Stigmatomma

Stigmatomma was long considered a junior synonym of Amblyopone (see Brown 1960) until it was recently revived (Yoshimura & Fisher, 2012). Members of this genus, as most amblyoponines, are specialised predators, which are thought to hold several ancestral anatomical and behavioural character states (Fisher, 2003). Stigmatomma species are known to live a hypogaeic lifestyle as predators of chilopods (Gotwald & Levieux, 1972) or occasionally other arthropods and in addition, are known as "dracula ants" that feed on their own larvae (Fisher, 2003; Saux et al., 2004). Queens can be observed to perform a form of non-destructive cannibalism by cutting a hole in the larval integument to feed on the exuding hemolymph. This however does not seem to harm the larvae, which continue growing and eventually emerge as normal adults. (Hita Garcia, Wiesel and Fischer 2013)

Identification
Head without mandible rectangular or trapezoidal mesosoma and gaster more or less cylindrical in form; mandibles elongate narrow, pointed and slightly curved at apex, middle to basal part of inner margin of mandible with triangular teeth arranged in two rows, or with more or less bifid teeth arranged in a single row, eye moderate to minute to vestigial, placed at the sides on above or below the mid length of head; antennae 11-12 segmented, filiform, the apex only slightly incrassate. Mesosoma narrower than head, promesonotal suture distinct, mesosoma strongly constricted at promesonotal suture and divided in to almost two equal halves; meso-metanotal sutue effaced; metanotum obliquely truncate posteriorly, basal portion passing into the apical portion by a more or less rounded curve, apical face of metanotum broadened, sides submargined; legs short, robust, tibiae of the posterior legs with two calcaria. Petiole cubical, broadly attached to ABIII, gaster narrow, not wider than the mesosoma, constriction between the basal two segments deep, giving the basal segment a nodiform appearance; sting exserted. (Bharti and Rilta 2015)

Within the United States workers and queens of Stigmatomma are recognized by their long, narrow jaws, set with sharp, triangular teeth. The margin of the clypeus has a row of small teeth, and the petiole is broadly attached to the gaster. Males also have the petiole broadly attached to the gaster, and lack the conspicuous punctures of male Platythyrea, the other poneromorph genus with a broadly attached petiole.

Species by Region
Number of species within biogeographic regions, along with the total number of species for each region.

Biology
Esteves and Fisher (2016) - Our understanding of the biology of the species assigned to Stigmatomma  is far from comprehensive as it is based on generalizations from limited observations of a few species. One of the major culprits for our lack of observations is the cryptobiotic lifestyle of these ants, which hampers access to their colonies and studies on their behavior (Ward and Fisher 2016).

Predominantly, Stigmatomma  species nests in the soil or in rotten logs of humid forest habitats (Brown 1960). Workers are usually solitary hunters (Masuko 1993, Traniello 1978), but Stigmatomma reclinatum, from Indomalaya region, has been found to recruit aid to recover prey (Billen et al. 2005, Ito 1993a). Stigmatomma prey upon other arthropods, especially geophilomorph centipedes (Gotwald and Lévieux 1972, Brown 1960)— observations indicate that up to 80% of the diet of Stigmatomma silvestrii (Palearctic region) is composed of such centipedes (Masuko 1993).

Larvae feed directly on prey when positioned on this food source (Brown 1960). We are not aware of any report of trophallaxis between larvae and adults of Stigmatomma . Instead, studies indicate that female adults perform nondestructive cannibalism on their own larvae. This practice, also known as Larval Hemolymph Feeding (LHF), consists of ingestion of hemolymph dripping from punctures made by adults in the larval integument. It was described for the Nearctic Stigmatomma pallipes and for Stigmatomma silvestrii (Haskins 1928, Masuko 1986, respectively). In the latter species, queens of mature colonies seem to feed exclusively on larval hemolymph. LHF was also reported for species of other Amblyoponinae genera (e.g., Amblyopone, Myopopone, Mystrium, and Prionopelta; Ito 2010, Ito and Billen 1998, Wheeler and Wheeler 1988).

The majority of the species produces winged gynes. However, some members of the Stigmatomma reclinatum species-group does not present a morphologically distinct queen, and reproduction is performed by gamergates (Ito 1993b, Ito 1991). Within these colonies, dominance is established through chemical and aggressive interactions (Ito 1993b).

Eguchi et al. (2014) - Vietnam - Stigmatomma spp. usually occur in well-developed forests but sometimes in forest edges. They nest under rotting logs, under stones, and in litter and soil. We often found their nests with remnants of small centipedes probably eaten by their larvae.

Within the United States most species are found in mesic areas, and forage underground or in litter; species of semiarid regions tend to remain deep in the soil and are seldom observed (Brown 1960). All species appear to be exclusively predatory. The prey is often large arthropods, especially centipedes: at least one species appears to be a specialized predator of geophilomorph centipedes (Gotwald and Levieux 1972). There is evidence that in some cases larvae may be transported to a large, paralyzed prey, rather than bringing the prey to the nest (Brown 1960).

While their primary prey is centipedes, James Trager observed Stigmatomma pallipes workers and larvae under a stone in Missouri USA, feeding on an Elateridae (beetle) larva, which appeared to have been killed by them (fresh, no external damage evident on the larva, except where the larvae were chewing into the intersegmental membranes).

Phylogeny
Note that Stigmatomma is not currently monophyletic and some species are more closely related to those of other genera than to each other.

Nomenclature

 *  STIGMATOMMA  [Amblyoponinae]
 * Stigmatomma Roger, 1859: 250. Type-species: Stigmatomma denticulatum, by subsequent designation of Bingham, 1903: 36.
 * Stigmatomma junior synonym of Amblyopone: Emery & Forel, 1879: 455; Mayr, 1887: 546.
 * Stigmatomma revived from synonymy: Dalla Torre, 1893: 14.
 * Stigmatomma subgenus of Amblyopone: Forel, 1900c: 55; Clark, 1934b: 27; Brown, 1949c: 87.
 * Stigmatomma revived status as genus: Bingham, 1903: 36; Emery, 1911d: 23; Creighton, 1950a: 31.
 * Stigmatomma senior synonym of Fulakora: Brown, 1949c: 87.
 * Stigmatomma junior synonym of Amblyopone: Brown, 1960a: 155.
 * Stigmatomma revived status as genus: Yoshimura & Fisher, 2012: 17.
 * Stigmatomma senior synonym of Arotropus, Ericapelta, Fulakora, Lithomyrmex: Yoshimura & Fisher, 2012: 17.
 * AROTROPUS [junior synonym of Stigmatomma]
 * Arotropus Provancher, 1881a: 205. Type-species: Arotropus binodosus (junior synonym of Typhlopone pallipes), by monotypy.
 * Arotropus junior synonym of Amblyopone: Provancher, 1887: 240; Brown, 1960a: 155.
 * Arotropus junior synonym of Stigmatomma: Yoshimura & Fisher, 2012: 17.
 * BANNAPONE [junior synonym of Stigmatomma]
 * Bannapone Xu, 2000b: 299. Type-species: Bannapone mulanae, by original designation.
 * Bannapone junior synonym of Stigmatomma: Ward & Fisher, 2016: 691.

Esteves and Fisher (2016) - Workers of Stigmatomma in the Malagasy bioregion – characters of the Amblyoponinae as described by Brown (1960) and the following characters:

1. Mandible elongate and linear, not as long as the head, pointed at the apex. Masticatory and basal margins running parallel to each other along baso apical axis, resulting in two rows of teeth. Teeth of the same pair generally basally fused.

2. Median portion of clypeal anterior margin anteriorly projected (generally convex). Anterior clypeal margin armed with single row of dentiform setae, arising from tubercle- like cuticular projections or from the flat cuticle. Pair of long setae on clypeus, generally arising from its anterior margin.

3. Genal teeth present or absent.

4. Number of antennomeres: 10–12.

5. Under the stereomicroscope, pilosity similar present on all antennomeres.

6. Palpal formula: 4:3; 4:2; or 2:2 (two maxillary and two labial).

7. Metanotal suture well developed to absent.

8. Mesepisternum generally divided into anepisternum and katepisternum.

9. Number of mesotibial spurs: 0–2.

10. Anterodorsal face of mesobasitarsus generally with a longitudinal sulcus.

11. Number of metatibial spurs: 1–2.

12. Anterior face of metabasitarsus generally without a longitudinal sulcus.

13. Pretarsal claw simple; arolium present on pro-, meso-, and metapretarsi.

14. Petiole (abdominal segment II) sessile. Subpetiolar process present; fenestra present or absent on its lateral face.

15. Constriction, generally scrobiculate, present between pretergite and postergite of abdominal segment III.

16. Prora present.

17. Scrobiculate constriction present between presclerites and postsclerites of abdominal segment IV.

18. Stout spiniform setae on apex of hypopygium present or absent.

Comments on worker characters: The list of characters above forms an inclusive diagnosis of the genus, but no character can currently be pointed as unique for Stigmatomma.

1. In Stigmatomma, the total dental count (including teeth arranged in pairs) recorded for Malagasy species is 11–15, distributed from base to apex as follows: 1–3 single teeth, followed by 3–6 teeth pairs (generally fused at the base), a pre apical (generally single) tooth, and an apical pointy tooth. Tooth number and arrangement may be constant within some species, but not for all species we evaluated: it varies within nest series and even between left and right mandibles of the same specimen. Given that, we did not use these characters alone to isolate individual species.

The most basal tooth is enlarged in the majority of species we studied, but not in all. This contradicts the opinion of Yoshimura and Fisher (2012b), which is that all Malagasy Stigmatomma species possess an enlarged basal tooth in their mandibles.

Teeth coupling generally occurs between teeth with similar dimensions. However, in two species (Stigmatomma bolabola and Stigmatomma sakalava), dorsal teeth increase in size towards the mandible’s apex. In that case, the dorsal tooth is smaller than the ventral paired tooth, but at the mandible's apex. This also contradicts Yoshimura and Fisher (2014) and Yoshimura and Fisher (2012b), who were of the opinion that dorsal teeth are smaller than ventral teeth in the XMMAS clade genera. In their view, the genus Amblyopone would generally present mandibles with no teeth pairs, but if teeth were present, the dorsal tooth would be larger than the respective ventral pair. A species noteworthy in this discussion is Stigmatomma pluto, whose mandible has no basal teeth paired with mandibular teeth, thus resembling the mandible of Amblyopone.

Among the other Amblyoponinae genera distributed in the Malagasy bioregion: Prionopelta  has short and subtriangular mandibles, which are usually armed with three teeth on the apical half, so that basal and mastigatory margins are distinct. The mandibles of Mystrium are similar to those of Stigmatomma in their indistinct basal and mastigatory margins, but are longer than its head, and have blunt apex (Bolton 1994). Also in Mystrium, the ventral row of teeth is set far apart from the dorsal row (Yoshimura and Fisher 2014). Adetomyrma and Xymmer, like Stigmatomma, present mandibles that shorter than the head, with indistinct basal and masticatory margins and a pointy apex. While teeth are not disposed in pairs along the mandibles of Adetomyrma (Yoshimura and Fisher 2012b), the mandibles of Xymmer do have pairs of teeth.

In addition to the similarities and differences among the shape and configuration of the mandibles, an enlarged mandibular basal tooth is absent in all other Malagasy Amblyoponinae genera (Yoshimura and Fisher 2012a, Yoshimura and Fisher 2012b).

2. Number and configuration of clypeal cuticular processes and associated dentiform setae vary among the evaluated species of Stigmatomma. All species present three to ten cuticular processes on the anterior margin of the clypeus. Each medial process bears one dentiform seta.

In half of the species (tsyhady species-complex members and Stigmatomma janovitsika), the seta on the lateral-most process is laterodistally followed by a row of dentiform setae. These lateral rows extend laterad on the anterior clypeal margin, where it arises from flat cuticle. In few species (S. bolabola and S. sakalava), the lateral-most cuticular process is smaller, and does not bear any dentiform setae. S. besucheti presents three medial cuticular processes that are followed laterodistally by a notch on the anterior clypeal margin. This notch is succeeded by a row of dentiform setae arising from flat cuticle (or from reduced cuticular processes.

However, the number of medial cuticular processes may vary within some species and sometimes within nest series. Thus, we did not use such variations to isolate individual species.

Among the other Amblyoponinae genera present in the Malagasy region, Mystrium, like Stigmatomma, also presents a single row of cuticular projections bearing dentiform setae on the anterior clypeal margin. On the other hand, Xymmer has neither specialized setae nor cuticular tubercle- like projections; in Adetomyrma, all dentiform clypeal setae arise from flat cuticle; and Prionopelta seems to have cuticular projections welded onto an anterior clypeal apron.

A pair of long setae is present on the anterior margin of the clypeus of all genera in the XMMAS clade in the Malagasy region, however, they are reduced and stouter in Mystrium.

3. The presence or absence of genal teeth is uniform within Stigmatomma species, and this character has relative importance to group species with similar morphology. In the Malagasy bioregion, this trait is present in all Mystrium species and absent in Adetomyrma, Prionopelta, and Xymmer.

4. Despite the variation among species, the number of antennomeres is constant within the Stigmatomma species we studied. Adetomyrma, Mystrium, and Xymmer species present no variation for this character, with all having twelve antennomeres.

5. Under the stereomicroscope, the whole antenna is equally covered by setae in Adetomyrma, Prionopelta, Stigmatomma, and Xymmer . In Mystrium, the four apical-most antennomeres are covered with denser pilosity. SEM images show that the apical antennomeres in Mystrium are actually covered by a different type of setae (ANTWEB1008554).

6. Without dissection, the maxillary and labial palpomeres are often extremely difficult to count in the species we studied.

Regarding the number of maxillary and labial palpomeres in other Amblyoponinae members in the Malagasy region, the palpal formula is constant within Mystrium (4:3) and Prionopelta (2:2) (Yoshimura and Fisher 2014), but not in Adetomyrma and Xymmer.

The palpal formula published for the Adetomyrma worker caste is 3:3, but some species are only known by the male caste, which, depending on the species, may present palpomere counts of 2:2 and 2:3 (Yoshimura and Fisher 2012a). The palpal formula for Xymmer males is 4:3/3:3/3:2 (Yoshimura and Fisher 2012b). Since published records indicate that the number of palpomeres is generally constant across castes of Amblyponinae species (Brown 1960), we expect the females of Xymmer and Adetomyrma to reflect a similarly diverse combination.

Finally, Yoshimura and Fisher (2012b) presented 4:3/4:2/3:3 as palpal formula for Stigmatomma males in the Malagasy region, differing from the numbers we counted for workers. However, mouthpart dissections on several male specimens of the same morphotypes used by Yoshimura & Fisher revealed that, for Stigmatomma, the number of palpomeres is the same in males and females (4:3/4:2; not evaluated for S. besucheti, as males are unknown).

7. The presence or absence of the metanotal suture, and the degree of its impression, may vary within species, as well as within nest series of Stigmatomma in the Malagasy region. Given this, we did not use those variations to isolate individual species.

9. The number of mesotibial spur(s) is difficult to determine under stereomicroscopes when the anterior spur is reduced in size, and also because the posterior spur may be “replaced” by an enlarged, stout spiniform seta. SEM images allowed comparisons between the texture of enlarged spinifom processes and surrounding cuticle, thus enabling us to differentiate spur and seta.

In the Stigmatomma we studied, the number of mesotibial spurs ranged from zero to two, and were generally constant within species. In one species, Stigmatomma liebe, the number of mesotibial spurs visible under the stereomicroscope ranges from one to two. The anterior spur may be visible and developed, but it is vestigial in the majority of the specimens we evaluated. This variation was observed in specimens from the same nest series.

Regarding other members of the XMMAS clade, Stigmatomma pallipes (Nearctic region), Stigmatomma pluto, Adetomyrma caputleae (Malagasy region), Fulakora mystriops (Neotropical region), Myopopone castanea (Indomalaya and Australasia regions), and Xymmer muticus (Afrotropical region) have two mesotibial spurs. Adetomyrma venatrix Malagasy region) possesses one spur (Ward 1994), as well as Fulakora chilensis (Neotropical region) and Mystrium voeltzkowi (Malagasy region). All Xymmer morphospecies from Madagascar evaluated under a stereomicroscope presented one spur/stout seta on the apex of the mesotibia. One species clearly seems to have a spur, while the others apparently present an enlarged stout seta.

Among the Amblyoponinae genera outside the XMMAS clade, Amblyopone australis (Australasia region), Amblyopone mercovichi (Australasia region), and Apomyrma stygia possess two mesotibial spurs. Onychomyrmex doddi (Australasia region) possesses two vestigial spurs at the apex of the mesotibia. Each spur is a small, stout, conic seta totally or partially concealed by a fovea. Prionopelta aethiopica (Afrotropical region) and Prionopelta antillana (Neotropical region) have one vestigial spur, while Prionopelta concenta (Afrotropical region) presents no spurs on the mesotibia.

10. We confirm the presence of a longitudinal sulcus on the antero dorsal face of the mesobasitarsus in all species of Stigmatomma in the Malagasy region save S. tsyhady.

Within the XMMAS clade, this sulcus is present on the mesobasitarsus of Stigmatomma pallipes, S. pluto, Adetomyrma caputleae, Fulakora chilensis, F. mystriops, Myopopone castanea, and Xymmer muticus. We confirm present of this sulcus in only one Xymmer  species in the Malagasy region. However, this character is difficult to visualize under a stereomicroscope when specimens are too small, as it occurs with Xymmer species, and its presence or absence may be better evaluated with an SEM microscope. This sulcus is absent in all Mystrium species we evaluated in the Malagasy region.

The sulcus on the anterior face of the mesobasitarsus is absent in ''Amblyopone australis, A. mercovichi, Apomyrma stygia, Onychomyrmex doddi, Prionopelta aethiopica, P. antillana, and P. concenta''.

11. All Stigmatomma species present in the Malagasy bioregion present two well-developed metatibial spurs save S. liebe. In this species, the number of metatibial spurs visible under the stereomicroscope ranges from one to two. The anterior spur is visibly smaller than the posterior spur, and may be vestigial in some specimens. This variation was observed in specimens from the same nest series. A similar condition is found in Onychomyrmex hedleyi (Australasia region). In this species, metatibial spurs are vestigial and may be present or absent in specimens from the same colony (Brown 1960).

In the XMMAS clade, Stigmatomma pallipes, S. pluto, Adetomyrma caputleae, A. venatrix, Fulakora chilensis, F. mystriops, Myopopone castanea, Mystrium voeltzkowi, and Xymmer muticus possess two spurs on the metatibia. Amblyopone australis, A. mercovichi, and Apomyrma stygia possess two metatibial spurs. Onychomyrmex doddi possesses two vestigial spurs at the apex of the metatibia. These spurs are small, stout, conic seta totally or partially concealed by a fovea. Prionopelta aethiopica and P. antillana have one spur, while P. concenta presents no spurs.

12. Only one Stigmatomma species evaluated in this study (S. roahady) presents a longitudinal sulcus on the anterior face of the metabasitarsus. The metabasitarsus of S. besucheti, while not presenting a sulcus on its anterior face, possesses two raised, parallel, not- well -developed longitudinal carinae with convergent apexes on its dorsal face.

This sulcus is present on the metabasitarsus of Myopopone castanea, and absent in Stigmatomma pallipes, S. pluto, Adetomyrma caputleae, A. venatrix, Fulakora chilensis, F. mystriops, and Xymmer muticus. It seems to be absent on the metabasitarsus of Xymmer in the Malagasy region. However, we cautiously affirm that, since this character is difficult to visualize under a stereomicroscope when specimens are too small, like those of Xymmer, it would be better evaluated under higher magnification. This sulcus is absent in all Mystrium species we evaluated in the Malagasy bioregion.

Among the Amblyoponinae genera that are not part of the XMMAS clade, the sulcus on the metabasitarsus is absent on Amblyopone australis, A. mercovichi, Apomyrma stygia, Onychomyrmex doddi, Prionopelta aethiopica, P. antillana, and P. concenta.

13. Arolium present on pro-, meso-, and metapretarsi in all species we studied. The same seems to apply to the other Amblyoponinae genera in the Malagasy region.

14. The petiole is sessile to sub sessile in Adetomyrma and Mystrium; and subsessile to penduculate in Xymmer. Also, within the XMMAS clade in the Malagasy region, Xymmer is the only genus in which the subpetiolar process is absent.

15. The constriction between pretergite and postergite of the abdominal segment III is scrobiculate in all Stigmatomma species but one. In Adetomyrma such a constriction is not visible; in Xymmer species the constriction is alveolate; and in Mystrium it is scrobiculate.

16. A prora is visible under a stereomicroscope in all Stigmatomma and Mystrium species in the Malagasy region; it seems to be absent in Adetomyrma and Xymmer .

17. Adetomyrma does not possess a constriction between the presclerite and postsclerite of abdominal segment IV. The constriction is scrobiculate in Mystrium and Stigmatomma, and alveolate in Xymmer.

18. Stout spiniform setae may be located on the apex of the hypopygium, surrounding the sting. The number of setae varies from six to nine, when present in Stigmatomma species in the Malagasy region. This contradicts the opinion of Yoshimura and Fisher (2014), which states that the number of stout setae ranges from three to nine.

In the Malagasy region, all Mystrium species present two or four stout setae on the hypopygium (Yoshimura and Fisher 2014), while Xymmer and Adetomyrma have no such setae. Yoshimura and Fisher (2014) affirmed that two or four stout setae on the apex of the hypopygium are uniquely observed in Mystrium; however, ''Fulakora mystriops'' also presents four stout setae on the hypopygium.

Stout spiniform setae are also present on the hypopygium of Stigmatomma pluto (twelve setae), Fulakora chilensis (eight setae), and in Fulakora cleae and Fulakora agostii, which have ten setae each (both from the Neotropical region). These setae are absent in S. pallipes, the Neotropical Fulakora heraldoi, Myopopone castanea, Xymmer muticus, Amblyopone australis, A. mercovichi, Apomyrma stygia, Onychomyrmex doddi, Prionopelta aethiopica, and P. concenta.