Tetramorium dysalum species group

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Based on Hita Garcia and Fisher 2012.

Key to Tetramorium dysalum-group species

Tetramorium species groups

Most species of this group are restricted to rainforests or montane rainforests along eastern and northern Madagascar, and some species are also found on Nosy Be. However, one species seems to prefer montane shrubland.

The group is comparatively heterogeneous in its species composition, and it is likely that it does not represent a monophyletic group. The species T. ambatovy is a very unusual member of the group since it is the only species without mainly longitudinally rugose or rugulose sculpture on the mesosomal dorsum, which is present in all other species of the group. This character is generally very stable within this and other species groups, and in most species of the T. dysalum group well-developed with often very regular and distinct longitudinal rugae without any cross-meshes. Instead T. ambatovy has irregularly arranged rugulae that often cross each other. Another character that separates T. ambatovy from the others is eye size. In T. ambatovy they are comparatively large (OI 25 - 26) while they are usually much smaller in the remaining species of the group (OI 19 - 24). Therefore, it is possible that T. ambatovy is not a natural member of the T. dysalum group, and belongs either to another species group or represents an independent development. At present, we do not see any close affinities between T. ambatovy and other Malagasy groups, and considering most morphological characters, T. ambatovy fits best within the T. dysalum group. However, it cannot be ruled out that the species might be excluded from the group in the future. All other species seem to be morphologically much closer to each other, and most are comparatively similar to T. steinheili and T. dysalum. The identification of T. dysalum, T. mallenseana, T. sargina, T. steinheili, T. yammer, and T. vohitra is sometimes difficult.

The T. dysalum group should not be confused with another Malagasy species group with 11-segmented antennae, although several group members have character combinations that are morphologically very close to other groups. The petiolar node shape is comparatively flexible in the T. dysalum group since it can be squamiform, triangular, cuneiform, high rounded nodiform, or rarely almost nodiform with a well-developed and moderately sharp anterodorsal margin but a rounded posterodorsal margin that is situated much lower. The latter condition could be confused with the rectangular nodiform node shape seen in the T. plesiarum, T. ranarum, or T. tortuosum groups. However, all members of the T. plesiarum group have a distinct antennal scrobe, which is absent in the T. dysalum group. The T. ranarum group members that could resemble T. dysalum species in petiolar node shape have decumbent to appressed gastral pilosity, thus are unlikely to be confused with each other. More difficult is differentiating between larger T. dysalum specimens and a few species in the T. tortuosum group (T. pleganon Bolton and few undescribed species) due to their very similar petiolar node shapes and general appearance. Nevertheless, T. pleganon and allied species have either a strongly sculptured petiolar node, a sculptured first gastral tergite, or both, while the larger specimens of T. dysalum display very little sculpture on the petiolar node and none on the first gastral tergite.

Some species of the T. dysalum group with a high rounded nodiform petiolar node in profile could be confused with some members of the T. naganum or T. ranarum groups. However, the species in the latter two groups that could be confused have decumbent to appressed pilosity on the first gastral tergite, whereas the pilosity in the species in question in the T. dysalum group is erect to suberect.

Within the Malagasy Tetramorium with 11-segmented antennae, the groups with reduced sculpture on head and mesosoma, T. bessonii, T. marginatum, T. tsingy, and T. weitzeckeri, differ significantly from the T. dysalum group, which always has distinct sculpture on head and mesosoma. The T. naganum, T. schaufussi, and T. severini groups cannot be confused with the T. dysalum group since the former groups usually have decumbent to appressed gastral pilosity which is often completely reduced, whereas the gastral pilosity of the T. dysalum group is usually erect to suberect and never reduced. In addition, the T. bonibony group, despite having a triangular or cuneiform petiolar node, is easily differentiated from the T. dysalum group thanks to the well-developed anterior face of the mesosoma and/or the distinct reticulate-rugose posterior head and anterior pronotum in the T. bonibony group. Furthermore, most species of the T. ranarum and T. tortuosum groups (except the ones mentioned above) have a distinctly rectangular nodiform petiolar node shape which is not developed in the T. dysalum group. The absence of a sharply defined antennal scrobe in the latter separates it also from the T. plesiarum group. Finally, the T. kelleri group with its single representative is very unlikely to be mistaken for any T. dysalym group member because of the clublike petiolar node shape in T. kelleri.

Diagnosis

Eleven-segmented antennae; anterior clypeal margin medially impressed; frontal carinae usually well-developed and ending at or shortly before posterior head margin, rarely ending halfway between posterior eye margin and posterior head margin; anterior face of mesosoma not well-developed; mesosoma moderately to strongly marginate dorsolaterally; mesosoma in profile comparatively high (LMI 38 - 46); propodeal spines medium-sized to long, elongate-triangular to spinose; propodeal lobes variable, from very short and inconspicuous to comparatively long and spinose; petiolar node in profile squamiform and anteroposteriorly compressed to high nodiform, in profile much higher than long, anterior and posterior faces generally approximately parallel with anterodorsal and posterodorsal angles situated at about same height, in few species posterodorsal angle lower than anterodorsal, then dorsum tapering backwards posteriorly, in dorsal view always distinctly wider than long, often transverse; postpetiole approximately rounded to weakly anteroposteriorly compressed; mandibular sculpture variable; cephalic dorsum between frontal carinae with predominantly longitudinal sculpture, rarely irregularly rugulose, posterior head and mesosoma with well-developed longitudinal sculpture, rarely with irregular rugulae; waist segments generally unsculptured, in a few species weakly but distinctly sculptured; gaster unsculptured, smooth, and shiny; all dorsal surfaces of head, mesosoma, waist segments, and first gastral tergite with abundant suberect to erect hairs; sting appendage spatulate.

References

  • Hita Garcia and Fisher. 2012. The ant genus Tetramorium Mayr (Hymenoptera: Formicidae) in the Malagasy region—taxonomy of the T. bessonii, T. bonibony, T. dysalum, T. marginatum, T. tsingy, and T. weitzeckeri species groups. Zootaxa 3365: 1-123. PDF