Tetramorium decem species group
Based on Hita Garcia and Fisher 2014.
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The T. decem species group is endemic to the Afrotropical region where it is widely distributed. Tetramorium raptor and T. uelense are found in West and Central Africa and T. venator occurs through most of the equatorial rainforest belt from Liberia in West Africa to Western Kenya. By contrast, T. decem and T. ultor are species from eastern and southeastern Africa. Surprisingly, the group seems to be absent from South Africa based on the material available to us, but T. decem or T. ultor are likely to be found there or in neighbouring Botswana or Namibia. Furthermore, we expect the distribution ranges of T. decem, T. uelense, and perhaps T. ultor to expand with further ant inventory or collecting projects in Afrotropical savannahs, dry forests, and other arid habitats. These were sparsely sampled in sub-Saharan Africa in the past since most modern ant inventories have focused on rainforests (e.g. Belshaw and Bolton 1994; Watt et al. 2002; Fisher 2004; Yanoviak et al. 2007; Hita Garcia et al. 2009), whereas only a few studies have examined ant faunas from drier localities (e.g. Robertson 1999, 2002; Braet and Taylor 2008). The separation of the T. decem species group from all other Tetramorium species groups is straightforward and easy. So far, only the members of the T. decem group have ten-segmented antennae, whereas all other Afrotropical Tetramorium have either eleven or twelve. Consequently, the T. decem species group is unlikely to be confused with another Afrotropical group. The morphology of the five species of the group is very uniform, likely due to their strongly specialised lifestyle, which makes the taxonomy of the group challenging at first sight. However, good diagnostic characters separate them fairly well from each other, especially eye size, propodeal spine/teeth length, petiolar node shape, mesosomal sculpture, and body colouration. These characters are remarkably consistent within each species throughout its whole distribution, as are the species-specific habitat preferences.
Ten-segmented antennae; antennal scape relatively short (SI 67–76); anterior clypeal margin with distinct but often shallow 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 usually ventral scrobe margins; antennal scrobes very well developed, deep and usually with clearly defined margins all around, median scrobal carina absent; eyes relatively large (OI 32–40); mesosoma relatively flat, low, and elongated, margination between lateral and dorsal mesosoma moderately developed (LMI 33–38); propodeum armed with short triangular to elongate-triangular teeth (PSLI 9–19); propodeal lobes short, rounded to triangular; tibiae and femorae strongly swollen; petiolar node nodiform with moderately rounded antero- and posterodorsal margins, petiolar dorsum weakly to strongly convex, node in profile between 1.0 to 1.3 times higher than long (LPeI 77–100), node in dorsal view around 1.1 to 1.3 times longer than wide (DPeI 76–92); postpetiole in profile globular, around 1.1 to 1.4 times higher than long (LPpI 71–88); mandibles and clypeus unsculptured, smooth, and shiny; sculpture on cephalic dorsum between frontal carinae and dorsal mesosoma variable, ranging from unsculptured, smooth, and shiny to longitudinally rugose/rugulose, often punctate or puncticulate; petiole usually weakly sculptured, postpetiole unsculptured to weakly sculptured; gaster unsculptured, smooth, and shiny; pilosity greatly reduced, head with several pairs of standing hairs, mesosoma with one pair, waist segments sometimes with one long pair each, and sometimes first gastral tergite with one pair; sting appendage triangular.
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 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.