Tetramorium decem

Based on Arnold (1917) and the collection label from some material from Arabuko Sokoke, T. decem nests in sandy soil. Like other members of its species group, they prey on termites.

Identification
Hita Garcia & Fisher (2014) - A member of the Tetramorium decem species group. Tetramorium decem can be recognised by the following combination of characters: relatively larger species (HW 0.59–0.62; WL 1.02–1.16); propodeal teeth relatively longer (PSLI 17–19); petiolar node in profile around 1.2 to 1.3 times higher than long (LPeI 77–82); dorsum of promesonotum unsculptured, smooth, and very shiny; strongly bicoloured species with dark brown or black gaster contrasting with light brown to reddish brown remainder of body.

Tetramorium decem is the core species of the group (Tetramorium decem species group), and was the type species for the description of the subgenus Decamorium by Forel (1913a). It is perhaps the most conspicuous species of the group. Its bicolouration, larger size, lack of sculpture on the mesosomal dorsum, and a higher petiolar node render it immediately recognisable. The mostly unsculptured, smooth and shiny mesosomal dorsum distinguishes T. decem from Tetramorium raptor and Tetramorium uelense, in which the dorsum of the mesosoma is clearly longitudinally rugose/rugulose. Tetramorium ultor and Tetramorium venator both share the lack of sculpture on the mesosomal dorsum with T. decem, but can still be easily separated from the latter. Tetramorium decem is generally larger in size (WL 1.02–1.16), has longer propodeal spines (PSLI 17–19) and is also conspicuously bicoloured, whereas T. ultor and T. venator are smaller species (WL 0.85–0.98) with significantly shorter propodeal teeth (PSLI 9–13) and a more uniform brown to black body colouration. In addition, T. decem also has a higher petiolar node, in profile around 1.2 to 1.3 times higher than long (LPeI 77–82), compared to the other two, in which the node in profile is only around 1.0 to 1.2 times higher than long (LPeI 86–100). The species that appears to be morphologically closest to T. decem is T. uelense. Both species share the large body, bicolouration, and preference for arid habitats. However, in addition to the sculpture on the mesosoma, T. uelense also has a lower petiolar node, in profile around 1.1 times higher than long (LPeI 88–93). Another character that is shared between T. decem and T. uelense but absent in the other species of the group is the development of the ventral margin of the antennal scrobe. In T. raptor, T. ultor, and T. venator the margin is clearly and well defined, while in T. decem and T. uelense it is less so and merges more with the surrounding rugose sculpture.

Based on the available material we did not observe any significant form of intraspecific variation in T. decem.

Distribution
Hita Garcia & Fisher (2014) - Based on the redefined species definition, T. decem is only known from the type locality in Zimbabwe and two additional localities in East Africa: Arabuko Sokoke in Kenya and Mkomazi in Tanzania. Nevertheless, if more extensive sampling efforts are undertaken in East Africa, T. decem is likely to be found in more localities in Kenya, Tanzania, and Zimbabwe.

Distribution based on Regional Taxon Lists
Afrotropical Region: Ivory Coast, Kenya, United Republic of Tanzania, Zimbabwe.

Habitat
Like Tetramorium uelense and Tetramorium ultor, T. decem prefers arid habitats, such as savannah and woodland.

Biology
Hita Garcia and Fisher (2014) - If one considers the whole tribe Tetramoriini, then it becomes apparent that the specialized habitus of Decamorium is not unique. Several authors have stated that Decamorium (now synonymized with Tetramorium) are specialised termite hunters, and that their specialised morphology could be an adaptation to such a dangerous lifestyle (Arnold 1917; Bolton 1976; Longhurst et al. 1979). Interestingly, both Arnold (1917) in the original description and later Bolton (1980) noted the similarities in general body shape and diet between members of Decamorium and the species Tetramorium simulator from South Africa. We agree that the similarities in morphology are indeed obvious, especially in profile view. However, at present it is not clear whether the shared morphology is based on a close phylogenetic relationship between Decamorium and T. simulator or a result of convergent evolution due to a similar lifestyle hunting termites. We believe the latter more likely since the twelve-segmented antennae, the much broader head, and sculptured clypeus of T. simulator suggest a closer relationship to another group with twelve-segmented antennae than to Decamorium. Therefore, we hypothesise that both have evolved from different Tetramorium lineages and acquired the specialised habitus independently from each other. Another remarkable aspect is the lack of a strong and sharp clypeal shield in T. simulator, which seems to have been reduced in a manner almost similar, though less pronounced, to Decamorium.

Nomenclature

 *  decem. Tetramorium (Decamorium) decem Forel, 1913a: 121 (w.) ZIMBABWE. Arnold, 1917: 350 (q.m.). Combination in Decamorium: Wheeler, W.M. 1922a: 906; in Tetramorium: Hita Garcia & Fisher, 2014: 78.

Worker
Hita Garcia & Fisher (2014) - (N=15). HL 0.71–0.74 (0.72); HW 0.59–0.62 (0.60); SL 0.42–0.45 (0.43); EL 0.19–0.21 (0.20); PH 0.33–0.37 (0.35); PW 0.47–0.50 (0.48); WL 1.02–1.16 (1.06); PSL 0.12–0.14 (0.13); PTL 0.25–0.27 (0.26); PTH 0.31–0.34 (0.33); PTW 0.22–0.24 (0.23); PPL 0.24–0.27 (0.25); PPH 0.32–0.36 (0.34); PPW 0.32–0.36 (0.34); CI 83–85 (84); SI 70–76 (72); OI 32–34 (33); DMI 41–47 (45); LMI 32–34 (33); PSLI 17–19 (18); PeNI 46–51 (48); LPeI 77–82 (80); DPeI 85–92 (88); PpNI 67–76 (70); LPpI 71–77 (75); DPpI 128–138 (133); PPI 143–149 (147).

Head much longer than wide (CI 83–85); posterior head margin weakly concave. Anterior clypeal margin with distinct, but often shallow median impression. Frontal carinae strongly developed and noticeably raised forming dorsal margin of very well-developed antennal scrobes, curving down ventrally and anteriorly halfway between posterior eye margin and posterior head margin and forming posterior and parts of ventral scrobe margins; antennal scrobes very well developed, deep and with clearly defined margins, but ventral margin less strongly developed, median scrobal carina absent. Antennal scapes short, not reaching posterior head margin (SI 70–76). Eyes very large (OI 32–34). Mesosomal outline in profile flat to weakly convex, relatively low and elongate (LMI 32–34), moderately to strongly marginate from lateral to dorsal mesosoma; promesonotal suture absent; metanotal groove present, distinct, and clearly impressed. Propodeal spines short, elongate-triangular, and moderately acute (PSLI 17–19), propodeal lobes short, triangular, and usually blunt, always significantly shorter than propodeal spines. Tibiae and femorae strongly swollen. Petiolar node nodiform with moderately rounded antero- and posterodorsal margins, around 1.2 to 1.3 times higher than long (LPeI 77–82), anterior and posterior faces approximately parallel, anterodorsal and posterodorsal margins situated at about the same height, petiolar dorsum clearly convex; node in dorsal view between 1.1 to 1.2 times longer than wide (DPeI 85–92), in dorsal view pronotum around 2.0 to 2.2 times wider than petiolar node (PeNI 46–51). Postpetiole in profile globular to subglobular, approximately 1.3 to 1.4 times higher than long (LPpI 71–77); in dorsal view around 1.3 to 1.4 times wider than long (DPpI 128–138), pronotum between 1.3 to 1.5 times wider than postpetiole (PpNI 67–76). Postpetiole in profile usually appearing less voluminous than petiolar node, postpetiole in dorsal view around 1.4 to 1.5 times wider than petiolar node (PPI 143–149). Mandibles and clypeus usually fully unsculptured, smooth, and shining, mandibles sometimes with few traces of rugulae apically; cephalic dorsum between frontal carinae mostly unsculptured and shiny, median ruga present and distinct, cephalic dorsum also puncticulate to punctate throughout its length, posteriorly close to posterior head margin especially pronounced; scrobal area partly unsculptured, smooth and shiny and partly merging with surrounding rugose sculpture on sides of head. Ground sculpture on head usually weak to absent. Dorsum of mesosoma mostly unsculptured, smooth and shiny with scattered punctures, rarely with few traces of rugulae; lateral mesosoma longitudinally rugose and very conspicuously reticulate-punctate except for mostly unsculptured lateral pronotum and katepisternum. Forecoxae unsculptured, smooth, and shining. Petiolar node and postpetiole superficially longitudinally rugulose or irregularly rugulose superimposed on conspicuous but relatively weak reticulate-punctate ground sculpture. Mesosoma and waist segments appearing mostly matt. First gastral tergite unsculptured, smooth, and shiny. Pilosity and pubescence greatly reduced: head with few pairs of moderately long, standing hairs, anterior pronotum with one long pair, waist segments sometimes with one long pair each, and sometimes first gastral tergite with one long pair; appressed pubescence present everywhere on body, but noticeable only on antennae, cephalic dorsum, legs, and first gastral tergite. Anterior edges of antennal scapes and dorsal (outer) surfaces of hind tibiae with appressed hairs. Body strongly bicoloured with dark brown to black gaster contrasting with light brown to reddish brown remainder.

Determination Clarifications
The distribution range of T. decem is far smaller than previously thought. Indeed, most of the material listed in the literature as T. decem or labelled as such in museum collections turned out to be either Tetramorium ultor or Tetramorium venator, while only a few collections proved to be genuine T. decem.

References based on Global Ant Biodiversity Informatics

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