Tetramorium mixtum species group
A group that has had seen its validity rise and fall, but it remains as a useful group as of 2017...
Agavekar et al. (2017) - Twelve-segmented antennae; anterior clypeal margin unspecialized, usually entire, rarely notched; eyes of moderate size; antennal scapes short, not surpassing posterior head margin; antennal scrobes variably developed; frontal carinae well-developed and surpassing eye level; base of first gastral tergite not concave in dorsal view, without tubercles or teeth on each side; pilosity on dorsal surfaces of body erect with long and fine hairs; sting appendage dentiform (Fig. 18).
Bolton (1977) defined and included 4 species in this Oriental group. Roncin (2002), describing two new species, abandoned this group and transferred the species to other groups. Agavekar et al 2017 then revived the group.
Agavekar et al. (2017) - This is a small group with seven species occurring in India, Sri Lanka, and Vietnam. The group was initially proposed by Bolton (1977) to include the species with a strongly concave base of the first gastral tergite that do not belong to the Tetramorium inglebyi group. Its validity was questioned by Roncin (2002) who while describing two new species of the group from Vietnam opined that the Tetramorium mixtum group is artificial and its members should be placed in different groups. Without going into detail, we admit that Roncin (2002) was likely correct in doubting the Tetramorium mixtum group as a whole. However, it is quite probable that the four Indian and the Sri Lankan species form a natural group based on their shared morphology, whereas the species from Vietnam might belong to another Indomalayan group. Consequently, we prefer to maintain the Tetramorium mixtum group and defer any decision of its validity until a molecular phylogenetic analysis is conducted on this group.
Roncin (2002) - Bolton defined this group by the combination of the following characters: 12-segmented antennae; frontal carinae extending back beyond the eyes; eyes of moderate size; and especially the development of a pair of anterolateral tubercles (gastral horns) on the first tergite of the gaster which surround the posterior portion of the postpetiole in dorsal view. This structure is also visible in profile as a tooth protruding forward above the anterodorsal limit of the sternite. This last character that led Bolton to create the mixtum-group, is also shared with the the inglebyi group. But species of this latter group differ by their reduced frontal carinae and eyes, smaller node of petiole, shorter propodeal spines, and are more related to the palaearctic caespitum group. Some African species of the solidum group also exhibit a similar development of the anterior part of the first gastral tergite as figured by Prins (1973) for Tetramorium peringueryi dichroum (presently Tetramorium dichroum) and Tetramorium rutilum (a junior synonym of Tetramorium glabratum). However this group is not related to the Asiatic groups discussed in this paper.
Well-developed gastral horns are one of the defining features of the mixtum-group, as defined by Bolton (1977). There are large specific differences in the degree of development of these structures in the mixtum-group. The most developed tubercles occur in the largest species: Tetramorium rugigaster and Tetramorium mixtum, and the less developed ones, in the smallest species: Tetramorium transversarium and in the two new species described here (Tetramorium kieti and Tetramorium securis). In fact, the state of development of this character in transversarium, kieti and securis is transitional with that observed in species of the tonganum- or of the scabrosum group. Bolton (1977) already stated the close proximity of these latter groups to the mixtum-group. Tetramorium seneb of the tonganum group has indeed the anterolateral parts of its gaster angled/slightly dentate, as clearly as transversarium of the mixtum-group; this character is a little less marked but still present in T. kraepelini of the scabrosum group. The anterolateral gastral margin varies from regularly curved to angled and at the extreme markedly tuberculate in each of these groups, and cannot be used as a supra-specific diagnostic character. We therefore propose to abandon the mixtum-group and to transfer its species into the scabrosum and the tonganum groups, both groups being defined according to Bolton (1977), except for the gaster morphology. Even so, as already stated by this author, the tonganum and scabrosum groups remain weakly defined and are more taxonomically practical groups than monophyletic units.
- Bolton, B. 1977. The ant tribe Tetramoriini (Hymenoptera: Formicidae). The genus Tetramorium Mayr in the Oriental and Indo-Australian regions, and in Australia. Bulletin of the British Museum (Natural History). Entomology. 36:67-151.
- Roncin, E. 2002. Two new Tetramorium species (Hymenoptera: Formicidae) from Vietnam with a discussion of the mixtum, tonganum, and scabrosum groups. Sociobiology. 40:281-292.
- Gaurav Agavekar, G., Hita Garcia, F., Economo, E.P. 2017. Taxonomic overview of the hyperdiverse ant genus Tetramorium Mayr (Hymenoptera, Formicidae) in India with descriptions and X-ray microtomography of two new species from the Andaman Islands. PeerJ 5:e3800 (DOI 10.7717/peerj.3800).