Tetramorium electrum

Tetramorium electrum is one of the most common and conspicuous genus members encountered in the eastern rainforests of Madagascar. Its distribution ranges from Andohahela in the southeast to Marojejy in the northeast. It is found at elevations from 25 to 1080 m, and appears to prefer the leaf litter stratum. (Hita Garcia and Fisher 2012)

Identification
Hita Garcia and Fisher (2012) - Tetramorium electrum is easily recognisable within the T. tortuosum group in the Malagasy region due to the following combination of characters: propodeal spines very long to extremely long (PSLI 46 - 52); petiolar node around 1.3 to 1.6 times higher than long (LPeI 64 - 74); posterodorsal corner of petiole not strongly protruding posteriorly; body dark brown to black in colour.

Within the species complex T. electrum can be grouped together with Tetramorium elf and Tetramorium isoelectrum on the basis of shared morphology. Tetramorium elf is not easily confused with T. electrum, however, since the first is yellowish in colour with longer antennal scapes (SI 78 - 83) and petiolar node (DPeI 88 - 96), whereas T. electrum is very dark brown to black in colour and the scapes and petiolar node are shorter (SI 67 - 74; DPeI 100 - 114). The head of T. elf (CI 92 - 96) is also narrower than that of T. electrum (CI 98 - 104). More challenging is the separation of T. electrum from T. isoelectrum since they are superficially very similar. Much like T. elf, T. isoelectrum also has longer antennal scapes (SI 81 - 84), a longer petiolar node (DPeI 87 - 97), and a less broad head (CI 93 - 96) compared to T. electrum. Another difference is the petiolar node, which is lower and more square in T. isoelectrum (LPeI 77 - 86) but higher and narrows towards the dorsum in T. electrum (LPeI 64 - 74). Furthermore, T. electrum is not likely to be misidentified with the remaining species of the complex due to the combination of very long to extremely long propodeal spines (PSLI 46 - 52), low and inconspicuous propodeal lobes, and comparatively high petiolar node.

It must be noted that some morphological variation exists within T. electrum. This fact is not surprising considering its large distribution range in eastern Madagascar. The mandibles are usually finely striate, but some populations have completely unsculptured mandibles while only partly sculptured mandibles are found in other series. The shape of the petiolar node is also somewhat variable. The node is usually rectangular nodiform but comparatively high. In many specimens the anterodorsal margin is slightly higher than the posterodorsal, which causes the dorsum to taper backwards, even if weakly (as in the holotype CASENT0280589). In other specimens the node narrows distinctly towards the dorsum but with the anterior and posterior faces almost mirror-inverted (like in the paratype specimens CASENT0102350 and CASENT0280850).

Distribution based on Regional Taxon Lists
Malagasy Region: Madagascar.

Nomenclature

 *  electrum. Tetramorium electrum Bolton, 1979: 144, fig. 20 (w.) MADAGASCAR. See also: Hita Garcia & Fisher, 2012: 31.

Worker
Hita Garcia and Fisher (2012) - HL 0.89 - 1.20 (1.07); HW 0.87 - 1.25 (1.08); SL 0.65 - 0.84 (0.77); EL 0.16 - 0.22 (0.19); PH 0.49 - 0.63 (0.55); PW 0.63 - 0.82 (0.75); WL 1.14 - 1.59 (1.34); PSL 0.44 - 0.60 (0.53); PTL 0.28 - 0.35 (0.31); PTH 0.38 - 0.47 (0.44); PTW 0.29 - 0.37 (0.33); PPL 0.32 - 0.40 (0.36); PPH 0.38 - 0.48 (0.43); PPW 0.37 - 0.44 (0.40); CI 98 - 104 (100); SI 67 - 74 (71); OI 16 - 19 (17); DMI 52 - 58 (56); LMI 39 - 44 (41); PSLI 46 - 52 (50); PeNI 41 - 46 (44); LPeI 64 - 74 (71); DPeI 100 - 114 (105); PpNI 50 - 59 (53); LPpI 77 - 89 (84); DPpI 106 - 116 (110); PPI 115 - 128 (122) (20 measured).

Head approximately as long as wide (CI 98 - 104). Anterior clypeal margin medially impressed. Posterior head margin weakly to moderately concave. 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 very short, not reaching posterior head margin (SI 67 - 74). Eyes small (OI 16 - 19). Mesosomal outline in profile weakly convex, moderately marginate from lateral to dorsal mesosoma; promesonotal suture and metanotal groove absent; mesosoma comparatively stout and high (LMI 39 - 44). Propodeal spines very long, spinose, and acute (PSLI 46 - 52); propodeal lobes strongly reduced, generally vestigial, sometimes very short, broadly triangular and blunt. Petiolar node in profile rectangular nodiform with moderately to sharply defined margins, around 1.3 to 1.6 times higher than long (LPeI 64 - 74), anterior and posterior faces often not parallel narrowing towards dorsum, anterodorsal and posterodorsal margins generally at approximately same height, dorsum straight to weakly convex, sometimes anterodorsal margin slightly higher than posterodorsal with dorsum tapering weakly backwards posteriorly; node in dorsal view as wide as long to 1.2 times wider than long (DPeI 100 - 114). Postpetiole in profile subglobular, approximately 1.1 to 1.3 times higher than long (LPpI 77 - 89); in dorsal view around 1.1 times wider than long (DPpI 106 - 116). Postpetiole in profile usually appearing a bit more voluminous than petiolar node, in dorsal view approximately 1.1 to 1.3 times wider than petiolar node (PPI 115 - 128). Mandibles ranging from completely unsculptured, smooth, and shining to finely striate, often partly striate and partly smooth; clypeus with three to six longitudinal rugulae, rugulae often broken and irregularly arranged; cephalic dorsum between frontal carinae with 8 to 12 longitudinal rugae, most rugae running unbroken from posterior head margin to posterior clypeus, rugae only very rarely with cross-meshes; scrobal area mostly unsculptured; lateral and ventral head longitudinally rugose with few cross-meshes. Mesosoma laterally and dorsally distinctly longitudinally rugose, often rugae on lateral mesosoma more irregularly arranged than on the dorsum. Forecoxae usually unsculptured, smooth, and shining, sometimes with superficial sculpture, rarely weakly reticulate-punctate or reticulate-rugulose. Waist segments with reticulate-rugose/rugulose to longitudinally rugose/rugulose sculpture, often weaker laterally. 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 decumbent to erect standing hairs. Body very dark brown to black in colour, sometimes of lighter brown.

Type Material
Hita Garcia and Fisher (2012) - Holotype worker, MADAGASCAR, Rte d'Anosibe, km 33, forest humus and litter, AB 44, 4.-12.IV.1975 (A. Peyrieras) (: CASENT0280589) [examined]. Paratypes, 11 workers with same data as holotype, and one worker from vic. Andasibe (= Perinet) 950 - 980 m, 2.-6.II.1977 (W.L. & D.E. Brown) (: CASENT0102350; MCZ: CASENT0280850; MCZ_paratype_32378) [examined].