Asphinctopone

Hawkes, 2010: Asphinctopone is a rarely encountered genus of ponerine ants, previously known only from the wet forest zones of West and central Africa. In the most recent revision of the genus Bolton & Fisher (2008) synonymised the three previously described species under Asphinctopone silvestrii Santschi and described one new but very closely related species, Asphinctopone differens. A very distinctive new species, Asphinctopone pilosa, is described from Tanzania and represents the first record of this genus from East Africa.

Identification
Schmidt and Shattuck (2014) - Asphinctopone is morphologically distinctive and unlikely to be confused with any other genus. Important diagnostic apomorphies of the genus include the complex clypeus (see description below), the long apical antennomere, the strongly impressed metanotal groove, the divided mesopleuron, and the lack of differentiated presclerites in A4. Additional apomorphies of Asphinctopone include characters of the subpetiolar process and the helcium (described in detail by Bolton & Fisher, 2008a). The presence of a small process on the basal mandibular margin and strongly impressed promesonotal suture, previously thought to be apomorphies, do not occur in all known species and are therefore not of use in diagnosing this genus (Hawkes, 2010).

Superficially, workers of Asphinctopone perhaps most resemble small Brachyponera, due to their similarly impressed metanotal grooves, strongly narrowed propodeal dorsa, round or ovoid propodeal spiracles, squamiform petioles, and absent or weak gastral constriction. These genera strongly differ in many characters, however, including those of the mandibles (triangular and with a basal pit in Brachyponera, subtriangular and with a unique process on the basal margin in Asphinctopone) and clypeus (broadly convex in Brachyponera, complex in Asphinctopone), their metatibial spur count (two in Brachyponera, one in Asphinctopone), and many other characters.

Key to Asphinctopone species.

Distribution
Schmidt and Shattuck (2014) - Asphinctopone is rarely collected and seems to be at low density where it occurs, but is widespread in central and western Africa with a single species known from eastern Africa, having been collected in the Ivory Coast, Nigeria, Cameroon, the Central African Republic, Ghana, Guinea, Gabon and Tanzania (Bolton & Fisher, 2008a; Hawkes, 2010).

Species richness
Species richness by country based on regional taxon lists (countries with darker colours are more species-rich). View Data



Biology
Asphinctopone is one of the most rarely collected and least known small ponerine genera of the Afrotropical region. Specimens are seldom found and most samples recovered consist of only one or two workers. As a measure of its rarity, a survey of leaf litter in Ghana (Belshaw & Bolton, 1994) recorded 43,824 individual ants, of which only 5 (about 0.01%) were Asphinctopone. Despite this rarity, the genus is widespread in wet forest zones in leaf litter, topsoil, pieces of rotten wood and rotting vegetation on the forest floor. One worker has been found foraging in a fallen, abandoned termitary (Dejean et al., 1996). Beyond this nothing is known of its biology. Its specialised morphology implies that it may be prey-specific, but in reality its victims remain unknown. (Bolton and Fisher 2008)

Castes
Males are not known for the genus.

Nomenclature

 *  ASPHINCTOPONE [Ponerinae: Ponerini]
 * Asphinctopone Santschi, 1914d: 318. Type-species: Asphinctopone silvestrii, by monotypy.
 * Asphinctopone senior synonym of Lepidopone: Brown, 1953h: 2.
 * LEPIDOPONE [junior synonym of Asphinctopone]
 * Lepidopone Bernard, 1953b: 207. Type-species: Lepidopone lamottei (junior synonym of Asphinctopone silvestrii), by monotypy.
 * Lepidopone junior synonym of Asphinctopone: Brown, 1953h: 2.

Schmidt and Shattuck (2014):

Santschi (1914) described Asphinctopone as a monotypic genus to hold his new species A. silvestrii. Bernard (1953) later erected the genus Lepidopone for his new species L. lamottei. Bernard differentiated his new genus from the obviously closely related Asphinctopone by supposed differences of the coxae, petiole and gaster. Brown (1953c) concluded that the justification for separating Lepidopone from Asphinctopone was weak, and synonymized them. Bolton & Fisher (2008a) revised the species-level taxonomy of Asphinctopone.

Schmidt (2013) was unable to include Asphinctopone in his phylogeny of Ponerinae, and the morphological traits of the genus give only a few clues to its phylogenetic position. Ouellette et al. (2006) included an unidentified Asphinctopone species in their 28S phylogeny of the poneroid subfamilies, and found weak support for a close relationship between Asphinctopone and Odontomachus or Anochetus, suggesting membership of Asphinctopone in the Odontomachus Genus Group. Morphological evidence does not give a strong indication of the phylogenetic relationships of Asphinctopone, though after considering the various possibilities we conclude that Asphinctopone most likely is a member of the Odontomachus Group, as suggested by Ouellette et al.’s (2006) molecular results. Bolton & Fisher (2008a) found similarities in the structure of the petiolar sternite and helcium in Asphinctopone, Phrynoponera, and Brachyponera, though they argued against these similarities representing synapomorphies of all three genera or any given pair of them. Still, the possibility cannot be rejected, especially given other superficial similarities between Asphinctopone and both Phrynoponera and Brachyponera.

The presence of only a single metatibial spur in Asphinctopone would be unusual among Odontomachus group genera, but some species of Anochetus have a similar reduction in spur count. Spur count is roughly correlated with body size, so the loss of the second spur in Asphinctopone could be the result of a reduction in body size, and would not exclude its placement in the Odontomachus group. Other morphological arguments for a placement in the Odontomachus group include the unconstricted gaster (present to a less extreme degree in most members of the group), the impressed metanotal groove (more commonly impressed in the Odontomachus group than in other genus groups), and the relatively large frontal lobes, which would argue against a placement in the Ponera group, nearly all of which have very small frontal lobes. A placement of Asphinctopone within the Plectroctena group is unlikely, given its posteriorly-opening metapleural gland orifice, impressed metanotal groove, and narrowed propodeal dorsum. The possibility of a close relationship between Asphinctopone and Hypoponera cannot be rejected, but is not supported by any particular putative synapomorphy. Finally, the biogeography of Asphinctopone (restricted to central and western Africa) lends credibility to a placement in either the Plectroctena or Odontomachus groups, which are apparently Afrotropical in origin. Weighing all the evidence, we find it most likely that Asphinctopone is simply an unusually small member of the Odontomachus group, and we therefore tentatively include it there.

Worker
Small (TL 3.3–3.7 mm) ants with the standard characters of Ponerini. Mandibles subtriangular, with five teeth, a small process on the basal margin near the mandibular articulation present in some species, and a faint basal groove. Clypeus projecting anteriorly, with a small rounded lobe medially, on either side of which is a shallow concavity and then an angular projection. Frontal lobes closely approximated and of moderate size. Antennae with the three or four apical antennomeres forming a weak club, the apical antennomere longer than the preceding five (or four) segments combined. Eyes small, located anterior of head midline. Promesonotal suture sometimes relatively deeply impressed, the metanotal groove always deeply impressed. Mesopleuron divided by a transverse groove. Propodeal dorsum strongly narrowed and relatively short, the posterior face relatively long. Propodeal spiracles ovoid. Metatibial spur formula (1p). Petiole squamiform, the scale thin in side view but broad in dorsal view. A4 without differentiated presclerites, and hence the gaster without a girdling constriction. Head and body shiny to very sparsely punctate, with sparse pilosity and pubescence. Color orange to reddish brown. See description by Bolton & Fisher (2008a) and Hawkes (2010) for further details and for discussion of additional important characters, such as those of the helcium and subpetiolar process.

Queen
Schmidt and Shattuck (2014) - Similar to worker but slightly larger, winged, with ocelli and larger eyes, and with the other characters typical of winged ponerine queens.

Bolton and Fisher (2008):

Diagnosis of worker and queen (gyne)

1 Mandible with 5 teeth; sometimes with an additional denticle between teeth 4 and 5.

2 Mandible oblique, not triangular; without a basal pit; with a weakly developed basal groove.

3 Masticatory margin of mandible somewhat oblique so that there is a gap between the mandibles basally when they are closed.

4 Basalmost tooth is at the rounded basal angle; basal margin shallowly convex; near the articulation the inner margin with a small tooth-like process (overlapped by anterolateral angles of clypeal lobe when mandibles closed).

5 Palp formula 3,3; second maxillary palpomere elongate and slender.

6 Clypeus complex: in full-face view the median portion projects anteriorly as a broad lobe which terminates in a distinct angle on each side; these angles overlap the basal margins of the mandibles; the anterior clypeal margin has a small median rounded projection, on each side of which the anterior margin is shallowly concave; above the median projection the central portion of the clypeus forms a narrow ridge to the frontal lobes.

7 Frontal lobes small, closely approximated, with only a linear median impression between them; in fullface view anterior margins of frontal lobes are behind anterior margin of clypeus.

8 Antenna with 12 segments, gradually incrassate apically and the terminal three segments form a weak club; apical antennomere hypertrophied, longer than the five preceding segments together.

9 Promesonotal suture deeply impressed, cross-ribbed on extreme anterior mesonotum; metanotal groove deeply impressed and cross-ribbed; mesonotum conspicuously isolated by these two impressions (worker only).

10 Mesopleuron with a transverse suture that divides it into anepisternum and katepisternum.

11 Metapleural gland orifice simple, posterolateral.

12 Propodeal spiracle very small, its sclerite almost circular but the orifice itself a small ellipse.

13 Propodeal lobes very reduced, rounded.

14 Mesosternal and metasternal processes present, the former narrowly bidentate, the latter narrowly bilobate and very long.

15 Mesotibiae and metatibiae each with a single pectinate spur.

16 Dorsal (outer) surfaces of middle and hind tibiae and basitarsi without cuticular spines or peg-like traction setae.

17 Pretarsal claws small and simple.

18 Petiole surmounted by a high, unarmed scale that is narrow in profile and broad in dorsal view.

19 Subpetiolar process complex: in profile with a short anterior prominence, a small submedian tooth and a posterior flange.

20 Sternite of petiole in posterior view complex (see discussion below).

21 Helcium with a narrow transverse crest of cuticle between the ventral apices of the tergal arch (see discussion below).

22 Prora present, long in profile, broad in anterior view with a thick outer annulus and a deep central concavity; prora in profile projects anteriorly to about the midlength of the helcium (see discussion below).

23 Gastral segment 2 (abdominal segment IV) without trace of differentiated presclerites on tergum or sternum.

24 Gastral tergite 2 (abdominal tergite IV) without a stridulitrum.

25 Queen (gyne) only: fractionally larger than the worker and with 3 ocelli present (absent in workers). Mesosoma with full complement of flight sclerites.

Discussion of characters

The characters in italics above are considered autapomorphic for the genus. Some of the other characters are probably also apomorphies of which analogues have apparently developed convergently elsewhere in tribe Ponerini. Some of these are discussed below. Characters that involve disarticulation or dissection are from Asphinctopone silvestrii. Only the holotype of Asphinctopone differens is known and it has not been dissected, so the presence of character 21 awaits confirmation in that species.

1 Most silvestrii workers had five teeth on each mandible, but in two workers examined the left mandible had the usual 5 teeth while the right mandible had an additional denticle present between teeth 4 and 5. Conversely, one worker had 5 teeth plus a denticle on the right mandible and 5 teeth on the left. The holotype of differens has 6 teeth on the left mandible (right overlapped and not visible), of which the second tooth is low and rounded, probably through wear.

4 The second part of this characterisation appears autapomorphic. Nothing matching it has yet been observed elsewhere in Ponerini.

5 Palp formula is not 4,4, as was erroneously recorded in Bolton (2003).

8 The apical antennomere is conspicuously elongated but is not markedly broader than the preapical. Its relative length is greater in Asphinctopone than in any other Ponerini.

15 Members of the Pachycondyla group of Ponerini genera, and of Odontomachus, almost universally have two spurs on the mesotibia and two on the metatibia. In each case the anterior spur is small and simple to barbulate, the posterior spur is larger and pectinate.

19 Subpetiolar process in ventral view shows a blunt prominence anteriorly, from which arises a pair of short, divergent carinae that extend posteriorly and terminate in a small tooth on each side just in front of the midlength (appearing single in absolute profile). Posterior to this the remainder of the sternite forms a curved plate that terminates in a posteroventrally directed elevated flange, which is the termination of the externally visible ventral plate mentioned under 19.

20 When the petiole is disarticulated from the gaster, the petiole sternite, in posterior view, is revealed as a complex structure. In the posterior third of its length the sternite bifurcates into an externally visible ventral plate and an internally projecting sclerite that terminates in a concave arc, which forms the actual articulation with the helcium; this is not visible in normally mounted specimens. Within Ponerini this specialised structure has also been detected only in Phrynoponera and to a somewhat lesser extent in those species of Pachycondyla which formerly constituted the genus Brachyponera. The morphology is currently suspected to have developed independently in each case and is not a synapomorphy of the three, or of any two of the three, but this awaits proof. The internal part of the sclerite corresponds to the normal articulatory surface seen in other Ponerini; the outer plate that conceals it is the secondary development.

21 and 22 In anterior view the helcium has a narrow crest of cuticle between the ventral apices of the arched tergite. There appear to be two possible origins for this crest. One possibility is that a ridge of cuticle has arisen de novo from the first gastral sternite adjacent to the helcium and anterior of the prora. Alternatively, this crest could be the remnant of the original prora and the sternal structure that now appears to be the prora is really a secondary development. Reduction and insertion of the prora between the apices of the helcium tergite also occurs in Phrynoponera and Brachyponera. However, in both these groups the inserted cuticle apparently represents the remnants of the prora itself, which is otherwise entirely absent from the sternum. Asphinctopone retains a large, externally visible prora that appears to be genuine and not a secondary structure that developed after the reduction of the original prora and its insertion between the apices of the helcium tergite. Based on these comparisons, the first option appears more likely, but in either case the combined structure of the helcium and prora appears to be a unique development of Asphinctopone.

23 The complete lack of presclerites on both tergite and sternite of abdominal segment IV (second gastral segment) means that there is no trace of a girdling constriction on the segment. The disappearance of presclerites, and of the constriction, is certainly an apomorphy by reduction.

24 In most genera of Ponerini a stridulitrum is generally universally absent or universally present. A polymorphic condition for this character occurs in Pachycondyla, but that genus is certainly polyphyletic as presently understood and much in need of a detailed analysis.

Characters 6 and 23 immediately and very obviously separate Asphinctopone workers from all other members of the Pachycondyla group of genera. Characters 21 and 22 together are unique to the genus, and the second halves of characters 4 and 8 also appear unique. Characters 1 – 24 together form an inclusive diagnosis that isolates Asphinctopone from all other genera in the tribe.

In general appearance Asphinctopone species resemble small members of the melanaria-group of Pachycondyla, which are those species that formerly constituted most of the genus Mesoponera. However, melanaria-group species can be distinguished by the following characters: the mandible is triangular to elongate-triangular and not oblique; the mandible has more than 6 teeth on the apical margin and lacks a tooth-like process on the basal margin; the apical antennomere is not hypertrophied; the clypeus is simple; the promesonotal suture is not deeply impressed nor cross-ribbed; the mesotibiae and metatibiae each have two spurs; the petiole sternite has a simple posterior structure; the helcium and prora are not modified as described above; and presclerites are present on the second gastral segment (= abdominal segment IV).