Paltothyreus

A small genus that is widespread in Sub-Saharan Africa. It is sister to Buniapone and is notable for huge colonies distributed over several distinct nests, noxious alarm pheromones and chemical recruitment during foraging. Paltothyreus tarsatus, the only valid species name for the genus, has a large number of junior synonyms and subspecies, suggesting the occurrence of significant morphological variation.

Identification
Schmidt and Shattuck (2014) - 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.

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) included Madagascar in the range of the genus, but this is incorrect (Fisher & Peeters 2019)

Biology
Modified from Schmidt and Shattuck (2014) - 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 (for Ponerinae), 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 encompass surface areas of as much as 1,200 m2, with multiple entrance holes and extensive tunnel systems (Braun et al., 1994). Some colonies inhabit multiple distantly-separated 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 canopy patterns 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., 1993a, 1993b, 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).

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).



Castes
Queens and workers exhibit a pronounced dimorphism in size. Large numbers of winged gynes and males are produced annually in colonies, and this is a substantial energetic investment that favours non-claustral independent colony foundation (Peeters et al. 2013).

Nomenclature

 *  PALTOTHYREUS  [Ponerinae: Ponerini]
 * Paltothyreus Mayr, 1862: 735. Type-species: Formica tarsata, by monotypy.
 * Paltothyreus junior synonym of Pachycondyla: Brown, in Bolton, 1994: 164.
 * Paltothyreus revived from synonymy: Schmidt & Shattuck, 2014: 124.

Description
Schmidt and Shattuck (2014):

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).