Centromyrmex

Centromyrmex is a compact, easily recognised genus with 15 species. A recent revision (2008) includes species keys (see identification section below). Centromyrmex is distributed throughout the world’s tropics but the majority of species (10) are found in the Afrotropics. All the species in the genus appear to be termitophagous. Members of the feae group, apparently due to morphological modifications evolved as part of their dietary specialization, appear strangely helpless away from their normal habitat. Weber (1949) described finding a worker “just beneath the soil surface under a thin cover of dead leaves”. The ant was “completely helpless when exposed to the daylight and writhed about when placed on the ground or in my palm. It made no attempt to run away, curling and uncurling without stinging, though it had a long, stout sting”. In other words, it seemed unable to walk when removed from its specialised habitat and placed on a surface where it could not use its specialised legs. When not in termitaries workers may be found singly or in small numbers in the top soil or the root-mat below the leaf litter layer. The workers possess short, powerful, spiny legs that facilitate their movement through the substrates they inhabit. (Kempf, 1967; Dejean & Fénéron, 1999; Bolton & Fisher, 2008).

Identification
Schmidt and Shattuck (2014) - Despite significant morphological heterogeneity within the genus, Centromyrmex workers are readily diagnosed by their relatively smooth cuticle, lack of eyes, strongly flattened scapes, obsolete metanotal grooves, laterally-opening metapleural gland orifices situated just below the propodeal spiracle, relatively high helcium (located near the midheight of the first gastral segment), and spiniform setae on mesotibiae and meso-/metabasitarsi. Of these characters, only the location of the metapleural gland orifice is truly autapomorphic for Centromyrmex. Centromyrmex bears some morphological resemblance to Promyopias, Feroponera, Buniapone, and Cryptopone, all of which are also adapted to a cryptic lifestyle. In addition to differences in the locations of their metapleural gland orifices, these genera are easily differentiated from Centromyrmex as follows. Promyopias has linear mandibles and a blunt medial clypeal projection. Feroponera has a pair of teeth projecting from the anterior clypeal margin, closely approximated frontal lobes which overhang the anterior clypeal margin, and strongly clubbed antennae. Buniapone has vestigial eyes, a blunt medial clypeal projection, a complex metapleural gland orifice, a squamiform petiole, and lacks spiniform setae on the mesotibiae and meso-/metabasitarsi. Cryptopone lacks the smooth and relatively hairless cuticle of Centromyrmex and spiniform setae on the meso- and metabasitarsi (though they are present on the mesotibiae), has basal mandibular pits (in most species), small closely approximated frontal lobes, eyes, a distinct metanotal groove, and a narrowed propodeal dorsum.

Distribution
The genus contains 10 African, three Neotropical and two Oriental/Malesian species.

Biology
Schmidt and Shattuck (2014) - Very little is known about the Ecology and Behavior of Centromyrmex, with virtually all information on the genus stemming from anecdotal observations, with the exception of Centromyrmex bequaerti (see below). Centromyrmex are clearly well adapted to a hypogeic and fossorial lifestyle (confirmed by direct field observations), as their relatively smooth cuticles, low pigmentation, lack of eyes, flattened scapes, and short thick legs with traction setae are all features commonly found in other hypogeic or fossorial ants. Workers are found in termitaries, upper soil layers, beneath leaf litter, or in rotten logs (Weber, 1949; Bolton & Fisher, 2008c). Nesting sites are usually in close proximity to termitaries or even inside the termitaries themselves (e.g., Centromyrmex alfaroi: Delabie, 1995; C. bequaerti: Déjean et al., 1996, 1997; Déjean & Fénéron, 1999; Centromyrmex brachycola: Mann, 1934; Centromyrmex feae: Wheeler, 1936; Centromyrmex gigas: Luederwaldt, 1926; Delabie, 1995; Centromyrmex sellaris: Lévieux, 1976, 1983; Déjean & Fénéron, 1996; Déjean et al., 1996, 1997; but see Arnold, 1915). All Centromyrmex species for which there are ecological data appear to be obligate termite specialists (Luederwaldt, 1926; Mann, 1934; Wheeler, 1936; Lévieux, 1983; Delabie, 1995; Déjean & Fénéron, 1996, 1999; Bolton & Fisher, 2008c). Some Centromyrmex species are known to prey on a wide range of termite species; other species may be even more specialized.

The details of Centromyrmex social organization and foraging behavior are generally unstudied except for two African species representing distinct species groups: C. sellaris, whose nest architecture was studied by Lévieux (1976); and C. bequaerti, whose social organization and foraging behavior were examined by Déjean & Fénéron (1996, 1999). Nests of C. sellaris consist of 10 or more small chambers distributed in the soil across an area of about 8 m2 and connected to each other by narrow tunnels (Lévieux, 1976). The single colony examined had over 400 workers and a single queen. Workers apparently always travel in the soil when foraging and can range up to 20 m from the nest. C. sellaris is known to prey on termites, but no additional details of its foraging behavior are known.

Centromyrmex bequaerti exhibits a rather different suite of behaviors, which are unusual among ponerines but similar to those displayed by some termitolestic myrmicines (Déjean & Fénéron, 1996, 1999). This species nests inside the termitaries of a wide range of termite species and preys exclusively on its hosts. Colonies are polygynous and relatively large, with up to 13 queens and several hundred workers, and they inhabit multiple cavities within host termitaries. The worker caste displays strong size polymorphism, and queens are substantially larger than even the major workers. The size ratio (4x) of C. bequaerti queens and minor workers is the largest known within the Ponerinae. Major workers principally act as guards, while smaller workers perform most of the hunting and basic nest activities, though there is overlap in these tasks. When a C. bequaerti scout detects a termite gallery, it attacks and paralyzes several termites, then returns to its nest to recruit a small number of nestmates, which it leads to the termites using chemical trails (Déjean & Fénéron, 1999). Workers might also recruit nestmates by tapping their heads on the substrate, though the actual function of this behavior has not been determined. Once they arrive at the termite gallery, the ants attack and paralyze large numbers of termites and stack them into piles before transporting them back to their nest. The response of a C. bequaerti worker to a termite depends on the caste of the termite: workers are seized and then stung, while soldiers are stung first, presumably to minimize the risk of a damaging counterattack. Déjean & Fénéron (1999) found that the gaster of C. bequaerti workers is shaped such that the mandibles of termite soldiers slip off without causing injury.

Castes
Workers are known for all species.

Queens are known for the following:

Afrotropical species
 * Centromyrmex bequaerti
 * Centromyrmex decessor
 * Centromyrmex fugator
 * Centromyrmex sellaris
 * Centromyrmex raptor
 * Centromyrmex angolensis

Neotropical species
 * Centromyrmex alfaroi
 * Centromyrmex brachycola
 * Centromyrmex gigas

Oriental and Malesian species
 * Centromyrmex feae
 * Centromyrmex hamulatus

Males have been noted in taxonomic studies for:
 * Centromyrmex alfaroi
 * Centromyrmex angolensis
 * Centromyrmex bequaerti
 * Centromyrmex decessor
 * Centromyrmex feae
 * Centromyrmex hamulatus
 * Centromyrmex sellaris

Nomenclature

 *  CENTROMYRMEX [Ponerinae: Ponerini]
 * Centromyrmex Mayr, 1866b: 894. Type-species: Centromyrmex bohemanni (junior synonym of Ponera brachycola), by monotypy.
 * Centromyrmex senior synonym of Spalacomyrmex: Emery, 1890b: 40.
 * Centromyrmex senior synonym of Typhloteras: Brown, 1953c: 8.
 * Centromyrmex senior synonym of Glyphopone, Leptopone: Brown, 1963: 9.
 * GLYPHOPONE [junior synonym of Centromyrmex]
 * Glyphopone Forel, 1913b: 308. Type-species: Glyphopone bequaerti, by monotypy.
 * Glyphopone junior synonym of Centromyrmex: Brown, 1963: 9.
 * LEPTOPONE [junior synonym of Centromyrmex]
 * Leptopone Arnold, 1916: 163 [as subgenus of Glyphopone]. Type-species: Glyphopone (Leptopone) rufigaster (junior synonym of Glyphopone bequaerti), by original designation.
 * Leptopone raised to genus: Wheeler, W.M. 1922a: 647.
 * Leptopone junior synonym of Centromyrmex: Brown, 1963: 9.
 * SPALACOMYRMEX [junior synonym of Centromyrmex]
 * Spalacomyrmex Emery, 1889b: 489. Type-species: Spalacomyrmex feae, by monotypy.
 * Spalacomyrmex junior synonym of Centromyrmex: Emery, 1890b: 40.
 * TYPHLOTERAS [junior synonym of Centromyrmex]
 * Typhloteras Karavaiev, 1925b: 128. Type-species: Typhloteras hamulatum, by monotypy.
 * Typhloteras junior synonym of Centromyrmex: Brown, 1953c: 8.

The following is based on Bolton and Fisher (2008)

Workers and Queen
1 Mandible triangular (MI 28–38) to elongate-triangular (MI 52–84), with 4–12 teeth; with a distinct basal groove but without a basal pit.

2 Palp formula 4,3.

3 Eyes absent in worker, present in queen.

4 Frontal lobes with their anterior margins considerably posterior to the anterior clypeal margin.

5 Antenna with 12 segments; scape very strongly dorsoventrally flattened in its basal half, the leading edge extremely thin; funiculus gradually incrassate towards the apex but without a differentiated club.

6 Mesotibia, mesobasitarsus and metabasitarsus with strongly sclerotised spiniform or peg-like traction setae.

7 Pretarsal claws small, simple.

8 Metanotal groove absent.

9 Orifice of metapleural gland a small pore or short slit that opens laterally, located well above the ventral margin of the metapleuron and far anterior of the posteroventral angle of the mesosoma.

10 Propodeum unarmed.

11 Helcium located close to mid-height on anterior face of the first gastral segment (abdominal segment III).

12 Prora present but of unusual form and sometimes very weak; see discussion below.

13 Girdling constriction between presclerites and postsclerites of second gastral segment distinct.

14 Stridulitrum absent.

15 Queen only. Eyes and ocelli present. Transverse suture present on the mesopleuron that divides the sclerite into anepisternum and katepisternum. Mesosoma with full complement of flight sclerites. Hind wing with jugal lobe present.

Discussion of female characters
Character 9, in italics, is autapomorphic. Some of the other characters may also be apomorphies but have analogues that have apparently developed convergently elsewhere in tribe Ponerini. Characters 1–14 together form an inclusive diagnosis that isolates Centromyrmex workers and queens from all other genera in the tribe.

1 In the bequaerti group the mandibles are triangular and relatively short, with a small number of strongly defined teeth and a distinctly inflected apical tooth. In all other groups the mandibles are elongate-triangular, pointed apically but without an inflected apical tooth as the latter continues the line of the long axis of the mandible, and with more weakly defined but more numerous teeth on the masticatory margin. The small teeth on the elongate-triangular mandibles are commonly very worn and rounded, leaving the margin with a crenulate or even an almost edentate appearance.

2 The consistent palp formula count of 4,3 has been confirmed in alfaroi (worker and queen), angolensis (worker and queen), bequaerti (worker (all size morphs) and queen), brachycola (worker and queen), decessor (worker and queen), ereptor (worker), feae (worker), fugator (worker and queen), hamulatus (worker), raptor (worker and queen) secutor (all worker size morphs), sellaris (worker and queen). PF 4,3 was earlier recorded for all Neotropical species by Kempf (1967). This consistent count is probably an apomorphy of the genus (the same count also applies in all known males, see below).

3 Loss of eyes in the worker caste but their retention in queens is also characteristic of Promyopias: see discussion of potential genus group, below.

5 Extreme flattening of the basal half of the scape allows it to fit tightly against the dorsum of the head when directed laterally or posteriorly. Presumably this is an adaptation that allows the scapes to remain easily mobile in very confined spaces.

6 The apparent cuticular spines on the mesotibia, mesobasitarsus and metabasitarsus are in reality hypertrophied sclerotised setae, with sockets at the base. Their function is to improve traction in the ant’s restricted habitat. They also occur in the same locations in Promyopias and Feroponera: see discussion of potential genus group, below.

8 All species lack any trace of a metanotal groove. Indeed, in all but alfaroi there is usually no trace of any suture across the dorsum at the junction of mesonotum and propodeum, so that the line of the posterior termination of the mesonotum is not demarcated in any way. In alfaroi a short, unimpressed, weak transverse suture is retained.

9 The unique position of the metapleural gland orifice is given as an unequivocal autapomorphy of Centromyrmex; it is a derived state not repeated anywhere else in the Ponerini, where the position of the orifice is always at or very near to the posteroventral corner of the mesosoma, opening laterally or posteriorly.

11 Position of the helcium is similar in Promyopias and Feroponera: see discussion of potential genus group, below.

12 In the C. bequaerti group the prora is a flat transverse plate, slightly indented medially, that traverses the first gastral sternite below the helcium. In all other species groups the prora is represented by a pair of longitudinal ridges on the anterior face of the first gastral sternite, one on each side below the helcium (extremely reduced in alfaroi and raptor); the space between the ridges is usually shallowly concave. The morphology of the former can easily be derived from that of the latter by emphasising and elevating the ridges and elevating the cuticle between the ridges. Prorae of this nature are not “normal” for Ponerini, which typically have a cuticular prominence, variously shaped, immediately below the helcium.

Worker
Schmidt and Shattuck (2014) - Small to large (TL 3.5–13 mm) robust ants with the standard characters of Ponerini. Usually monomorphic, but polymorphic in the C. bequaerti species group. Mandibles triangular to subtriangular with variable dentition and a faint basal groove. Frontal lobes moderately large. Scapes strongly flattened basally and with a sharp anterior edge. Eyes absent. Pronotum usually with rounded lateral margins, but with sharp lateral margins in the C. feae species group. Mesopleuron sometimes divided by a transverse groove. Metanotal groove almost always obsolete. Mesosomal profile usually continuous but with a distinct depression in the propodeum in the C. feae species group. Propodeal spiracles slit-shaped or ovoid. Metapleural gland orifice opening laterally just below the propodeal spiracle. Mesotibiae and meso-/metabasitarsi armed with stout traction setae. Metatibial spur formula (1p) or (1s, 1p). Petiole nodiform, becoming wider dorsally and posteriorly. Helcium projecting from near midheight of anterior face of A3. Gaster with a weak to strong constriction between pre- and postsclerites of A4. Head and body shining, sometimes sparsely punctate, with sparse to scattered pilosity and no pubescence. Color testaceous to ferrugineous. See Bolton & Fisher (2008c) for a detailed description of Centromyrmex structure.

Queen
Schmidt and Shattuck (2014) - Similar to worker but winged and with compound eyes and ocelli (Bolton & Fisher, 2008c); generally only slightly larger than conspecific workers, but much larger in C. bequaerti (Déjean & Fénéron, 1996; Bolton & Fisher, 2008c).

Male
1 Mandible very reduced, almost lobate; edentate or with a small apical tooth.

2 Palp formula 4, 3 (in situ counts).

3 Frontal lobes absent; antennal sockets fully exposed.

4 Antenna with 13 segments, filiform.

5 Second funicular segment very short, only as long as, or at most 1.10 × longer than, the short scape.

6 Eyes large, inner margin shallowly convex to shallowly concave, without a marked concavity or indentation in about the median third; ocelli prominent.

7 Notauli variable, see discussion below.

8 Parapsidal grooves present.

9 Mesonotum with a deep, transverse groove between mesoscutum and mesoscutellum.

10 Epimeral lobe present.

11 Metapleural gland orifice present.

12 Propodeal spiracle with orifice elliptical to slit-shaped.

13 Spurs of mesotibia and metatibia as in worker and queen, see below.

14 Mesotibiae, mesobasitarsi and metabasitarsi lack the spiniform setae that are so conspicuous in female castes.

15 Pretarsal claws simple.

16 A membranous arolium present between the pretarsal claws.

17 Hindwing with jugal lobe present.

18 Prora usually present, its structure as in respective worker and queen but reduced in size; absent in alfaroi.

19 Gastral segment 2 (= abdominal segment IV) with a distinct girdling constriction between presclerites and postsclerites.

20 Pygidium (= abdominal tergite VIII) without a median apical spine.

21 Cerci (= pygostyles) present.

Males are known for few Afrotropical species, and very few specimens of each exist. The paucity of material makes it impossible to predict which characters will be of value at species-rank in this sex. For this reason, formal descriptions are not presented in the treatment by species.

Discussion of male characters
None of the characters listed are unequivocally stated as apomorphic at present, but 2 is a strong contender for this status and 5 and 11 are possibilities.

1 A reduced mandible of this form appears apomorphic for the tribe Ponerini as a whole (Bolton, 2003).

2 The 4,3 palp formula of males and the female castes is the same in all Centromyrmex where both are known. Elsewhere in Ponerini it is usual for males to have higher palp formula counts than conspecific females (Bolton, 2003), so this equality of PF is most probably an apomorphy of the genus. The plesiomorphic maximum count in Ponerini males is 6,4 (as in Aculeata in general), as opposed to 4,4 in females (Brown, 1963; Bolton, 2003).

5 In Ponerini generally the second funicular segment of males is much longer than the scape (e.g. Ogata, 1987; Yoshimura & Fisher, 2007; Bolton & Fisher, 2008). It is not certain whether the very short condition in Centromyrmex is plesiomorphic or apomorphic.

6 In many groups of Ponerini the inner margin of the eye is distinctly concave or suddenly indented in approximately its median third (e.g. Ogata, 1987; Yoshimura & Fisher, 2007).

7 Notauli are distinctly present in decessor, angolensis and most sellaris. However, in one example of sellaris only the anterior portions of the notauli were developed and in another the notauli were present but superficial throughout. Notauli were entirely absent in all alfaroi and bequaerti males examined.

11 A metapleural gland orifice is visible in all males. Unlike the female castes, males have the orifice in the usual position for Ponerini, close to the posteroventral angle of the metapleuron. The presence of this structure in this sex is uncommon and may be an apomorphy of the genus.

13 In the male of bequaerti both the mesotibia and metatibia have two spurs, on each the anterior spur is small and simple, the posterior is large and pectinate. In alfaroi the mesotibia has a single small, barbulate spur and the metatibia a large pectinate spur. In angolensis, decessor and sellaris the mesotibia lacks spurs and the metatibia has only a single pectinate spur present. These arrangements correspond to those of the female castes of the same species.

16 The arolium is usually white, membranous and very conspicuous.

Larva
Described for two species by Wheeler & Wheeler (1952, 1976).