Stigmatomma trigonignathum

Stigmatomma trigonignathum is one of the most rarely collected species in the world. Described from a single individual collected in 1948, and observed a second time in 2008.

Identification
Distinguished from Stigmatomma pallipes, the only other eastern Stigmatomma, by the triangulate basal half of the mandibles, the small, regular, clypeal teeth, the almost straight (not strongly convex) clypeal margin, and the lack of conspicuous, sharp projections flanking the outer bases of the mandibles. The distinguishing feature of this species are the two enlarged blunt teeth, fused at their base, situated at the basal/apical border of the inner margin of each mandible.

Although this species is only known from a single specimen, it is clearly a distinct species, not some weird variant of S. pallipes. The kinds of modifications of the mandibles, clypeus, and the sculpture of the head, and other features described by Brown (1949) are most unlikely to be a single genetic or developmental aberration, nor are they "reduced" features of S. pallipes. Stigmatomma pallipes itself, as mentioned above, shows remarkably little structural variation through its large range, or between individuals of different sizes, so there is no pattern of variation in S. pallipes that one might extrapolate to produce S. trigonignathum as an extreme variant.

Distribution
Known only from a handful of collections in North Carolina.

Distribution based on Regional Taxon Lists
Nearctic Region: United States.

Biology
Little information is known about this species. Workers have been collected from leaf litter samples (Brown 1949) and under rocks during the cooler period of the year in October and January.

Castes
Known only from the worker caste.

Nomenclature

 *  trigonignathum. Amblyopone (Stigmatomma) trigonignatha Brown, 1949c: 81, fig. 1 (w.) U.S.A. Combination in Stigmatomma: Yoshimura & Fisher, 2012: 19.

Description
Holotype worker: Total length measured from lateral profile, mandibles included but sting excluded, 6.12 + .10 mm.: Weber's length of alitrunk, 1.60 + .05 mm.; maximum measurable length of the head from the center of the anterior clypeal border to a line connecting the posterior extremities of the occipital corners, 1.22 + .005 mm.; maximum width of head, 1.05 + .005 mm.; cephalic index, 86 + 1; left mandible, straightline distance, when closed, from the point of contact with the anterior border of head to apex, 0.80 +.01 mm., or, more roughly, about two thirds of the length of the head proper.

Head a little more slender than in Stigmatomma pallipes, sides gently convex, greatest width at about the anterior third, slightly convergent behind and passing into the rounded occipital corners through easy curves; posterior border of head moderately but distinctly concave in outline. "Amblyoponine teeth" at the anterolateral corners of the head reduced to small, bluntly rounded tubercles which are more or less hidden in dense pilosity; this reduction much greater than in any small specimens of the pallipes complex I have seen. Clypeus dorsally weakly convex, its anterior apron rather narrow, with a very feebly convex anterior border which appears straight at some angles of view. This apron is rather abruptly terminated on each lateral extremity by an angle which marks the boundary between position as in medium-sized pallipes workers, with only 3 or 4 of the facets actually pigmented in each. A faint median depression on the cephalic dorsum at about the midlength evidently represents a vestige of the anterior ocellus.

Mandibles distinctive; considering only the basal halves each forming an obtuse triangle, with the inner border of the basal part just before the midlength distinctly angulate at somewhat more than a right angle; this angle, which is capped by two massive, blunt double teeth partially fused at their bases, marks off fairly distinct basal and apical borders. The basal border bears two rows of teeth, a dorsal and a ventral; there are two teeth in each row, the ventral ones triangular, acute, the dorsal low and rounded and alternating with the ventral ones so that all may be seen from a position directly dorsad. The more basal of the two ventral teeth is the larger, and this fits into the notch mentioned above just lateral to the clypeal apron. The apical border distad of the two large double teeth at the angle with two more blunt, massive double teeth, which are well separated; these followed by a smaller acute tooth just before the acute, stoutly spiniform apex. Seen from the side, the mandibles are nearly twice as thick dorsoventrally as in pallipes, and not quite so strongly projecting anteriorly; the apices somewhat recurved.

The alitrunk is very similar to that of. small pallipes workers, but perhaps very slightly more slender; the rounded lamellae at the base of the propodeal declivity smaller. The petiolar node is very slightly longer than broad seen from above, much as in smaller workers of pallipes.

Sculpture throughout less pronounced than in pallipes, the head and thorax shining to the naked eye. Seen at very high magnifications, the dorsum of the head is sown with very small, regular punctures which are separated from each other by plane, shining surfaces which do not form the fine longitudinal costulation or rugulation seen in the forms of pallipes. Clypeus very indistinctly and irregularly longitudinally striate; mandibles striate longitudinally as in pallipes, but less regularly and distinctly. Sculpture of dorsum of alitrunk much like that of the head, but the sides posteriorly are longitudinally striae as in pallipes; propodeal declivity with a large central area devoid of transverse striae and smooth and shining.

Color rather uniform medium ferrugineous. Other characters of structure and pilosity within the range of variability shown by small to medium-sized pallipes workers from the eastern United States.