Paltothyreus 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 : 124-126

Paltothyreus Mayr

Fig. 22

Paltothyreus Mayr, 1862: 714 , 735 (as genus). Type-species: Formica tarsata Fabricius, 1798: 280 ; by monotypy. Gen. rev.

Paltothyreus is a monotypic genus (with five subspecies) widespread in Sub-Saharan Africa. It is sister to Buniapone and is notable for its huge nests, noxious alarm pheromones, chemical recruitment, and prey transfer behavior.

Diagnosis. Diagnostic morphological apomorphies of Paltothyreus workers include their striate sculpturing, blunt clypeal projection, complex metapleural gland orifice (with both anterior and posterior cuticular flanges), and hypopygium armed with stout setae. This combination of characters does not occur in any other ponerine genus. Striate sculpturing and an armed hypopygium occur in several other ponerine genera, but never together, and never in combination with a blunt clypeal projection or complex metapleural gland orifice. The apomorphies of the clypeus and metapleural gland orifice are shared with Buniapone , but that genus lacks striate sculpturing and an armed hypopygium.

Synoptic description. Worker. Very large (TL 17-20 mm; Arnold, 1915) ants with the standard characters of Ponerini . Mandibles triangular and long, with a distinct basal groove. Clypeus with a blunt squarish anteromedial projection. Frontal lobes widely separated anteriorly by an extension of the clypeus. Eyes large, located anterior to head midline. Ocelli sometimes present in workers. Metanotal groove at most present as a faint suture. Propodeum broad dorsally. Propodeal spiracle ovoid. Metapleural gland orifice complex, with a broad cuticular flange posteriorly and a small cuticular flange anteriorly, together forming a deep transverse groove. Metatibial spur formula (1s, 1p). Tarsal claws with a single preapical tooth. Petiole surmounted by a thick scale. Subpetiolar process a deep keel. Sternite of A3 with a large keel-like anteroventral process. Tergite of A3 with blunt dorsolateral angles. Gaster with a moderate girdling constriction between pre- and post-tergites of A4. Hypopygium armed with a row of stout setae on either side of the sting. Head and body striate, with scattered to abundant pilosity and moderate pubescence. Color black.

Queen. Similar to worker but larger (TL 23 mm; Arnold, 1915) and winged.

Male. See descriptions in Emery (1911), Arnold (1915), and Wheeler (1922b).

Larva. Not described.

Geographic distribution. Paltothyreus is widespread and common in Sub-Saharan Africa, though it is patchily distributed ( Arnold, 1915; Wheeler, 1922b). Wheeler (1922b) gives its range as encompassing all of Africa south of roughly 15° N latitude. Emery (1911) includes Madagascar in the range of the genus, but this has not been reported elsewhere and is presumably in error.

Ecology and behavior. Paltothyreus , the African stink ant, is so called because its workers produce powerfully smelly sulfur-containing alarm pheromones in their mandibular glands ( Casnati et al., 1967; Crewe & Fletcher, 1974; Crewe & Ross, 1975a, 1975b). Paltothyreus occurs mainly in forests and forest-savannah transition zones. Colonies are relatively large, with as many as 2,500 workers (mean = 926 workers; Braun et al., 1994) but only a single queen. Nests are constructed in the ground or in abandoned termite nests ( Kalule-Sabiti, 1980; Déjean et al., 1996, 1997) and are of exceptional size, encompassing surface areas of as much as 1,200 m 2, with multiple entrance holes and extensive tunnel systems ( Braun et al., 1994). Some colonies inhabit multiple distantlyseparated nests connected by underground tunnels, which Braun et al. (1994) characterize as a form of polydomy. The tunnels and multiple nest entrances allow workers to access a large foraging area with minimal time spent in the open. During nest relocations, workers employ a pygidial gland-derived pheromone to recruit nestmates for tandem running ( Hölldobler, 1984; Braun et al., 1994). Hölldobler (1980) discovered that Paltothyreus foragers visually navigate through their forest environments by memorizing the canopy overhead.

Paltothyreus are largely termite predators, though they do hunt or scavenge a broad range of other invertebrates, including other ants (Lévieux, 1977; Kalule-Sabiti, 1980; Déjean et al., 1993 a, 1993 b, 1999). Workers primarily forage singly, but will recruit nestmates to large prey or concentrated termite sources, using a trail pheromone produced by sternal glands ( Hölldobler, 1984; Déjean et al., 1993a). Workers often sting their prey upon capture, and when collecting termites will stack multiple individuals between the mandibles for one return trip to the nest ( Déjean et al., 1993b; López et al., 2000). Paltothyreus foragers often employ a unique form of group prey retrieval, termed “prey chain transfer behavior” ( López et al., 2000), by which successful foragers (”finders”) returning to the nest will transfer their termite prey to other workers (”receivers”) at locations between the prey capture site and the nest. The finders then go back to capture more termites, while the receivers head to the nest, sometimes transferring the prey to yet other workers. López et al. (2000) hypothesize that this behavior increases the efficiency of prey capture and also serves as a simple form of recruitment.

The mating and dispersal behavior of Paltothyreus were studied by Villet et al. (1989). The abdominal glands of Paltothyreus males were studied by Hölldobler & Engel-Siegel (1982). Queens apparently chemically suppress the production of eggs by workers ( Braun et al., 1994).

Phylogenetic and taxonomic considerations. Paltothyreus was described by Mayr (1862) to hold the single species Formica tarsata Fabricius. The genus experienced relative taxonomic stability until W. L. Brown (in Bolton, 1994) synonymized it under Pachycondyla without phylogenetic justification.

We are reviving Paltothyreus to full genus status based on both morphological and molecular evidence. Schmidt's (2013) molecular phylogeny of the Ponerinae places Paltothyreus with strong support within the Odontomachus group as sister to Buniapone , and not at all close to Pachycondyla .

A sister group relationship between Paltothyreus and Buniapone is initially a surprising and suspect result. Superficially, these taxa are remarkably different. Whereas Paltothyreus is a very large epigeic African ant with triangular mandibles and large eyes, Buniapone is a fairly small hypogeic ant restricted to Southeast Asia, with subtriangular mandibles and extremely reduced eyes. Paltothyreus would seem to most closely resemble other large African ponerines like Megaponera , Ophthalmopone or Hagensia . Buniapone , on the other hand, superficially bears a closer resemblance to Centromyrmex or Cryptopone .

A closer examination of the morphological structures of these taxa strongly supports a close relationship, however, as they share several apomorphies: a blunt squarish anteromedial clypeal projection, a nearly or completely obsolete metanotal suture, ovoid propodeal spiracles, a complex metapleural gland orifice with both posterior and anterior cuticular flanges, and a squamiform petiole with a large keel-like ventral process. The unusual metapleural gland orifice in particular is a strong synapomorphy for the two genera. See the discussion under Buniapone for more on the evolutionary implications of their close relationship.

Figures

FIGURE 22. Worker caste of Paltothyreus tarsatus: lateral and dorsal view of body and full-face view of head (CASENT0172430, April Nobile and www.antweb.org); world distribution of Paltothyreus.