Tetramorium marginatum, Forel, 1895

Garcia, Francisco Hita & Fisher, Brian L., 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, pp. 1-123 : 90-93

Tetramorium marginatum species group

Diagnosis

Eleven-segmented antennae; anterior clypeal margin medially impressed; frontal carinae well developed but generally of moderate length, usually ending approximately halfway between posterior eye margin and posterior head margin; anterior face of mesosoma not well developed and no distinct anterodorsal margin present; mesosoma moderately to strongly marginate, dorsum usually sharply separated from lateral mesosoma; mesosoma comparatively high (LMI 37–45); propodeal spines long to extremely long, and spinose; propodeal lobes triangular and short; petiolar node in profile generally triangular cuneiform or thickly cuneiform, usually strongly anteroposteriorly compressed dorsally, in profile much higher than long, anterior and posterior faces not parallel, often anterodorsal angle much better developed and situated higher than posterodorsal angle, dorsum then tapering backwards posteriorly, in dorsal view distinctly wider than long and transverse; postpetiole approximately rounded and weakly anteroposteriorly compressed; mandibular sculpture variable, but often reduced; cephalic sculpture to a large extent reduced and absent; mesosoma in most species completely unsculptured, in two species weak irregular longitudinal rugulae present; waist segments and gaster unsculptured, smooth, and shiny; all dorsal surfaces of head, mesosoma, waist segments and first gastral tergite with few to relatively abundant, whitish, long, fine standing hairs; never with short, dense, and appressed pilosity or pubescence; sting appendage spatulate.

Comments

The T. marginatum group is endemic to the island of Madagascar, and includes six species from which only T. marginatum was previously described. All species live in humid forest habitats, such as tropical rainforests, littoral rainforests, and montane rainforests.

All members of the group are fairly conspicuous and easily distinguishable from all the other Malagasy Tetramorium with 11-segmented antennae. The very strong reduction of sculpture on head, mesosoma, and waist segments, the generally well-developed margination from lateral to dorsal mesosoma, and the generally triangular or cuneiform petiolar node distinguish this group from others in the region.

The group can be further divided into the four species with a completely unsculptured mesosoma and very dark brown to blackish colouration, T. valky , T. hector , T. marginatum , and T. silvicola , and the two species, T. norvigi and T. shamshir , which are of yellowish to pale brownish colour and display a dorsally rugulose mesosoma.

Key to the species of the T. marginatum group (workers)

1. Mesosomal dorsum unsculptured, generally smooth and shining; body colouration usually a dark brown to black ( Fig. 128 ). 2

- Mesosomal dorsum irregularly to longitudinally rugulose; body colouration generally yellow, rarely a pale brown ( Fig. 129 )..................................................................................................... 5

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2. Head in frontal view distinctly much longer than wide (CI 87–90); propodeal spines long (PSLI 26–30) ( Fig. 130 ). T. silvicola

- Head in frontal view longer than wide to as long as wide, much wider than above (CI 92–100); propodeal spines long to very long (PSLI 32–52) ( Fig. 131 )............................................................................ 3

3. Petiolar node in profile thickly cuneiform and less anteroposteriorly compressed (LPeI 60–66; DPeI 134–147) ( Fig. 132 ).................................................................................................. T. hector

- Petiolar node in profile triangular cuneiform to squamiform and much more anteroposteriorly compressed than above (LPeI 16–47; DPeI 189–500) ( Fig. 133 )......................................................................... 4 4. Few pairs of hairs present on mesosomal dorsum, but generally restricted to dorsolateral margins of pronotum and mesonotum, rarely some hairs medially present at dorsal border between mesonotum and propodeum ( Fig. 134 ).......... T. marginatum

- Hairs on dorsal mesosoma more abundant than above, scattered throughout whole dorsum, and not restricted to any margin ( Fig. 135 )....................................................................................... T. valky

5. Propodeal spines long to extremely long and always distinctly strongly back-curved; frontal carinae moderately developed and either ending at posterior head margin or shortly before; petiolar node in profile high nodiform to weakly cuneiform, not strongly anteroposteriorly compressed dorsally (LPeI 43–53; DPeI 163–184) ( Fig. 136 )...................... T. shamshir

- Propodeal spines generally long and usually straight-lined, rarely weakly back-curved; frontal carinae often weakly developed and ending between posterior eye margin and posterior head margin, generally closer to posterior eye margin; petiolar node in profile triangular cuneiform and strongly anteroposteriorly compressed dorsally (LPeI 26–41; DPeI 195–325) ( Fig. 137 ).................................................................................................. T. norvigi

Figures

FIGURES 128 & 129. 128. Unsculptured, smooth, and shining mesosomal dorsum of T. valky—CASENT0496394 (Erin Prado 2010). 129. Irregularly to longitudinally rugulose mesosomal dorsum of T. shamshir—CASENT0467696 (Erin Prado 2010).

FIGURES 130 & 131. 130. Head of T. silvicola in frontal view distinctly much longer than wide—CASENT0042828 (Erin Prado 2010). 131. Head of T. hector in frontal view moderately longer than wide—CASENT0038460 (Erin Prado 2010)

FIGURES 132 & 133. 132. Petiolar node of T. hector in profile with a thickly cuneiform and less anteroposteriorly compressed shape—CASENT0038460 (Erin Prado 2010). 133. Petiolar node of T. valky in profile with a triangular cuneiform and and much more anteroposteriorly compressed—CASENT0496394 (Erin Prado 2010).

FIGURES 134 & 135. 134. Mesosoma of T. marginatum in profile with scarce pilosity, which is generally restricted to dorsolateral margins—CASENT0102398 (April Nobile 2005). 135. Mesosoma of T. valky in profile with abundant pilosity, which is not restricted to any margin—CASENT0496394 (Erin Prado 2010).

FIGURES 136 & 137. 136. Mesosoma and waist segments of T. shamshir in profile, propodeal spines strongly back-curved and petiolar node high nodiform to weakly cuneiform, not strongly anteroposteriorly compressed dorsally—CASENT0467696 (Erin Prado 2010). 137. Mesosoma and waist segments of T. norvigi in profile with triangular cuneiform and strongly anteroposteriorly compressed dorsally petiolar node—CASENT0489037 (Erin Prado 2010).