Centromyrmex Mayr

Schmidt, C. A. & Shattuck, S. O., 2014, The Higher Classification of the Ant Subfamily Ponerinae (Hymenoptera: Formicidae), with a Review of Ponerine Ecology and Behavior, Zootaxa 3817 (1), pp. 1-242 : 164-167

Centromyrmex Mayr

Fig. 35

Centromyrmex Mayr, 1866: 894 (as genus in Poneridae). Type-species: Centromyrmex bohemanni Mayr, 1866: 895 (junior synonym of Centromyrmex brachycola ( Roger, 1861)) ; by monotypy.

Spalacomyrmex Emery, 1889: 489 (as genus). Type-species: Spalacomyrmex feae Emery, 1889: 491 ; by monotypy. Emery, 1890: 40 ( Spalacomyrmex as junior synonym of Centromyrmex ).

Glyphopone Forel, 1913b: 308 (as genus). Type-species: Glyphopone bequaerti Forel, 1913b: 308 ; by monotypy. Brown, 1963: 9 ( Glyphopone as junior synonym of Centromyrmex ).

Leptopone Arnold, 1916: 163 (as subgenus of Glyphopone , in Ponerinae , Ponerini ). Type-species: Glyphopone (Leptopone) rufigaster Arnold, 1916: 163 (junior synonym of Centromyrmex bequaerti Forel, 1913 ); by original designation. Brown, 1963: 9 (Leptopone as junior synonym of Centromyrmex ).

Typhloteras Karavaiev, 1925: 128 (as genus). Type-species: Typhloteras hamulatum Karavaiev, 1925: 129 ; by monotypy. Brown, 1953a: 8 ( Typhloteras as junior synonym of Centromyrmex ).

Centromyrmex is a moderately sized genus (15 described species) distributed in the Neotropics, Afrotropics, and Asian tropics. They are superbly adapted to life underground and are specialist termite predators.

Diagnosis. 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.

Synoptic description. Worker. 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. 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. See description in Bolton & Fisher (2008c).

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

Geographic distribution. Centromyrmex is widespread in the tropics except the Malagasy and Australasian regions. Three species are known from the Neotropics (all in the C. brachycola species group), and two species in the C. feae and C. hamulatus species groups occur in the Asian tropics. There are almost certainly undescribed species from these regions ( Bolton & Fisher, 2008c). The Afrotropical fauna was recently revised ( Bolton & Fisher, 2008c) and consists of 10 known species in the C. bequaerti and C. feae groups.

Ecology and behavior. 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 C. 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., C. alfaroi: Delabie, 1995 ; C. bequaerti: Déjean et al., 1996, 1997 ; Déjean & Fénéron, 1999; C. brachycola: Mann, 1934 ; C. feae: Wheeler, 1936 ; C. gigas: Luederwaldt, 1926 ; Delabie, 1995; C. 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 m 2 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.

C. 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.

Phylogenetic and taxonomic considerations. Centromyrmex was erected by Mayr (1866) to house the single species C. bohemanni (now C. brachycola ). The morphological diversity within the genus (as presently defined) led to the creation of several other genera which were gradually synonymized under Centromyrmex : Spalacomyrmex ( Emery, 1889; synonymy by Emery, 1890), Glyphopone ( Forel, 1913b; synonymy by Brown, 1963), Promyopias ( Santschi, 1914; synonymy by Brown, 1973), Leptopone ( Arnold, 1916; synonymy by Brown, 1963), and Typhloteras ( Karavaiev, 1925; synonymy by Brown, 1953a).

Bolton & Fisher (2008c), in their recent revision of Centromyrmex , revived Promyopias as a distinct genus but retained the other synonymies. They arranged the species of Centromyrmex into several species groups: the C. bequaerti group, which corresponds to Glyphopone and Leptopone; the C. feae group, which corresponds to Spalacomyrmex ; the C. hamulatus group, which corresponds to Typhloteras ; and the C. brachycola group, which is poorly defined morphologically and may not be monophyletic. C. brachycola itself is the type species and represents Centromyrmex (s.s.). Schmidt's (2013) molecular phylogeny of the Ponerinae includes three Centromyrmex species : C. brachycola , C. hamulatus , and C. sellaris . These taxa form a tight clade, confirming the synonymy of Spalacomyrmex and Typhloteras under Centromyrmex . The C. bequaerti group is morphologically quite distinctive from the other species groups, but they do have the sole autapomorphy of the genus, the unusual location of the metapleural gland orifice ( Bolton & Fisher, 2008c). In the absence of any contradictory molecular evidence, it therefore seems prudent to retain Glyphopone and Leptopone as junior synonyms of Centromyrmex .

Emery (1911) placed Centromyrmex in its own subtribe within Ponerini , Centromyrmecini. Brown (1953a; also Bolton, 2003) synonymized Centromyrmecini under Ponerini , but Bernard (1953) considered it worthy of full tribal status. Schmidt's (2013) molecular phylogeny of the Ponerinae clearly places Centromyrmex within Ponerini , and among the taxa included in the phylogeny Centromyrmex is resolved as sister to Psalidomyrmex + Loboponera + Plectroctena . The actual sister group of Centromyrmex may be Feroponera , though molecular data for these taxa are currently lacking, and some morphological evidence argues against this relationship. See the previous discussion of phylogenetic relationships within the Plectroctena group for more on the possible sister of Centromyrmex .

Figures

FIGURE 35. Worker caste of Centromyrmex brachycola: lateral and dorsal view of body and full-face view of head (CASENT0178344, April Nobile and www.antweb.org); world distribution of Centromyrmex.