Hypoponera Santschi

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 : 57-60

Hypoponera Santschi

Fig. 5

Hypoponera Santschi, 1938: 79 (as subgenus of Ponera ). Type-species: Ponera abeillei André, 1881: 61 , xlviii; by original designation.

Hypoponera is a large (approximately 170 described extant species) cosmopolitan genus of small cryptic ponerines. Most Hypoponera are generalist predators, and some are widespread tramp species. Several are known to have highly unusual reproductive strategies, including the presence of combative ergatoid males.

Diagnosis. Hypoponera is morphologically the most generalized of the cryptobiotic ponerine genera, as it lacks any obvious autapomorphies. The genus also shows greater variability than most ponerine genera in many characters typically useful for generic diagnosis. Despite these complications, Hypoponera workers are generally diagnosable by the following combination of characters: mandibles triangular, with a variable number of small teeth and without basal pits or grooves; frontal lobes small and closely approximated; metanotal groove usually shallowly depressed; mesotibiae and meso-/metabasitarsi without stout traction setae; metatibial spur formula (1p); petiole squamiform; subpetiolar process a rounded lobe without paired teeth posteriorly, and usually without an anterior fenestra; and head and body without strong sculpturing and usually with a relatively dense pubescence. Hypoponera is morphologically most similar to Ponera , Cryptopone , Pseudoponera , Brachyponera , some Euponera , Belonopelta , and Emeryopone . Hypoponera most consistently differs from Ponera in the structure of the subpetiolar process: in Ponera the subpetiolar process has an anterior fenestra and a pair of teeth posteriorly, whereas in Hypoponera the subpetiolar process is a simple rounded lobe, only rarely with an anterior fenestra. Hypoponera differs from Cryptopone in lacking stout spines on the middle and hind legs, from Euponera , Pseudoponera and Brachyponera in having only a single metatibial spur, and from Belonopelta and Emeryopone in having triangular mandibles without a series of long attenuated teeth.

Synoptic description. Worker. Very small to small (TL 1.4–5.5 mm) slender to robust ants with the standard characters of Ponerini . Mandibles triangular, with a variable number of teeth and without a distinct basal groove (though a shallow pit or vestigial groove may be present). Frontal lobes small and closely approximated. Apical segments of antennae sometimes enlarged into a distinct club. Eyes sometimes absent, but usually present, very small and located far anterior on the sides of the head. Mesopleuron usually not divided by a transverse groove, though occasionally faintly present. Metanotal groove usually present and shallowly depressed, though sometimes reduced to a suture, and rarely absent altogether. Propodeum usually moderately to strongly narrowed dorsally, but occasionally broad. Propodeal spiracles round. Metatibial spur formula (1p). Petiole squamiform. Subpetiolar process a rounded lobe, without paired teeth posteriorly and only rarely with an anterior fenestra. Gaster with a weak to strong girdling constriction between pre- and postsclerites of A4. Stridulitrum sometimes present on pretergite of A4. Head and body occasionally glossy, usually finely punctate, sometimes lightly striate on the sides of the mesosoma. Head and body with sparse to scattered pilosity and usually a dense pubescence. Color variable, testaceous to black.

Queen. Similar to worker; usually alate, with ocelli and larger compound eyes. Queens are sometimes ergatoid or intermorphic ( Yamauchi et al., 1996). See further description in Taylor (1967) and Bolton & Fisher (2011).

Male. Usually winged, but sometimes ergatoid. See general description in Taylor (1967) and Bolton & Fisher (2011) and descriptions of ergatoid males in Yamauchi et al. (1996).

Larva. Hypoponera larvae were described generically by Taylor (1967) and for various species by Wheeler & Wheeler (1964, 1971a, 1990). Detailed histological or morphological studies of Hypoponera larvae were performed by Peeters & Hölldobler (1992) and Escoubas et al. (1987). Unusually among ponerines, sufficient research has been conducted on Hypoponera larval structure to yield a meaningful diagnostic character: Hypoponera larvae have two pairs of sticky tubercles on A4 and A5, whereas Ponera larvae have three or four pairs ( Taylor, 1967).

Geographic distribution. Hypoponera is the most cosmopolitan of all ponerine genera, occurring on every continent except Antarctica and extending into many temperate regions. Endemic species are apparently absent from many island groups (e.g., Polynesia), but several widespread tramp species have become established even in these locations ( Taylor, 1967; Ingram et al., 2006).

Ecology and behavior. Hypoponera is probably the most common and diverse ponerine genus worldwide and is consistently one of the most abundant and diverse ant genera collected in quantitative surveys of leaf litter and soil ant communities, especially in the tropics (e.g., Argentina: Theunis et al., 2005; Australia: King et al., 1998; Brazil: Soares & Schoereder, 2001; Costa Rica: Longino et al., 2002; Ghana: Belshaw & Bolton, 1994; Guyana: LaPolla et al., 2007; Madagascar: Fisher, 1999; Melanesia and New Caledonia: Wilson, 1976; generally: Ward, 2000). Though Leptogenys currently boasts many more described species than Hypoponera , the true species diversity of Hypoponera is probably grossly underestimated and may rival or exceed that of Leptogenys given their cryptobiotic habits, lack of revisionary taxonomic work, and the likely occurrence of cryptic species complexes. Fisher (1999) found that Hypoponera diversity was a good surrogate for total ant diversity in forests in eastern Madagascar, indicating that it may be a useful indicator genus for faunal surveys.

Except for their unusual reproductive and larval behaviors (see below), surprisingly little work has been done on the ecology and behavior of Hypoponera . They are cryptobiotic, nesting in soil, under rocks, or in rotting wood, and foraging there or in leaf litter and other sheltered microhabitats ( Wilson, 1958c; Onoyama, 1989; Yamauchi et al., 1996; Terayama, 1999; Foitzik et al., 2002; pers. obs.). Reports on the sizes of Hypoponera colonies are scarce, but colonies usually have fewer than 100 workers and only occasionally more (e.g., Wilson, 1958c; Villet et al., 1991; Peeters & Hölldobler, 1992; Peeters, 1993; Hashimoto et al., 1995; Yamauchi et al., 1996; Foitzik et al., 2002). They are typically reported to be generalist predators of small arthropods or scavengers (e.g., Wilson, 1958c; Agbogba, 1984; Escoubas et al., 1987; Brown, 2000; Seifert, 2004), though some species probably have stricter diets (e.g., one African species is reported to feed principally on collembolans; Lévieux, 1983). Foraging is probably generally performed by solitary workers, though at least one species is known to recruit nestmates to help dismember large prey ( Agbogba, 1984). Hölldobler (1985) observed tandem running in an unidentified Hypoponera species but did not clarify its purpose.

While in most respects Hypoponera are fairly typical ponerines, members of the genus exhibit many unusual social or reproductive traits, including one of only two known instances of adult trophallaxis in the Ponerinae ( Hashimoto et al., 1995) , as well as obligate worker sterility (Villet et al., 1991; Ito & Ohkawara, 1994; Yamauchi et al., 2001), male polymorphism, and larval cannibalism. As a genus Hypoponera displays an interesting diversity of reproductive strategies, with species variously having almost every conceivable combination of alate and intermorphic queens and alate and ergatoid males. At least three Hypoponera species are known to have both winged and ergatoid males ( H. eduardi , H. nubatama and H. opacior ), at least three species apparently have only ergatoid males ( H. ergatandria , H. gleadowi and H. punctatissima ,), and still other species are thought to have only winged males ( Yamauchi et al., 2001; Bolton & Fisher, 2011). A similar variation occurs in the queen caste, with some species having both alate and intermorphic queens and others probably having only one or the other (e.g., Hashimoto et al., 1995; Yamauchi et al., 1996, 2001). In what are probably the most complex mating systems known for any ponerine, H. nubatama and H. opacior have alate queens, intermorphic queens, alate males, and ergatoid males, often all in the same colony ( Yamauchi et al., 2001; Foitzik et al., 2002; Rüger et al., 2008). Some colonies of H. opacior are polygynous and polydomous, with 2 to 15 intermorphic queens and multiple nest sites connected by subterranean tunnels. Ergatoid males remain in their natal nest and mate with virgin queens (either alate or intermorphic) which are still in their puparia (Foitzik et al., 2002).

The behavior of ergatoid Hypoponera males is both unusual and varied within the genus. At one extreme is H. ergatandria , whose males are dimorphic, the minor males mimicking females and utilizing a sneaky mating strategy, and the major males fighting and killing one another to secure mating opportunities ( Yamauchi et al., 1996). Similar fighting was also observed among ergatoid males of H. punctatissima , though it’s unclear whether they are dimorphic ( Hamilton, 1979). Ergatoid males of H. nubatama do not fight directly but instead kill other males before they eclose from their puparia ( Yamauchi et al., 2001). Finally, ergatoid males of H. opacior do not fight but monopolize their mates by copulating with them for up to 40 hours, the longest copulations ever observed in ants (Foitzik et al., 2002). Given the presence of ergatoid males in virtually all tramp species of Hypoponera, Taylor (1967) hypothesized that the consequent simplification of the mating process in these species may facilitate their spread.

Hypoponera larvae have two pairs of sticky tubercles, with which they are stuck to the walls and ceilings of their nest cavities ( Taylor, 1967; Escoubas et al., 1987; Peeters & Hölldobler, 1992). This may assist the larvae in feeding or may function to protect the larvae from excess humidity ( Peeters & Hölldobler, 1992), but may also function to separate larvae sufficiently to prevent larval cannibalism, which has been observed in at least three species ( Rüger et al., 2008).

Phylogenetic and taxonomic considerations. Santschi (1938) erected Hypoponera as a subgenus of Ponera to house those Ponera species with shallow metanotal grooves. Taylor (1967) raised Hypoponera to full genus status, recognized that Santschi’s definition of the subgenus was phylogenetically meaningless, and instead differentiated the genus from Ponera by its palp formula (1,1 in Hypoponera ), absence of an anterior fenestra or posterior teeth in the subpetiolar process, several male characters, and the number of sticky tubercles on the larvae. Hypoponera workers are superficially quite similar to those of Ponera , but they are distantly separated in Schmidt’s (2013) molecular phylogeny of the Ponerinae , suggesting that their morphological similarities are the result of convergence due to their similar cryptobiotic lifestyles. Hypoponera is phylogenetically distinct from other major lineages of Ponerini , diverging from its closest relative (probably the Plectroctena group) quite early in the radiation of the tribe.

Given the absence of strong autapomorphies for the genus and its relatively high morphological diversity, it is possible that Hypoponera as presently defined is non-monophyletic. Schmidt (2013) included in his molecular phylogeny several Hypoponera species presenting maximum morphological diversity from across the range of the genus, to begin to test the monophyly of the genus. To his surprise these species formed a tight clade with strong phylogenetic support. Even more surprising was that three sympatric Hypoponera species from Malaysia, which he included in the phylogeny because they are morphologically quite divergent from one another (to the point that he expected them all to represent distinct genera) turned out to be very closely related to one another, indicating an impressive recent adaptive radiation.

Figures

FIGURE 5. Worker caste of Hypoponera eduardi: lateral and dorsal view of body and full-face view of head (CASENT0172333, April Nobile and www.antweb.org); world distribution of Hypoponera.