Tetramorium naganum species group


 * Tetramorium alperti
 * Tetramorium dalek
 * Tetramorium enkidu
 * Tetramorium gilgamesh
 * Tetramorium naganum

Based on Hita Garcia and Fisher 2014.

You may also be interested in: and the following keys:
 * Tetramorium species groups
 * Tetramorium
 * Key to Tetramorium naganum-group species
 * Key to Malagasy Tetramorium species groups

This small and compact group is restricted in its distribution to eastern and northern Madagascar. All five species are found only in rainforests or montane rainforests and seem to live mainly in leaf litter.

Within the 13 species groups with 11-segmented antennae, the T. naganum group shares the complete lack of sculpture on both waist segments with the majority of groups, but differs from the T. kelleri, T. ranarum, T. tortuosum, and parts of the T. dysalum groups. These groups contain only species in which either one or both waist segments are clearly sculptured. In addition, the very well developed sculpture on head and mesosoma distinguishes the T. naganum group from the groups with reduced sculpture: the T. bessonii, T. marginatum, and T. tsingy groups. Also, T. severini, the only member of the T. severini group, has a longer and lower mesosoma (LMI 35–38) with less margination from the sides to the dorsum, while the species of the T. naganum group have a higher (LMI 40–46), stouter, and more angled mesosoma. The T. plesiarum group is characterised by the presence of relatively deep and well-developed antennal scrobes with margins all around and a strongly developed median scrobal carina, a character always absent in the T. naganum group. The latter also cannot be mistaken for the T. bonibony group, in which some species possess a very conspicuous bump or protuberance on the pronotal dorsum while the remainder of the species have triangular or cuneiform petiolar nodes. The differentiation between the T. naganum group and the T. dysalum group can be more difficult however. The T. dysalum group is relatively heterogeneous and there are a few species that are morphologically close to some species in the T. naganum group. Nevertheless, the best method to discriminate between these two groups is to compare gastral pubescence and/or pilosity. In all species of the T. dysalum group appressed pubescence on the first gastral is scarce and inconspicuous, and pilosity consists of numerous long suberect to erect hairs. By contrast, pubescence and pilosity are very variable in the T. naganum group, but never with very reduced pubescence and long, standing pilosity as in the T. dysalum group.

The separation from the T. schaufussii group is likely the most difficult, and T. naganum was a member of that group until recently (Hita Garcia and Fisher 2012a). The five species of the T. naganum group have much stronger developed frontal carinae, a generally broader head (CI 92–99), a higher mesosoma (LMI 40–46), and usually longer propodeal spines (PSLI 25–37). In the T. schaufussii group most species (but not all) have weaker frontal carinae, a usually thinner head (CI 85–95), a lower mesosoma (LMI 35–42), and usually much shorter propodeal spines (PSLI 7–28). These values overlap in a few species. In fact, most members of the T. schaufussii group have relatively short spines with a PSLI below 20, a few species have slightly longer spines (PSLI 20–25), and only a few specimens of T. gladius and T. rumo have a higher PSLI of 26–28.

The five species of the group are morphologically very close to each other, and their delimitations are mainly based on different patterns of pilosity/pubescence on the waist segments and the first gastral tergite. Tetramorium dalek is easily separable from the other four species on the basis of the absence of standing hairs on the waist segments and shorter propodeal spines, but T. alperti, T. enkidu, T. gilgamesh, and T. naganum are morphologically very similar. Indeed, they are often difficult to discriminate and there are only very few reliable diagnostic characters, mainly gastral pilosity/pubescence. It is possible that two, three, or all four are conspecific and the abovementioned differences are just intraspecific variation. Nevertheless, we prefer to treat them as four distinct species since gastral pilosity/pubescence is usually very species-specific within the genus Tetramorium and other myrmicine genera and the material listed here can be moderately well distinguished by the diagnostics given in the key and the descriptions. However, we cannot rule out that gastral pilosity/ pubescence is highly variable in this group, and if so, our hypotheses may not reflect real species boundaries.

Diagnosis
Eleven-segmented antennae; anterior clypeal margin medially impressed; frontal carinae always well developed, diverging posteriorly and usually approaching or reaching corners of posterior head margin; antennal scrobe present, weakly to very well developed; mesosoma moderately to strongly marginate from sides to dorsum; propodeal spines medium-sized to long, elongate-triangular to spinose; propodeal lobes triangular and short; petiolar node in profile high rounded nodiform to rectangular nodiform with moderately to well-rounded margins, in profile 1.5 to two times higher than long (LPeI 50–68), in dorsal view between 1.0 and 1.4 times wider than long (DPeI 103–139), anterior and posterior faces parallel, anterodorsal and posterodorsal margins usually at about same height (sometimes anterodorsal margin higher); postpetiole in profile always more or less globular; mandibles variably sculptured, but mostly unsculptured; cephalic dorsum and dorsal mesosoma with distinct longitudinally rugose sculpture; waist segments and gaster always unsculptured, smooth, and shiny; dorsal surfaces of head, mesosoma, and usually waist segments with few to abundant long, standing hairs; pilosity/pubescence on first gastral tergite variable, but with tendencies to more inclined pilosity and dense pubescence; sting appendage spatulate.