Tetramorium occidentale

Tetramorium occidentale seems to be a widely distributed species found in West and Central African rain forests and on Principe Island. Surprisingly it was relatively rarely sampled since less than 50 specimens from a good number of localities over its wide distribution range are known, but never from more than 4 to 6 specimens per locality. Compared to other similarly widely distributed members of the species complex as Tetramorium muralti or Tetramorium susannae it appears as if T. occidentale lives a more cryptic lifestyle, perhaps in the lower vegetation. This would explain its rarity in litter samples, but it is also possible that it is simply a rare species compared to most other muralti complex members. (Hita Garcia et al. 2010)

Identification
Tetramorium occidentale can be well separated from the rest of the species complex by the following character set: anterior clypeal margin with median impression; metanotal groove not impressed; petiolar node moderately squamiform (DPeI 225 - 268, LPeI 36 - 43); postpetiole cuneiform without sharp dorsal margin (DPpI 153 - 182, LPpI 62 - 71) and only slightly wider than petiole (PPI 104 - 118).

A member of the Afrotropical muralti species complex, which is part of the weitzeckeri species group.

Distribution based on Regional Taxon Lists
Afrotropical Region: Benin, Cameroun, Central African Republic, Ghana, Ivory Coast, Kenya, Togo, Uganda.

Nomenclature

 *  occidentale. Xiphomyrmex occidentalis Santschi, 1916a: 50, fig. 1 (w.) CAMEROUN. Combination in Tetramorium: Bolton, 1980: 229. Senior synonym of insularis: Bolton, 1980: 229. See also: Hita Garcia, Fischer & Peters, 2010b: 56.
 * insularis. Xiphomyrmex insularis Menozzi, 1924b: 223, fig. 4 (w.) SÃO TOMÉ & PRINCIPE (Principe I.). Junior synonym of occidentale: Bolton, 1980: 229.

Hita Garcia et al. (2010) - T. kakamega has now been split from T. occidentale while Tetramorium akengense has been revived from synonymy. These actions have been undertaken to improve and facilitate the taxonomy of T. occidentale and allied species. However, the situation is far from being satisfactory since the examined material is still too scarce for a final and strongly evident conclusion. Even after its actual redefinition T. occidentale remains a species with an extraordinary intraspecific variability for the weitzeckeri species group. Generally, size variation is remarkable as can be seen in its morphometric range, though it is not related to any biogeographic pattern. In addition, there is a high variation observable in length and shape of the propodeal spines that seems to be related to body size. The spine length becomes shorter and the spine shape grows more triangular and less spinose with decreasing body size. Although this pattern was expected for most of the weitzeckeri group species, it could only be observed in T. occidentale. To conclude it has to be pointed out that examination of this observed variation did not reveal any argument to divide the material presented here as T. occidentale. How to distinguish T. occidentale from T. akengense and T. kakamega can be seen in the respective species descriptions in detail. Nonetheless, all three species share many characters and can be well distinguished from the rest of the species complex by their possession of an impressed anterior clypeal margin.

Worker
Hita Garcia et al. (2010) - HL 0.589 - 0.833 (0.686); HW 0.567 - 0.822 (0.665); SL 0.378 - 0.533 (0.443); EL 0.122 - 0.172 (0.142); PW 0.400 - 0.561 (0.484); WL 0.600 - 0.956 (0.780); PSL 0.089 - 0.217 (0.156); PTL 0.100 - 0.133 (0.115); PTH 0.239 - 0.344 (0.287); PTW 0.228 - 0.333 (0.279); PPL 0.156 - 0.222 (0.185); PPH 0.233 - 0.322 (0.271); PPW 0.267 - 0.367 (0.311); CI 92 - 100 (97); SI 63 - 72 (67); OI 18 - 24 (21); PSLI 14 - 26 (23); PeNI 48 - 63 (58); LPeI 36 - 43 (40); DPeI 225 - 268 (244); PpNI 54 - 69 (64); LPpI 62 - 71 (68); DPpI 153 - 182 (169); PPI 104 - 118 (112) (38 measured).

Head generally weakly longer than wide, rarely as long as wide (CI 92 - 100). Anterior clypeal margin with median impression, sometimes small but always distinct. Frontal carinae strongly developed and sinuate, curving down ventrally shortly before posterior margin of head to form the posterior and ventral margins of antennal scrobe; sometimes carinae growing weaker behind eye level and posterior margin of scrobe only weakly marginate; scrobe well-developed and broad, with distinct margin all around, dorsal and ventral margins always sharply defined. Antennal scape short (SI 63 - 72). Eyes relatively small to moderate (OI 18 - 24), with 9 to 11 ommatidia in longest row. Metanotal groove in profile never impressed. Propodeal spines very variable, short to moderate in length, triangular to elongate-triangular or spinose in shape (PSLI 14 - 26). Propodeal lobes short and triangular with a very broad base. Petiolar node squamiform, in dorsal view generally between 2.2 to 2.7 times wider than long (DPeI 225 - 268), in lateral view between 2.3 to 2.8 times higher than long (LPeI 36 - 43). Postpetiole cuneiform, rounded dorsally without sharp dorsal margin, in dorsal view between 1.5 and 1.8 times wider than long (DPpI 153 - 182) and usually only faintly wider than petiole (PPI 104 - 118); in profile generally around 1.3 to 1.6 times higher than long (LPpI 62 - 71). Mandibles unsculptured, smooth and shiny. Clypeus generally with 3 longitudinal rugae, median ruga always strongly developed, lateral rugae sometimes weak or effaced. Head unsculptured except for 1 median longitudinal ruga between frontal carinae and 1 median longitudinal ruga anteriorly within the antennal scrobe, the latter usually running to eye level, rarely 1 or 2 very weak additional rugae present between median ruga and frontal carinae. Mesosoma generally unsculptured, rarely few weak longitudinal rugulae present on anterior pronotal dorsum. Ground sculpturation on head and mesosoma smooth and shiny. Both waist segments and gaster completely unsculptured, smooth and shiny. All dorsal body surfaces with simple, fine, long, and erect hairs. Fine pubescence on tibiae and antennal scapes appressed to decumbent. Colour uniformly very dark brown to black, appendages brown.

Type Material
Hita Garcia et al. (2010) - Holotype worker, CAMEROON, no location [examined].

Xiphomyrmex insularis Syntype workers, SÃO TOMÉ & PRINCIPE, Principe I., Roca Infante Don Henrique, 100-300 m, 1.III.1901, leg. L. Fea [not examined].

Determination Clarifications
Earlier records of this species should be treated with caution since some specimens previously listed as  Tetramorium occidentale (Bolton 1980, Hita Garcia et al. 2009) proved to be either Tetramorium akengense or Tetramorium kakamega.

Additional References

 * Hita Garcia, F.; Fischer, G.; Peters, M. K. 2010. Taxonomy of the Tetramorium weitzeckeri species group (Hymenoptera: Formicidae) in the Afrotropical zoogeographical region. Zootaxa 2704:1-90.