Tetramorium tortuosum species group

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The Tetramorium tortuosum group is widely distributed in the New and Old World with its main diversity centered in the Malagasy, Oriental and Indo-Australian regions.

With its 50 species this is one of the most species-rich groups of all the Tetramorium species groups (Bolton 1977, 1979, 1980, Marques et al. 2011, Vásquez-Bolaños 2007, Vásquez-Bolaños et al. 2011). The New World has only seven native Tetramorium species and all belonging to the T. tortuosum group (Bolton 1977, Marques et al. 2011, Vásquez-Bolaños 2007, Vásquez-Bolaños et al. 2011). The group is also well-represented in the Oriental and Indo-Australian regions with eight and ten species respectively (Bolton 1977, Sheela and Narendran 1998). The group attains its highest species richness in the Malagasy region, where 22 species are known (Hita Garcia and Fisher 2012b). Despite having the highest diversity of species and species groups within the genus Tetramorium, the Afrotropical region harbours only three T. tortuosum group species, which seems very low compared to the other regions as already noted by Bolton (1980).

Overview

Diagnosis

Eleven-segmented antennae; anterior clypeal margin medially impressed; frontal carinae well-developed and usually running to posterior head margin; anterior face of mesosoma weakly developed; margination between lateral and dorsal mesosoma generally well-developed; propodeal spines always long to extremely long, spinose, and acute (PSLI 28–72); propodeal lobes usually well-developed, triangular to elongate-triangular, generally short and acute, rarely strongly reduced to almost absent; petiolar node rectangular nodiform, anterodorsal and posterodorsal margins usually well-defined, anterior and posterior faces often parallel, node longer than wide in most species, broader than long in few species; postpetiole usually globular to subglobular; mandibles strongly sculptured in most species; head and mesosoma with distinct and predominantly longitudinally rugose sculpture; waist segments with distinctly rugose, rarely rugulose, sculpture; gaster unsculptured, smooth and shiny in many species, but sculpture present on the first gastral tergite in several species; in most species all dorsal surfaces of head, mesosoma, waist segments and gaster with abundant, long, standing hairs; sting appendage spatulate. (Hita Garcia and Fisher 2012)

The main characters that unite the species in the group are 11-segmented antennae, the spatulate sting appendage, and the approximately rectangular nodiform shape of the petiolar node. Other group characters of less diagnostic value are the comparatively large body size, an armed propodeum, sculptured mandibles, and generally sculptured waist segments. Taking into consideration the large number of species and species groups worldwide, the above-listed characters might not be sufficient to justify a natural group.

Within the Malagasy region, the T. tortuosum group is easily recognisable within the groups with 11-segmented antennae. The distinction from the T. kelleri group id given in the notes for that group. The T. plesiarum group is characterised by its distinct and conspicuous antennal scrobes with well-defined margins all-around. By contrast, the scrobes are usually developed, but less conspicuous and without well-defined posterior and ventral margins in the T. tortuosum group. Due to the presence of distinct sculpture on both waist segments, the group cannot be mistaken for the T. bessonii, T. marginatum, T. naganum, T. schaufussii, T. severini, T. tsingy, or T. weitzeckeri groups, nor parts of the T. bonibony and T. dysalum groups. The species of the T. bonibony group with sculptured waist segments have reticulate-rugose sculpture on the posterior head and anterior mesosoma which is not present in the T. tortuosum group. Most species of the T. dysalum group with sculptured waist segments usually have a weakly sculptured petiolar node which distinguishes them from the T. tortuosum group species. Tetramorium dysalum has more sculpture, and could be misidentified with T. avaratra or T. pleganon on the basis of petiolar node shape, but the latter species have sculptured mandibles and sculpture on the first gastral tergite, whereas T. dysalum possesses smooth mandibles and lacks sculpture on the first gastral tergite. Moreover, the T. ranarum group, despite sharing the rectangular nodiform shape of the petiolar node in some species, differs in several other important aspects, making it easily separable from the T. tortuosum group. The sculpture on posterior head and mesosoma is usually reticulate-rugose in the T. ranarum group but longitudinally rugose in the T. tortuosum group. Also, the propodeal spines are also much longer in the latter group than in the T. ranarum group. In addition, the frontal carinae and antennal scrobes are often much weaker in the T. ranarum group, and several species have one or even both waist segments unsculptured, whereas all Malagasy T. tortuosum group species have both waist segments noticeably sculptured.

The one complex from the Afrotropical region, the Tetramorium capillosum species complex, includes notes under its heading (see below) regarding distinctive features, within that region, in comparison to other Tetramorium.

Agavekar et al. (2017) provided the following diagnosis and notes for their revision of the Tetramorium of India. Eleven-segmented antennae; anterior clypeal margin notched and unspecialized; eyes of moderate size; antennal scapes variable, ranging from short to long; antennal scrobes variable, ranging from very reduced to strongly developed, in the latter case without clear margin all-around; frontal carinae always strongly developed; base of first gastral tergite not concave in dorsal view, without tubercles or teeth on each side; pilosity on dorsal surfaces of body erect, usually with long and fine, rarely short and thick, hairs; sting appendage spatulate.

The Tetramorium tortuosum species group is the most species-rich group of the genus with more than 50 described species, of which six are known from India. It is also the most widespread with native faunas in the Afrotropical, Malagasy, Indomalayan, and Australasian regions, as well as the New World. There are some considerable doubts if all of these represent indeed a monophyletic clade, and it is probable that the morphological similarities upon which they were placed in the same group are based on convergent evolution (Hita Garcia & Fisher, 2012a, 2013).

This group can be arranged into the following species complexes:

Tetramorium andrei species complex

As defined by Hita Garcia and Fisher (2012).

Malagasy

This species complex is the largest within the Tetramorium tortuosum group. The forecoxae are usually completely or mostly unsculptured, smooth, and shining, although sculpture is sometimes present. What sculpture is present is mostly superficial and generally does not consist of strongly longitudinally arranged rugae. Instead, it is reticulate-punctate with few superimposed rugulae or traces of rugulae. In a few species (e.g. T. elf, T. voasary) the forecoxae are partly rugulose, but the sculpture is comparatively weak, and never covers the whole coxa as in the T. smaug species complex. The sculpture may appear longitudinal at first glance, but this is due to sections of linearly arranged reticulate-punctate sculpture, not true rugae/rugulae (e.g. T. isoelectrum). The first gastral tergite is always completely devoid of any sculpture, and fairly smooth and shining.

Tetramorium capillosum species complex

As defined by Hita Garcia and Fisher (2013).

Key to the Tetramorium capillosum species complex

Afrotropical. A moderately restricted distribution range, all 3 species are known from Equatorial rainforests in the Central African countries of Gabon, Cameroon, Democratic Republic of Congo, Central African Republic, and Uganda.

Figure 7. Geographic distribution maps for members of the Tetramorium capillosum species complex. Star symbols represent type localities while rectangles represent non-type localities.

Given all of the known African localities, the distribution of T. capillosum and T. tabarum appears fairly disjunctive. Most localities are located in the west of the distribution range in Gabon, Cameroon, and western parts of the Central African Republic, but few localities are found much further east in the northeastern Democratic Republic of Congo and northwestern Uganda. This represents a great gap between these two groups of localities. However, we think that this lack of occurrence records is very likely due to a sampling artefact since ant sampling has been relatively fragmentary in sub-Saharan Africa. The westernmost known distribution limit is the eastern coast of the Gulf of Guinea and the easternmost known locality appears to be the Budongo Forest in northwestern Uganda. It is unlikely that they occur further east, which is supported by an inventory of the myrmecofauna of the Kakamega Forest in Western Kenya (Hita Garcia et al. 2009). The latter study yielded 40 species of Tetramorium in this rainforest locality but no member of the T. tortuosum group could be collected, even though the sampling effort was comparatively high. The same is true for West Africa. Several ant sampling projects were carried out northwest of the known distribution range of the group; e.g. in Ghana (Belshaw and Bolton 1993, 1994, Majer 1972, 1976a, 1976b, Room 1971), Nigeria (Taylor 1977, 1978), or Ivory Coast (Kone et al. 2012, Yeo et al. 2011). However, none of these projects collected a single T. tortuosum group member. Consequently, it is safe to say that the Afrotropical members of the group are all restricted to the Equatorial rainforests of Central Africa.

Diagnosis

Eleven-segmented antennae; antennal scape short to moderately long (SI 73 - 86); anterior clypeal margin usually entire without median notch; frontal carinae very well developed and usually reaching posterior head margin; antennal scrobe present, weakly to very well developed; propodeal spines medium-sized to long, elongate-triangular to spinose; propodeal lobes short, triangular to elongate-triangular; petiolar node in profile nodiform, in profile as high as long to 1.3 times higher than long (LPeI 78 - 100), in dorsal view always longer than wide (DPeI 80 - 93); postpetiole sub-globular to moderately anteroposteriorly compressed; mandibular sculpture variable; cephalic sculpturation distinct, between frontal carinae longitudinally rugose to reticulate-rugose; mesosoma predominantly longitudinally rugose; petiolar node weakly to distinctly rugose, postpetiole ranging from unsculptured to longitudinally rugose; gaster unsculptured, smooth, and shiny; all dorsal surfaces of body with abundant, long, standing hairs; first gastral tergite without pubescence and pilosity never short, dense, and appressed; sting appendage spatulate.

Identification

In the Afrotropical region, members of the T. tortuosum group are unlikely to be misidentified with species from the other three groups having 11-segmented antennae. The 26 species of the T. weitzeckeri group all have a squamiform or high nodiform petiolar node, which is always significantly wider than long. This node shape strongly contrasts with the shape observed in the T. tortuosum group since all three members have a nodiform node which is much longer than wide. The second species group, the T. angulinode group, is morphologically closer to the T. tortuosum group since both groups share a nodiform petiolar node. However, they can be clearly separated by the pilosity/pubescence patterns on the first gastral tergite. In the T. tortuosum group pubescence is absent and pilosity is long and mainly erect, whereas in the T. angulinode group pubescence and pilosity are usually present, dense, appressed to decumbent, and often pointed towards a longitudinal midline of the tergite. The synonymisation of Triglyphothrix under Tetramorium (Bolton 1985) added an additional group, the T. ericae group, with few species that possess 11-segmented antennae. However, these species are all comparatively small and have branched pilosity on most of the body, thus not easily confused with the much larger T. tortuosum group species that all possess simple pilosity.

Tetramorium jedi species complex

As defined by Hita Garcia and Fisher (2012).

Malagasy

The T. jedi complex is relatively small with just three species. It is characterized by the absence of sculpture on the forecoxae and the presence of reticulate-punctate sculpture on the first gastral tergite.

Tetramorium jedi does not seem to be morphologically close to the other two species of the complex, and they were grouped together on the basis of the reticulate-punctate sculpture on the first gastral tergite present in all three, even though the development of the sculpture from T. jedi to T. avaratra and T. pleganon is fairly different. The latter two, as mentioned above, may not be closely associated with any other T. tortuosum group members. Tetramorium jedi, however, might be distantly related to T. andrei and allies since the main separating character is the conspicuous sculpture on the first gastral tergite present in T. jedi but absent in the T. andrei complex.

Tetramorium noeli species complex

As defined by Hita Garcia and Fisher (2012).

Malagasy

The characters that define the complex are the lack of sculpture on the forecoxae and the presence of basigastral costulae on the first gastral tergite. The four species are morphologically close to one another and it is likely that they represent a natural group with shared common ancestry.

Tetramorium smaug species complex

As defined by Hita Garcia and Fisher (2012).

Malagasy

Characterized by the presence of very strongly developed longitudinal rugae on the forecoxae, and the absence of sculpture on the first gastral tergite (except in some specimens of T. manongarivo, which have costulate sculpture on the basal half of the first gastral tergite).

The species within this group can be further divided into those with massive propodeal spines and reduced hairiness (T. latreillei, T. sabatra, and T. smaug), while the remaining three species are much hairier and have less massive propodeal spines ('T. adamsi, T. marojejy, and T. nazgul).

References