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Figures 1 - 2
Worker, measurements (mm), holotype (paratypes, n = 3): HW 0.48 (0.46 - 0.48); HL 0.57 (0.57 - 0.62); SL 0.37 (0.37 - 0 - 41); EL 0.03 (0.03); WL 0.72 (0.70 - 0.72); PL 0.25 (0.25); PPL 0.18 (0.18); GL 0.73 (0.73 - 0.80); TL 2.58 (2.58 - 2.61). Indices: CI 84 (74 - 84); SI 77 (77 - 89).
Head longer than wide. Sides of head parallel, very slightly wider anteriorly, broadly rounded into the posterior border, which is more or less straight. Mandibles with four teeth: the three apical separated from isolated basal by a large diastema. Maxillary palps 2 segmented. Median lobe of clypeus strong, bicarinate, narrowing posteriorly to narrow strip between frontal lobes. Clypeus with apical median seta and several paracarinal setae. Antenna 12 segmented with strong 3 segmented club; scapes fail to reach the vertexal border. Antennal insertions close together. Frontal carinae and antennal scrobes absent. Eyes reduced to indistinct ommatidia, placed in front of head midline.
In lateral view promesonotum slightly convex. Metanotal groove very deep, well marked. Propodeum unarmed, with a faint cuticular crest, from the meeting point of dorsal and declivity faces, extended and slightly higher, continuous with the small metapleural lobes. Propodeal spiracle circular, opened posteriorly. Bullae of metapleural glands large. Petiole with peduncle and node well differentiated, the node more or less with the sides parallel, dorsum convex. Petiolar spiracle at beginning of node. Petiolar ventral process tooth-like. Postpetiole subcampaniform, with a ventral strong transverse carina. Apical portion of penultimate tergite with four small pegs or teeth, hairs arising from outermost.
Mandibles, most of promesonotum, dorsum of petiole and postpetiole and gaster smooth and shining. Head with longitudinal rugulae mixed with dense foveae. Posterior promesonotum with feeble short rugulae. Most of mesopleura with irregular short longitudinal striae, mixed with fine reticulation. Most of propodeum and sides of petiole and postpetiole densely reticulated. Declivity of propodeum with several fine transverse carinae, the most posterior more marked. Moderate erect pilosity on head, promesonotum, petiole, postpetiole and gaster, very few on propodeum. Scapes with several erect hairs. Larger hairs about 0.15 mm, those of head shorter. Body brown, appendages lighter.
Queen and male unknown.
Type data: Holotype worker, Brazil, Bahia, Barrolandia, 16 - 23.07.1994 (S. Lacau) (CEPLAC).
Paratypes, 1 worker, same data as holotype, deposited in ICN; 3 workers, Brazil, Amazonas, Benjamin Constant , 21. ix. 1962 (W. L. Brown Jr.) (MZSP); 2 workers, Ecuador, Napo , Limoncocha , 1973 (M. Rettenmeyer) (MZSP, BMNH).
Distribution: Ecuador and Brazil.
Etymology: The name refers to the unusual traits of the ant, and the taxonomical difficulties for their generic placement.
This species presents a series of unusual characters that are interesting in the context of the SolenopsidiniHNS, and especially MonomoriumHNS and its allied genera. The long diastema between the 3 apical teeth and the basal tooth does not appear to be common in the genus or any other close genera.
The same can be said for the much reduced eyes, since large, multifaceted eyes are common in the genus, although some Afrotropical species have reduced eyes (Snelling, personal communication). The narrow carina or propodeal crest that extends down to join the metapleural lobes is reminiscent of EpelysidrisHNS. The anterior and posterior sides of the petiolar node are more or less straight; in other species in this and neighboring genera, they tend to be rounded. The postpetiole has a strong transverse keel in the ventral part, which is reminiscent of AdelomyrmexHNS. The head sculpturing is also unusual, with longitudinal rugules mixed with obvious foveae, a trait which is absent in other Neotropical MonomoriumHNS. The most outstanding feature is the series of minute teeth or pegs in the last abdominal tergum, from at least the outermost of which arise hairs. This appears to be an autapomorphic structure, absent in other SolenopsidiniHNS and probably in MyrmicinaeHNS; it is somewhat like the pygidial pegs of CerapachyinaeHNS and Pachycondyla crassinodaHNS workers.
The Brazilian specimens are uniform in size and general aspect. Nevertheless, the two Ecuadorian specimens differ somewhat in size and some measurements (slightly shorter dorsal side of the propodeum, for instance) and the visible micropegs of the last tergum are less conspicuous than those of Benjamin Constant, Brazil. These structures are well-developed in the type specimen, from Bahia, but the propodeum is intermediate between the other Brazilian specimens and those from Ecuador. Given that in the other essential characteristics all samples are similar, I prefer to consider these differences as within-species variability, instead of creating a new taxon that might result in a paraphyletic species.
Is M. inusualeHNS truly a MonomoriumHNS? I recently was loaned two interesting myrmicines from CEPLAC. The first of these, a worker from Guaramiranga (Brazil, Ceara) is reminiscent in general form of the Australian M. sublamellatum, although without a single apical clypeal seta and clypeus so conspicuously bicarinate. The Brazilian specimen could pass as an unusual MonomoriumHNS. However, the discovery of a preoccipital carina — supposedly apomorphic in RogeriaHNS — could require the inclusion of the species in the latter genus, in spite of the fact that other characteristics do not coincide (Kugler, 1994). Or, alternatively, the Australian species possibly does not belong in SolenopsidiniHNS; Heterick (2003) places sublamellatum in MonomoriumHNS by default, since it does not fit in any other genus of the tribe. The paraphyletic nature of MonomoriumHNS renders it a “ basket ” or “ catch-all ” genus. Maybe sublamellatum, like this Ceara worker, belongs to StenamminiHNS (Bolton, personal communication).
The second CEPLAC specimen from Bahia (Brazil, S. Jose Victoria, No. 2139, provisionally determined as MonomoriumHNS sp.) has a general aspect typical of MonomoriumHNS, the genus to which the Bolton (1994) key leads, if one ignores the absence of the apical clypeal seta (which is variable in SolenopsidiniHNS). Nevertheless, as in the above case, this specimen might better be associated with some StenamminiHNS, possibly RogeriaHNS, although it is impossible to confirm the preoccipital carina in the unique dry-mounted specimen, and the total absence of color and other traits make it difficult to place in RogeriaHNS (as defined by Kugler, 1994).
Bolton (personal communication) suggests that M. inusualeHNS might be a member of StenamminiHNS, which was my first suspicion due to the overall resemblance with members of this tribe. The traits of frontal lobes and toruli, as well as clypeal posterior border, places inusuale in the StenamminiHNS as diagnosed in Bolton (2003: 58). However, the first gastral tergite clearly overlapping the first sternite on ventral surface, and the single medial clypeal seta puts the taxon in the SolenopsidiniHNS, again sensu Bolton (2003: 59 - 60). Although the solenopsidine tribe group (Bolton 2003: 57) may be a monophyletic group, the tribes proposed by Bolton (2003) as StenamminiHNS and SolenopsidiniHNS currently lack synapomorphies. Perhaps the mixed traits of inusuale, the Australian species, and the Brazilian specimens referred to above may justify merging the two tribes. On the other hand, moving these taxa from MonomoriumHNS to StenamminiHNS simply transfers the problem of generic allocation from one tribe to the other.
Bolton (2003) has clarified some of the uncertainties in the systematics of the myrmicines allied to SolenopsidiniHNS and StenamminiHNS. However, there are many problems to be resolved only when the limits of the Myrmicine tribes are clearly defined. For these reasons, I think that the only option at the moment is to leave inusuale as a member of MonomoriumHNS.