Tetramorium isoelectrum

Only known from seven specimens from Betaolana, Befingotra, and Marojejy. All three localities are rainforests found in the north-eastern part of Madagascar at elevations of 450 to 880 m.

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

Hita Garcia and Fisher (2012) - Tetramorium isoelectrum differs from the remainder of the species group by the following character combination: propodeal spines extremely long (PSLI 58 - 63); petiolar node around 1.1 to 1.3 times higher than long (LPeI 79 - 86); posterodorsal corner of petiole not strongly protruding posteriorly; mandibles with distinct longitudinal sculpture; body dark brown to black in colour.

Tetramorium isoelectrum is morphologically very close to Tetramorium electrum and Tetramorium elf, as outlined in the descriptions of the latter two species. Tetramorium elf, however, is yellow in colour and has mandibles that are unsculptured, smooth, and shining, whereas T. isoelectrum is very dark brown and has mandibles that are very clearly sculptured. T. isoelectrum and T. electrum are more difficult to distinguish. Both species are very close in general gestalt and easy to confuse at first glance. Nevertheless, T. isoelectrum has a longer head (CI 93 - 96), longer antennal scapes (SI 81 - 84), and a longer and lower petiolar node (LPeI 77 - 86; DPeI 87 - 97) than T. electrum (CI 98 - 104; LPeI 64 - 74; DPeI 100 - 114). Furthermore, due to its extremely long propodeal spines (PSLI 58 - 63), T. isoelectrum is not likely to be misidentified with the remaining species of the complex.

Distribution based on Regional Taxon Lists
Malagasy Region: Madagascar.

Nomenclature

 *  isoelectrum. Tetramorium isoelectrum Hita Garcia & Fisher, 2012: 40, figs. 59, 64, 65, 93-95, 141 (w.) MADAGASCAR.

Worker
HL 1.09 - 1.19 (1.16); HW 1.03 - 1.13 (1.10); SL 0.84 - 0.94 (0.91); EL 0.19 - 0.21 (0.20); PH 0.53 - 0.68 (0.61); PW 0.74 - 0.84 (0.81); WL 1.36 - 1.57 (1.50); PSL 0.63 - 0.75 (0.71); PTL 0.35 - 0.38 (0.36); PTH 0.44 - 0.46 (0.45); PTW 0.32 - 0.34 (0.33); PPL 0.38 - 0.40 (0.39); PPH 0.44 - 0.46 (0.45); PPW 0.39 - 0.41 (0.40); CI 93 - 96 (95); SI 81 - 84 (82); OI 17 - 19 (18); DMI 53 - 55 (54); LMI 38 - 45 (40); PSLI 58 - 63 (61); PeNI 38 - 46 (41); LPeI 79 - 86 (81); DPeI 87 - 93 (90); PpNI 47 - 54 (49); LPpI 83 - 91 (87); DPpI 101 - 105 (103); PPI 118 - 127 (123) (seven measured).

Head longer than wide (CI 93 - 96). Posterior head margin weakly to moderately 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 comparatively short to moderately long, not reaching posterior head margin (SI 81 - 84). Eyes small (OI 17 - 19). 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 38 - 45). Propodeal spines extremely long, spinose, and acute (PSLI 58 - 63); propodeal lobes short, triangular, and blunt. Petiolar node in profile rectangular nodiform with comparatively rounded angles, around 1.1 to 1.3 times higher than long (LPeI 79 - 86), anterior and posterior faces approximately parallel, anterodorsal and posterodorsal margins approximately at same height, dorsum straight to weakly convex; node in dorsal view around 1.1 times longer than wide (DPeI 87 - 93). Postpetiole in profile subglobular, approximately 1.1 to 1.2 times higher than long (LPpI 83 - 91); in dorsal view feebly wider than long (DPpI 101 - 105). Postpetiole in profile appearing a bit more voluminous than petiolar node, in dorsal view approximately 1.2 to 1.3 times wider than petiolar node (PPI 118 - 127). Mandibles distinctly longitudinally rugose; clypeus with four to six longitudinal rugulae, median rugula better developed than lateral rugulae; cephalic dorsum between frontal carinae with eight to ten longitudinal rugae, most rugae running unbroken from posterior head margin to posterior clypeus, rugae never with cross-meshes; lateral and ventral head longitudinally rugose with very few cross-meshes. Mesosoma laterally and dorsally distinctly longitudinally rugose, rugae laterally more irregularly arranged. Forecoxae mainly reticulate-punctate with superficial, superimposed rugulae. Waist segments with reticulate-rugose to longitudinally rugose sculpture. Ground sculpture everywhere on body faint to absent. First gastral tergite unsculptured, smooth, and shining. All dorsal surfaces of head, mesosoma, waist segments, and gaster with abundant, long, and fine standing hairs. First gastral tergite without appressed pubescence. Anterior edges of antennal scapes with subdecumbent to erect standing hairs. Body of uniform very dark brown to black colour.

Type Material
Holotype worker, MADAGASCAR, Antsiranana, Betaolana Forest, along Bekona River, 14.52996 S, 49.44039, 880 m, rainforest, on low vegetation, collection code BLF22647, 5.III.2009 (B.L. Fisher et al.) (: CASENT0152199). Paratype, one worker with same data as holotype except sampled from ground and collection code BLF22648 (CASC: CASENT0152192).

Etymology
The name of the new species is composed of "iso", which is Old Greek meaning "equal", and "electrum", which is the species epithet of T. electrum. Tetramorium electrum and T. isoelectrum are very similar, and the new species name reflects this. The species epithet is a noun in apposition, and thus invariant.