The collection abbreviations follow Evenhuis (2014). The material upon which this study is based is located and/or was examined at the following institutions:

BMNH The Natural History Museum (British Museum, Natural History), London, U.K.

CASC California Academy of Sciences, San Francisco, California, U.S.A.

HLMD Hessisches Landesmuseum Darmstadt, Darmstadt, Germany

MCZ Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts, U.S.A.

MHNG Muséum d’Histoire Naturelle de la Ville de Genève, Geneva, Switzerland

NHMB Naturhistorisches Museum Basel, Basel, Switzerland

The Tetramorium ant fauna of the Malagasy region can be divided into 19 species groups that represent different major lineages within this hyper-diverse genus. However, not all groups are native to the region. A proper assessment of the biogeographical affinities of a region, such as Madagascar and its surrounding South West Indian Ocean island systems, is only possible if comprehensive knowledge on the origin and distribution of each species group is available. This is of special importance when dealing with hyper-diverse genera that possess hundreds of species and dozens of evolutionary lineages throughout most or all zoogeographical regions. Fortunately, in the case of Malagasy Tetramorium we are able to assess the whole fauna and classify the groups into native, exotic, or shared with the Afrotropical region. Six of the 19 groups are either completely exotic, contain partly global tramps, or species of African origin. Two of these, the T. bicarinatum and T. obesum groups, are only present in the region with a few very successful global tramp species that very likely originated in the Oriental and Indo-Australian regions (Bolton 1977, 1979; McGlynn 1999; Wetterer 2010; Hita Garcia and Fisher 2011). Four other groups are clearly Afrotropical in origin: the T. sericeiventre, T. setigerum, T. simillimum, and T. weitzeckeri groups. The Malagasy representatives of these groups are either species that have likely been recently transferred from eastern and/or southern Africa by humans, or species that are Malagasy endemics that have evolved from much older colonisation events from Africa, or combinations of both. The T. simillimum group in the Malagasy region is a good example of the latter. It contains two global tramp species of African origin (T. caldarium (Roger) and T. simillimum (Smith), one very widespread non-tramp species of African origin (T. delagoense Forel), as well as some species endemic to the Malagasy region (T. anodontion Bolton, T. scytalum Bolton, and a few undescribed species). The T. sericeiventre group has one species that is widespread in the southern Palaearctic, Afrotropical, and Malagasy regions (T. sericeiventre Emery) and one species endemic to Madagascar (T. mahafaly Hita Garcia & Fisher). The T. weitzeckeri group is represented by T. humbloti Forel, a widespread species distributed in eastern and southern Africa that is probably a more recent introduction to the Malagasy region. The T. setigerum group, here recorded for the first time from the Malagasy region, contains one recently discovered species endemic to Madagascar that we describe below.

Most of the abovementioned, mostly non-native, groups possess twelve-segmented antennae and a triangular to dentiform sting appendage (except the T. weitzeckeri group, which has eleven-segmented antennae and a spatulate sting appendage). The only other Malagasy species group with twelve-segmented antennae is the T. tosii group, which seems to be endemic to Madagascar (Bolton 1979; Hita Garcia and Fisher 2011). In Hita Garcia and Fisher (2011) we discussed the very strong morphological similarities between the T. tosii group and some Afrotropical members of the T. setigerum group, which was not known from Madagascar at the time. At present, we propose to keep both groups separate until more data becomes available, but provide a more thorough discussion in the species group treatment of the T. setigerum group below.

One intriguing finding of the recent revisions (Hita Garcia and Fisher 2011, 2012a, 2012b, 2014b) is that the vast majority of native species groups (T. bessonii, T. bonibony, T. dysalum, T. kelleri, T. marginatum, T. naganum, T. plesiarum, T. ranarum, T. schaufussi, T. severini, T. tsingy, and T. tortuosum) described from Madagascar share the three following key characters: eleven-segmented antennae, anterior clypeal margin notched, and sting appendage spatulate. These twelve groups together contain more than 110 species that are endemic to Madagascar, plus three species found only on Mayotte or the Comoros. The only other group with eleven-segmented antennae, notched anterior clypeal margin, and spatulate sting appendage is the T. weitzeckeri group. However, as noted above, T. humbloti represents a recent arrival from the Afrotropical region and did not evolve independently in Madagascar (Hita Garcia and Fisher 2011). Not considering the latter group, most native species groups could have originated from one or a few ancient colonisation events, with subsequent adaptive radiation that is most pronounced in the humid forests of eastern and northern Madagascar. Assessing the geographic origins of the first colonists with eleven-segmented antennae appears challenging at first glance. A number of species groups have eleven-segmented antennae and a spatulate sting appendage in the Afrotropical, Oriental, and Indo-Australian regions. However, the species morphologically closest to the Malagasy groups are mostly found in Africa.

The most closely related ants seem to belong to the comparatively species-rich T. weitzeckeri species group (Bolton 1980; Hita Garcia et al. 2010). The group is very widespread and ecologically successful in sub-Saharan Africa, and has both eleven-segmented antennae and a spatulate sting appendage. Many of its species resemble a number of members of Malagasy groups, such as the T. bessonii group, parts of the T. bonibony group, parts of the T. dysalum group, and parts of the T. marginatum group. The older species of these groups were initially even placed in the T. weitzeckeri group (Bolton 1979) until recent rearrangements of the Malagasy species group system (Hita Garcia and Fisher 2011, 2012a, 2012b, 2014b). In Hita Garcia and Fisher (2011) we proposed to treat the Malagasy members (with the exception of T. humbloti that stayed in the group) as independent developments from the T. weitzeckeri group and created the new T. bessonii, T. bonibony, T. dysalum, and T. marginatum groups. These groups contain a high degree of morphological diversity often differing from the Afrotropical T. weitzeckeri group. One reason for the distinctiveness of the Malagasy groups was the shape of the petiolar node. We stated that in the T. weitzeckeri group the node is often squamiform with anterior and posterior faces approximately parallel, whereas in Madagascar this form of node is only present in few species of the T. dysalum group. Most other species of the groups in question have petiolar nodes that are anteroposteriorly compressed, often very strongly so, but with a more triangular to cuneiform shape, the node narrowing towards the dorsum, or the anterodorsal margin much more angled than the posterodorsal margin causing the dorsum to strongly taper backward posteriorly. The dorsum of the node is greatly reduced in a number of species, especially from the T. bonibony and T. marginatum groups. Despite being absent in the T. weitzeckeri group, this more triangular node shape is also found in the Afrotropical T. squaminode group, which is also predominantly East and South African. This group also has a spatulate sting appendage but twelve-segmented antennae, and despite this difference seems to be closely related to the T. weitzeckeri group (Bolton 1980; Hita Garcia et al. 2010). Nevertheless, the argument about the difference in node shape is less valid now after the recent description of T. mpala Hita Garcia & Fischer (Hita Garcia and Fischer 2014). This interesting species from Kenya belongs to the T. weitzeckeri group but has a more triangular squamiform node shape like in the Malagasy groups mentioned above. However, instead of reuniting several species groups with more than 70 species, we prefer to wait until more data is available. Morphological similarities in as diverse a genus as Tetramorium can be misleading, and better taxonomic resolution on a supraspecific level can only be achieved with a large-scale analysis that combines morphology with informative molecular phylogenetic or phylogenomic data. Regardless, the Malagasy T. bessonii, T. bonibony, T. dysalum, and T. marginatum groups are very likely part of a larger Tetramorium lineage that also includes the Afrotropical T. weitzeckeri and T. squaminode groups, even though the relationships remain unclear at present.

The Malagasy T. naganum, T. schaufussi, and T. severini groups (and parts of the T. dysalum group) also appear to have a strong African influence since they share a spatulate sting appendage, high nodiform petiolar node shape, and a lack of any sculpture on the waist segments with the South African T. grassii group, even though the latter group has twelve-segmented antennae. Bolton (1980) stated that the T. grassii group is related and possibly ancestral to the T. weitzeckeri and T. squaminode groups. We concur that these three groups are very likely closely related. Based on their unique morphology, the T. plesiarum, T. ranarum, and T. tsingy groups seem to be independent Malagasy developments since there are no species groups with similar morphology in any region. However, they are very probably also part of the same larger lineage as the other species groups with eleven-segmented antennae and a spatulate sting appendage. This leads us to hypothesise that there is a larger Afrotropical and Malagasy Tetramorium clade/lineage that contains the following groups: T. bessonii, T. bonibony, T. dysalum, T. grassii, T. marginatum, T. naganum, T. plesiarum, T. ranarum, T. schaufussi, T. severini, T. squaminode, T. tsingy, and T. weitzeckeri species groups. The situation for the T. kelleri and T. tortuosum groups is less clear. These two groups are very close and were separated recently on the basis of their distinctiveness in Madagascar (Hita Garcia and Fisher 2012b). However, the T. tortuosum group is present in the Neotropical, Afrotropical, Malagasy, Oriental, and Indo-Australian regions, and displays great differences in morphological diversity and species richness from region to region (Bolton 1977, 1979, 1980; Hita Garcia and Fisher 2012b, 2013). It is difficult to assess if all the species now listed as T. tortuosum group indeed belong to one very widespread, possibly very old, monophyletic clade, or some have evolved independently to share several key morphological characters. Consequently, we cannot assess with any certainty whether the Malagasy T. kelleri and T. tortuosum groups are more closely related to the African or Asian members of the T. tortuosum group, or represent a more independent lineage.

In summary, we were able to identify a highly unique Malagasy Tetramorium fauna. We recognise 12 of the 19 species groups and an astonishing 113 of the 125 species as Malagasy endemics. This results in an endemism rate of 93%, which is more or less in agreement with the published value for the whole Malagasy ant fauna (ca. 96% in Fisher 2003). In Table 1 we provide an overview of the Malagasy species groups with data on their biogeography, key characters, taxonomic revisions, and preferred habitats.

The species group key presented here is based on the one published in Hita Garcia and Fisher (2011). Although the key in that publication still works for most species in Madagascar, it does not accommodate them all. The recent revisions of most species groups, with the establishment of some new groups (Hita Garcia and Fisher 2012a, 2012b, 2014b), require an updated and improved key. The following key applies to the 19 groups we currently recognise, which contains around 125 species (the T. ranarum and T. simillimum groups will be revised in a future publication and the species count for these two groups is temporary).

Tetramorium cavernicola Hita Garcia & Fisher, sp. n.

Twelve-segmented antennae; antennal scapes very long (SI 120–123); anterior clypeal margin entire and clearly convex; frontal carinae well-developed, ending at or approaching posterior head margin; eyes moderate (OI 23–26); anterior face of mesosoma weakly developed, no distinct margination between lateral and dorsal mesosoma; propodeum armed with short triangular to elongate-triangular teeth (PSLI 7–11), propodeal lobes moderately developed, triangular to elongate-triangular, slightly longer and broader than propodeal teeth; petiolar node relatively small, nodiform, with weakly angled anterodorsal and posterodorsal margins, and comparatively long peduncle, petiolar dorsum flat to very weakly convex, node in profile between 1.2 to 1.4 times higher than long (LPeI 73–79), node in dorsal view between 1.2 to 1.3 times longer than wide (DPeI 121–127); postpetiole in profile approximately globular, around 1.0 to 1.1 times higher than long (LPpI 90–98); mandibles striate; clypeus longitudinally rugose/rugulose with well-developed median ruga and usually one or two weaker, sometimes irregular, lateral rugae/rugulae on each side; sculpture on cephalic dorsum irregularly longitudinally rugose to reticulate-rugose; mesosoma laterally irregularly rugulose, dorsally reticulate-rugulose to irregularly rugulose; petiole and postpetiole conspicuously rugulose; ground sculpture on mesosoma and waist segments distinctly reticulate-punctate, much weaker on head; gaster unsculptured, smooth, and shiny; all dorsal surfaces of body with short to moderately long, thick, and apically blunt pilosity; sting appendage triangular to dentiform.

Prior to this study, the T. setigerum species group appeared endemic to the Afrotropical region where it is widely distributed. Of the 13 species recognised by Bolton (1980), most are found in more arid areas of eastern and southern Africa, a few are distributed in the rainforests of Central Africa, while two species are also found in Ethiopia and the southwestern Arabian Peninsula. The recent finding of T. cavernicola in Madagascar was unexpected since there was no previous indication of the presence of the group on Madagascar or any of the surrounding islands of the South West Indian Ocean. However, as outlined above, considering the strong biogeographical affinities of the Tetramorium ant fauna of Madagascar with the Afrotropical region, this is less of a surprise. Indeed, the T. setigerum group has its highest abundance and diversity in South and Southeast Africa, which is geographically comparatively close to Madagascar. As outlined above, other species or species groups that made it from Africa to Madagascar are often of predominantly eastern and southern African origin; examples include T. humbloti from the T. weitzeckeri group and T. delagoense from the T. simillimum group.

The T. setigerum group cannot be mistaken for any other Malagasy species group. Its possession of twelve-segmented antennae, an entire and convex clypeal margin, and simple pilosity distinguish it from most other groups, except the T. sericeiventre, T. simillimum, and T. tosii groups. In the T. sericeiventre group the clypeus is distinctly modified, with the lateral portion being very prominent and raised into a tooth/denticle in full-face view while the clypeus of the T. setigerum group lacks such a tooth/denticle. Also, the species of the T. simillimum group possess much shorter antennal scapes (SI always much shorter than 100) than the T. setigerum group (SI over 120). The differentiation of the latter from the T. tosii group is more problematic. Despite the fact that the only representative of the T. setigerum group in Madagascar and the two species of the T. tosii group are easily separable (see key couplets 4 to 6), only a few morphological characters separate both groups if one also considers all members of the T. setigerum group from the Afrotropical region. Nevertheless, we prefer to keep both groups separate for the following reasons. First, the shape of the petiolar node is low, elongate, clublike, and always longer than high in the T. tosii group (Fig. 29A), whereas it is variably nodiform in the T. setigerum group, but usually higher than long and never low and elongate (Fig. 29B, C, D, E, F). Second, the standing pilosity in the T. tosii group consists of long, fine, acute hairs (Fig. 29A), whereas the pilosity in most members of the T. setigerum group is thick, short to moderately long, and usually blunt apically (Fig. 29B, E, F). Nevertheless, this is not the case in T. metactum Bolton and T. youngi Bolton since they have long and fine pilosity (Fig. 29C, D). This may seem contradictory, but leads to our next argument. Third, we strongly suspect that the T. setigerum group is not a monophyletic group, but might be composed of different lineages that share a number of morphological characters that have evolved convergently. For example, the morphology of T. cavernicola from Madagascar is certainly closer to the species complex around T. setigerum Mayr and allies (Bolton 1980) than to T. metactum or T. youngi. In sum, the relationships between the T. tosii and the T. setigerum groups, as well as within the latter group, remain unclear, and we prefer to keep the groups as they are until additional data can provide better resolution of the groupings.