Centromyrmex

Centromyrmex is a compact and easily recognised genus that contains only 15 species to date, distributed through the world’s tropics but with a preponderance of species (10) in the Afrotropical region. As noted below, all of its species appear to be termitophagous and all are superbly adapted to this specialised predatory life style. Indeed, members of the feae group are so well adapted that they appear strangely helpless away from their normal habitat. When not in termitaries ants of this fossorial group are usually found singly or in small numbers in the top soil or the root-mat below the leaf litter layer, where their short, powerful, spiny legs facilitate their movement. Weber (1949) described what happened when he found 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. (Bolton and Fisher 2008).

Identification
Key to Centromyrmex species

Centromyrmex species groups

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

Biology
Bolton and Fisher (2008) - Members of Centromyrmex all seem to be obligate termitophages and are quite active in termitaries (e.g. Wheeler, 1936; Kempf, 1967; Lévieux, 1983; Delabie, 1995; Dejean & Fénéron, 1996, 1999, Dejean et al. 1996, 1997).

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.

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.