Taxonomic history
See also: Alpert, 2007 PDF: 10.Figures 5, 6, 11, 12, 17, 18
Gregg (1958) described this species and provided an illustration of the petiole and postpetiole in lateral view. Additional figures are provided here of the entire ant for comparison purposes. Gregg based his description on two workers found within a series of termites collected 12 miles from Perinet, Madagascar on June 28 , 1935 by Harold Kirby . Kirby attached a locality code T- 4503, his record label T specifying a collection of termites. Many of Kirby's termite collections were later identified to species in a series of papers including Moszkowski's 1955 paper on Malagasy termites. T- 4503 however has never been listed as a locality code for Cryptotermes kirbyi, or any other termite, leaving in question the identification of the host termite for M. emersoni . One worker of M. emersoni has been collected subsequently from a more northern location, ( Toamasina, P. N. Mantadia , 18 & # 730; 47° 30' S , 48 & # 730; 25° 36' E , 895 m, rainforest, 25 Nov- 1 Dec 1998 , coll. H. J. Ratsirarson , HJR 020 , CASENT 0003116 , CASC ) .
A single queen assigned to M. emersoni was collected from the Andohahela Special Reserve ( 24 & # 730; 46° 35'S , 45 & # 730; 42° 19' E ), in southern Madagascar ( P. Rabeson , 4. II. 1993 , MCZC ). When M. emersoni is rediscovered and queens are collected in association with workers, then the status of this single queen can be confirmed .
Type locality: Perinet, Madagascar (H. Kirby)
Worker: Length, 8.33 mm.; head length (excluding mandibles), 1.58 mm.; head width, 1.25 mm.; head index 0.79; thorax length, 2.33 mm.
This species has many similarities to M. madagascarica , but as these traits are common to the genus Metapone and are given in detail in the foregoing description, they will not be repeated here. The most important features of this ant are as follows. Head about 1 1/4 times longer than broad, tapering slightly from the rear to the mandibular insertions, very convex both longitudinally and transversely, occipital border only weakly and broadly excised. Clypeus separated from the frons by a definite suture, subquadrate, though narrowing slightly at the anterior end, its median lobe projecting as a very short, anterior process, transversely truncated in front but not bidentate, with blunt but distinct anterolateral angles and notably concave lateral edges. The median lobe stands quite high above the lateral lobes, and is bordered by anterior extensions of the frontal carinae flush with its dorsal surface. At the antennal insertions, the carinae diverge sharply and then extend backward as strong ridges overarching rather deep facial scrobes which receive the antennae. Scapes fairly slender at the base and widening to broad, flattened expansions distally. Funiculus with joints 2 to 7 wider than long and gradually expanding in size toward the apex; club composed of the last three segments which are spatulate and concavo-convex as in madagascarica . Eyes reduced to a group of 8 to 10 minute, flat ommatidia at the posteroventral border of the scrobe, and % the distance from mandibles to occiput. Mandibles narrow, masticatory border rounded, bearing 7 weak teeth (approaching denticles), and without any trace of a basal lobe. Ocelli and ocellar pits absent. Frontal groove distinct.
Thorax subrectangular, exactly twice as long as wide, humeri well-developed, and the epinotum tapering slightly to its posterior border. Pro-mesonotal suture absent; meso-epinotal suture distinct and somewhat impressed. Dorsum of thorax straight longitudinally, convex transversely, with vertical, concave pleurae; strongly margined at the sides, and across the anterior border of the pronotum. Epinotal teeth reduced to rather sharp ridges; basal face of the epinotum a little longer than the declivity, and joining the latter through a rounded angle; posterior border of the epinotum transverse and slightly excavated. Petiolar node from above strongly transverse, exactly twice as wide as long; anterior border faintly excised, lateral borders diverging posteriorly to well-marked, but rounded and backward-pointing lobes, and the posterior border deeply excised; the whole structure subtrapezoidal in shape, and strongly margined on the front and sides; antero-posteriorly convex. In profile, the petiole has a short anterior peduncle, a thin, translucent, longitudinal, ventral plate or keel with a rounded edge, the anterior and posterior faces of the node concave, the entire node rising and flaring laterally through concave sidewalls to the flat summit that expands into wing-like lateral lobes. From before, the node is decidedly cuneate in shape. Postpetiole also strongly transverse, but barely twice as wide as long (slightly narrower than the petiole); subrectangular in outline, the anterior border straight, posterior border weakly convex, and the sides diverging noticeably to the rear; summit of node nearly flat; sides tapering ventrally but not concave, converging obliquely to the petiole-postpetiolar joint, and continuing into a stout, ventrally directed spine, which curves slightly backward.
Gaster of the usual shape in these ants, rounded anteriorly and sloping to a rather narrow point apically. First segment exactly twice as long as either the petiole or the postpetiole. Sting well-developed, protruding.
Legs similar to those of madagascarica . Femora moderately inflated, tibiae stout, but metatarsi slender.
Sculpture. - Entire head, including scrobes, longitudinally striate, except for the occiput; striae very fine on the clypeus and mandibles, heavier elsewhere on the cephalic dorsum and genae, coarse over the entire gula. Thorax longitudinally striate on the dorsum and pleurae, the sculpture of about the same texture as on the head. Head and thorax with scattered, hair-bearing punctures, and brightly shining. Petiole and postpetiole plentifully supplied with piligerous punctures, dorsally; shining. Gaster likewise punctate and shining, faintly shagreened or coriaceous.
Pilosity.- Very similar to that of the previously described species.
Color. - Reddish brown, the head and antennae darker, mandibles black; gaster lighter, yellow-brown toward the tip.
Holotype. - Worker; collected 12 miles from Perinet, Madagascar, on June 28, 1935, by Harold Kirby (?). As with the preceding species, they were recorded under a field number, T-4503, and presumably were associated with termites. Deposited in the author's collection.
Paratype. - One worker bearing the same collection data as the holotype.
In Wheeler's key, this species goes to couplet 5 because of the scarcely projecting median lobe of the clypeus, and thence to tillyardi because the petiole is broadly excised behind and the body is longitudinally striate. Though emersoni in morphology resembles tillyardi most closely, it can be distinguished from that species by lacking the small, blunt clypeal teeth, by the presence of 7 mandibular teeth instead of 5, a postpetiole which is virtualy as broad as the petiole (more transverse and less elliptical) and which has one stout, curved spine (instead of two transverse processes), and by its larger size (8.33 mm. vs. 5.5-6 mm.).
Prom bakeri it is easily separated by having 7 instead of 4 mandibular teeth, by the longitudinal striation of the head and thorax ( bakeri is very smooth and shining except for dense sculpture in the scrobes), the very broad petiole (petiole longer than broad and narrower than the postpetiole in bakeri ), and by its larger size (worker of emersoni 8.33 mm.; female of bakeri 6.4 mm.).
Metapone gracilis differs from emersoni by its 5-toothed mandibular dentition, but especially by its petiole which is about 1 1/3 times as long as broad, somewhat longer than high, and the anterior face of the node which is straight and perpendicular.
From jacobsoni , the new species can be recognized by its shorter and stouter head (1 1/4 longer than broad rather than 1 1/2), 7 instead of 5 mandibular teeth, and by its very broad petiolar node (twice as wide as long in contrast to 1 1/3 times as long as wide).
To distinguish it from johni , it is necessary only to compare the form and proportions of the petiole and postpetiole, both of which are strongly transverse and twice as wide as long in contrast to the petiole of johni which is trapezoidal, and the postpetiole which is transversely oval and about 1.6 times as wide as long; it also lacks the prominent ventral spine.
Finally, the two new species can he readily differentiated by the form of the anterior clypeal margin, the shape of the scapes, the epinotum, and particularly the petiole and postpetiole which are strikingly unlike in these ants.
It will be noticed that madagascarica and emersoni belong to that division of the genus in which the anterior clypeal lobe is short, only slightly produced over the mandibular bases, and either somewhat truncate or else bidentate. The preceding comparisons have all concerned other members of this group, while the remaining species of Metapone are in another division, represented by M. greeni , etc., and agree in having the clypeus notably extended, truncated in front, and furnished with sharp, tooth-like lateral corners.
A list of the known species of Metapone will serve to summarize the taxonomy of this group, and also to give some conception of its distribution so far as present information allows.
