Tetramorium xanthogaster

Tetramorium xanthogaster shows a patchy distribution. This could possibly be explained by this species nesting and/or foraging in the vegetation since almost all of the available material was collected either from beating low vegetation or Malaise traps.

Identification
A member of the Tetramorium schaufussii species complex of the Tetramorium schaufussii species group. Hita Garcia and Fisher (2014) - The following character set discriminates T. xanthogaster from the other species of the T. schaufussii complex: moderate to large eyes (OI 22–25); short antennal scapes (SI 70–75); frontal carinae weakly developed, only faintly raised, usually becoming much weaker around eye level and fading out halfway between posterior eye margin and posterior head margin; propodeal spines/teeth very short to short (PSLI 10–16), propodeal spines and lobes not strongly inclined towards each other; petiolar node in profile around 1.5 to 1.7 times higher than long (LPeI 59–67) and in dorsal view around 1.2 to 1.3 times wider than long (DPeI 119–133); dorsum of promesonotum with numerous pairs of long, standing hairs (10+), propodeum usually with one or two pairs, sometimes with up to five; waist segments with long, standing pilosity.

Despite its pronounced variability in colouration and petiolar node shape, the determination of T. xanthogaster is fairly straightforward. The species was described by Santschi (1911) and later redescribed by Bolton (1979) as a bicoloured species with dark head and mesosoma contrasting with the yellowish to light brown remainder of the body. Indeed, the material from the southwest and the Central Highlands is consistently strongly bicoloured, and even though one should not overly rely too heavily on body colouration, no other species of the T. schaufussii species group is similarly bicoloured. However, material from the northern localities shows remarkable diversity in colouration. The populations from Andranobe, Marojejy, and Binara are of a fairly bright, yellowish brown. By contrast, the material from Manongarivo and Montagne d’Ambre is in parts bicoloured like the southern populations, partly bicoloured with only the head or mesosoma darker than the rest of the body, or just uniformly light brown to dark brown. But even misregarding colouration, T. xanthogaster cannot be misidentified with another member of the complex. Due to the presence of long, standing pilosity on its waist segments it cannot be mistaken for Tetramorium obiwan (partly), Tetramorium pseudogladius, Tetramorium sikorae, or Tetramorium rala, and the broader than long petiolar node (DPeI 119–133) distinguishes it from Tetramorium nassonowii (DPeI 87–98). In addition, the petiolar node of T. xanthogaster in profile is around 1.5 to 1.7 times higher than long (LPeI 59–67), which separates it from T. rala and Tetramorium scutum that have nodes which are 2.0 to 2.2 times higher than long (LPeI 45–50). The petiolar nodes of the latter two are also thinly cuneiform to weakly squamiform while the node of T. xanthogaster is variably shaped, but lower and less angled. Tetramorium obiwan possesses well-developed frontal carinae and cephalic sculpture, as well as relatively long antennal scapes (SI 77–82), whereas T. xanthogaster has much weaker frontal carinae and cephalic sculpture and shorter antennal scapes (SI 70–75). The remaining three species, Tetramorium merina, Tetramorium monticola, and Tetramorium schaufussii, are more difficult to separate from T. xanthogaster. Tetramorium merina is only found in the central Highlands and co-occurs there with T. xanthogaster. In this region T. xanthogaster is always strongly bicoloured with head and mesosoma very dark brown to black, which contrasts with the yellow to light brown waist segments and gaster. In addition, T. merina lacks any standing pilosity on the propodeum while T. xanthogaster usually has long, fine, standing pilosity on the propodeal dorsum. As mentioned above, there are not too many other diagnostic characters to separate these species, but the fact that they co-occur in sympatry in several localities without any intermediate forms supports their heterospecificity. Tetramorium monticola is also sympatric with T. xanthogaster, this time in the northern part of Madagascar, where the colouration of the latter species is not reliable. However, in T. monticola the frontal carinae are more strongly developed and the cephalic dorsum between the frontal carinae has nine to thirteen relatively regularly shaped, mostly unbroken rugae while the frontal carinae of T. xanthogaster are weakly developed and the cephalic dorsum has only six to eight widely separated, often irregularly shaped rugulae. Also, the propodeal spines of the latter are shorter (PSLI 10–16) than in T. monticola (PSLI 18–22). The last and most common and abundant species of the complex, Tetramorium schaufussii, is also usually found in sympatry with T. xanthogaster, but both species can be well separated by head shape. The head of T. schaufussii is much longer and thinner (CI 86–90) than in T. xanthogaster (CI 92–94). Additional differentiating characters are the long, standing pilosity on the propodeal dorsum (usually present in T. xanthogaster but absent in T. schaufussii) and the median clypeal ruga (present in T. schaufussii but absent in T. xanthogaster).

It has to be noted that T. xanthogaster is relatively variable in the shape of the petiolar node, which can be high nodiform, nodiform, cuneiform, or in intermediate stages. The node shape varies strongly even within material from the same collection event. Another noteworthy variation can be seen in the material from Analavelona. In contrast to the rest of the material from all other localities it seems that the sculpture on the cephalic dorsum and the mesosomal dorsum is much better developed, and does not differ significantly from other species like T. schaufussii. However, the frontal carinae are still weaker than in the latter.

Key to Tetramorium schaufussii  species complex workers

Distribution
Tetramorium xanthogaster has a relatively patchy distribution range. The southernmost locality is Analavelona in the southwest. The next known localities are located in the Central Highlands much further north and east (Ambohitantely, Ankokoy, and Marotandrano). The remaining localities are situated in the northern part of Madagascar (Andranobe, Marojejy, Binara, Montagne d’Ambre, and Manongarivo). Many of these localities are widely separated from each other. (Hita Garcia and Fisher 2014)

Distribution based on Regional Taxon Lists
Malagasy Region: Madagascar.

Habitat
Tetramorium xanthogaster prefers montane rainforests since most collections are from elevations around or above 1000 m, whereas it was only occasionally collected from lower elevations.

Nomenclature

 *  xanthogaster. Tetramorium (Xiphomyrmex) sikorae var. xanthogaster Santschi, 1911e: 124 (w.) MADAGASCAR. [Original spelling of xantogaster justifiably emended to xanthogaster by Wheeler, W.M. 1922a: 1032.] Raised to species: Bolton, 1979: 139.

Worker
Hita Garcia and Fisher (2014) - (N=25). HL 0.54–0.75 (0.64); HW 0.59–0.80 (0.70); SL 0.38–0.53 (0.46); EL 0.13–0.18 (0.16); PH 0.27–0.41 (0.32); PW 0.39–0.56 (0.46); WL 0.72–1.05 (0.86); PSL 0.06–0.13 (0.09); PTL 0.12–0.19 (0.15); PTH 0.21–0.29 (0.25); PTW 0.16–0.24 (0.19); PPL 0.14–0.21 (0.18); PPH 0.19–0.29 (0.24); PPW 0.19–0.29 (0.24); CI 92–94 (93); SI 70–75 (72); OI 22–25 (24); DMI 51–55 (54); LMI 36–39 (38); PSLI 10–16 (13); PeNI 37–46 (41); LPeI 59–67 (62); DPeI 119–133 (126); PpNI 48–56 (51); LPpI 68–80 (75); DPpI 125–140 (135); PPI 117–129 (125).

Head clearly longer than wide (CI 92–94); posterior head margin weakly concave. Anterior clypeal margin with distinct median impression. Frontal carinae weakly developed, only faintly raised, usually becoming much weaker around eye level and fading out halfway between posterior eye margin and posterior head margin. Antennal scrobes very weak, shallow and without clear and distinct posterior and ventral margins. Antennal scapes short, not reaching posterior head margin (SI 70–75). Eyes moderate to relatively large (OI 22–25). Mesosomal outline in profile flat to weakly convex, comparatively low and long (LMI 36–39), weakly to moderately marginate from lateral to dorsal mesosoma; promesonotal suture absent; metanotal groove weakly developed or absent. Propodeal spines/teeth very short to short, varying from triangular and blunt to elongate-triangular and acute (PSLI 10–16), propodeal lobes short, triangular, and blunt, always much shorter than propodeal spines, spines and lobes not strongly inclined towards each other. Petiolar node in profile high nodiform, nodiform or weakly cuneiform, always with relatively well rounded anteroand posterodorsal margins, around 1.5 to 1.7 times higher than long (LPeI 59–67), anterior and posterior faces often approximately parallel and often not, anterodorsal and posterodorsal margins usually at about same height, sometimes anterodorsal margin situated slightly higher than posterodorsal, petiolar dorsum weakly to moderately convex; petiolar node in dorsal view around 1.2 to 1.3 times wider than long (DPeI 119–133), in dorsal view pronotum between 2.2 to 2.7 times wider than petiolar node (PeNI 37–46). Postpetiole in profile globular to subglobular, approximately 1.3 to 1.5 times higher than long (LPpI 68–80); in dorsal view around 1.3 to 1.4 times wider than long (DPpI 125–140), pronotum between 1.8 to 2.1 times wider than postpetiole (PpNI 48–56). Postpetiole in profile appearing more or less as voluminous as petiolar node, postpetiole in dorsal view around 1.2 to 1.3 times wider than petiolar node (PPI 117–129). Mandibles unsculptured, smooth, and shining; generally sculpture on clypeus very much reduced, median area usually unsculptured with one or two weak, irregular, and broken rugulae laterally, very rarely median rugula present but then weak and broken; cephalic dorsum between frontal carinae irregularly longitudinally rugulose with six to eight widely separated rugulae, rugulae running from posterior clypeal margin to posterior head margin, but irregularly shaped, often broken or with cross-meshes, and becoming weaker or fading out towards posterior head margin; scrobal area partly unsculptured, but mostly merging laterally with surrounding reticulate-rugose to longitudinally rugose sculpture present around eyes, most of lateral head predominantly unsculptured, smooth, and shiny; ground sculpture on head weakly to moderately punctate. Dorsum of mesosoma weakly to moderately longitudinally rugulose, sometimes irregularly so, often rugulae very weak and parts of dorsal mesosoma smooth; lateral mesosoma anteriorly often only weakly sculptured and shiny, katepisternum and lateral propodeum usually irregularly longitudinally rugulose to reticulate-rugose, sometimes almost completely unsculptured, smooth, and shiny; ground sculpture on mesosoma usually only weakly developed, mostly absent. Forecoxae unsculptured, smooth, and shining. Waist segments and gaster unsculptured, smooth, and shining. Dorsum of head with several pairs of long, fine, standing hairs; dorsum of promesonotum always with more than ten pairs of long, standing hairs, propodeum usually with one or two pairs, sometimes with up to five, rarely without any standing pilosity; waist segments each with several pairs; first gastral tergite with short, scarce to moderately abundant, appressed to decumbent pubescence in combination with scattered, long, standing hairs. Anterior edges of antennal scapes and dorsal (outer) surfaces of hind tibiae with decumbent to subdecumbent hairs. Body colour variable, usually bicoloured with head and mesosoma of dark brown to blackish colour contrasting with yellowish or light brown appendages, waist segments, and gaster; very rarely only head of dark brown contrasting with yellowish remainder of body, sometimes body uniformly coloured, ranging from yellow to dark brown.

Type Material
Hita Garcia and Fisher (2014) - Holotype, pinned worker, MADAGASCAR (M.J. de Gaulle) (: CASENT0101146) [examined].