright orange in colour. Nevertheless, T. isectum was found to live in sympatry with T. andrei throughout most of its distribution range, and remained remarkably recognisable. The diagnostic characters of T. isectum are very consistent throughout all the material studied, and no intermediate forms seem to exist.

Apart from the similarities with T. andrei mentioned above, T. isectum is easily recognisable within the species complex. The very small eyes (OI 15 - 16) are very conspicuous with the caveat that Tetramorium electrum, Tetramorium elf, Tetramorium isoelectrum, and Tetramorium nify also have generally smaller eyes (OI 16 - 19). However, T. electrum, T. elf, and T. isoelectrum have very long to extremely long propodeal spines (PSLI 46 - 64), whereas the spines of T. isectum are much shorter (PSLI 28 - 35). Tetramorium nify is also unlikely to be misidentified with T. isectum since the latter is bright orange in colour and the former is very dark brown to black in colour. In addition, they have fairly differently shaped petiolar nodes. The node of T. nify has antero- and posterodorsal margins at the same height and is only faintly higher than long (LPeI 91 - 100), whereas in T. isectum the anterodorsal margin is usually weakly higher and the node is between 1.2 to 1.4 times higher than long (LPeI 71 - 85). The remaining species, Tetramorium ala, Tetramorium andohahela, and Tetramorium voasary, have much larger eyes (OI 20 - 24) and differently shaped petiolar nodes, and are thus not easy to confuse with T. isectum.

Distribution based on Regional Taxon Lists
Malagasy Region: Madagascar.

Nomenclature

 *  isectum. Tetramorium isectum Bolton, 1979: 145, figs. 17, 18 (w.) MADAGASCAR. See also: Hita Garcia & Fisher, 2012: 37.

Worker
Hita Garcia and Fisher (2012) - HL 0.90 - 1.10 (0.96); HW 0.85 - 1.50 (0.93); SL 0.65 - 0.83 (0.72); EL 0.14 - 0.18 (0.15); PH 0.42 - 0.54 (0.47); PW 0.58 - 0.76 (0.68); WL 1.08 - 1.39 (1.17); PSL 0.26 - 0.36 (0.29); PTL 0.25 - 0.34 (0.28); PTH 0.33 - 0.40 (0.35); PTW 0.24 - 0.29 (0.26); PPL 0.27 - 0.35 (0.31); PPH 0.31 - 0.39 (0.34); PPW 0.32 - 0.40 (0.34); CI 94 - 99 (97); SI 74 - 79 (77); OI 15 - 16 (16); DMI 54 - 60 (58); LMI 39 - 42 (40); PSLI 28 - 33 (30); PeNI 36 - 42 (38); LPeI 71 - 85 (81); DPeI 86 - 98 (92); PpNI 49 - 54 (50); LPpI 82 - 97 (91); DPpI 103 - 120 (111); PPI 128 - 136 (132) (12 measured).

Head usually weakly longer than wide (CI 94 - 99). Posterior head margin concave. Anterior clypeal margin medially impressed. Frontal carinae strongly developed, diverging posteriorly, and ending at corners of posterior head margin. Antennal scrobes weakly developed, shallow, narrow, and without defined posterior and ventral margins. Antennal scapes short, not reaching posterior head margin (SI 74 - 79). Eyes very small (OI 15 - 16). Mesosomal outline in profile flat to weakly convex, moderately marginate from lateral to dorsal mesosoma; promesonotal suture and metanotal groove absent; mesosoma comparatively stout and high (LMI 39 - 42). Propodeal spines long, spinose, and acute (PSLI 28 - 33); propodeal lobes short, triangular to elongate-triangular, and acute. Petiolar node in profile rectangular nodiform, around 1.2 to 1.4 times higher than long (LPeI 71 - 85), anterior and posterior faces approximately parallel, anterodorsal margin situated weakly higher and more angulate than posterodorsal margin, dorsum tapering weakly backwards posteriorly; node in dorsal view between 1.0 to 1.2 times longer than wide (DPeI 86 - 98). Postpetiole in profile globular, approximately 1.0 to 1.2 times higher than long (LPpI 82 - 97); in dorsal view between 1.0 and 1.2 times wider than long (DPpI 103 - 120). Postpetiole in profile appearing approximately as voluminous as petiolar node, in dorsal view approximately 1.3 to 1.4 times wider than petiolar node (PPI 128 - 136). Mandibles distinctly longitudinally rugose; clypeus generally with three distinct longitudinal rugae/rugulae, sometimes with few more but much weaker rugulae present, and very rarely clypeus with irregular rugulation; cephalic dorsum between frontal carinae with 8 to 11 longitudinal rugae, most rugae running unbroken from posterior head margin to posterior clypeus, rugae very rarely with cross-meshes; lateral and ventral head longitudinally rugose with very few cross-meshes. Mesosoma laterally and dorsally distinctly longitudinally rugose. Forecoxae unsculptured, smooth, and shining. Waist segments irregularly rugulose with distinct punctate ground sculpture, laterally ground sculpture better developed and rugulae weaker. First gastral tergite unsculptured, smooth, and shining. All dorsal surfaces of head, mesosoma, waist segments, and gaster with abundant, long, and fine standing hairs. Anterior edges of antennal scapes with decumbent to suberect standing hairs. Body colour bright yellow to orange.

Type Material
Holotype worker, MADAGASCAR, Beforona, 500 m, forest humus and litter, IX.1974 (A. Peyrieras) (: CASENT0172829) [examined].