Plectroctena

Plectroctena is a medium sized genus (16 described species) widespread in Sub-Saharan Africa. They are cryptobiotic predators of millipedes, millipede eggs and termites.

Identification
Schmidt and Shattuck (2014) - Plectroctena can be readily identified by its linear mandibles, which have dorsal longitudinal grooves, and by the anteromedial and lateral excavations of its clypeus, all of which are autapomorphic within the Ponerinae. Plectroctena is most similar to Loboponera, Boloponera, and Promyopias, which all have expanded frontal lobes and an overall similar gestalt. Plectroctena differs from Loboponera most obviously in the shape of the mandibles (triangular in Loboponera). Boloponera and Promyopias both have linear mandibles, but they both lack the autapomorphies of Plectroctena given above, among several other differences. Plectroctena also bears some resemblance to Myopias, given their linear mandibles, but Plectroctena lacks an anteromedial projection of the clypeus and has only a single metatibial spur.

Distribution
Plectroctena ranges throughout most of Sub-Saharan Africa, from Sierra Leone to Ethiopia and south to South Africa (Bolton & Brown, 2002).

Species by Region
Number of species within biogeographic regions, along with the total number of species for each region.

Biology
Schmidt and Shattuck (2014) - More is known about the habits of Plectroctena than of any other genus in the Plectroctena group, but data on most species are still scarce. Like other members of the group, Plectroctena are primarily cryptobiotic, nest in soil or rotting wood, and forage in these same microhabitats as well as among leaf litter (Arnold, 1915; Bolton, 1974; Bolton et al., 1979; Peeters & Crewe, 1988; Bolton & Brown, 2002; Déjean et al., 2002). They have also been found nesting in abandoned termitaries (Déjean et al., 1996). Very little is known about the social and reproductive behavior of Plectroctena. Colony sizes are unknown for most species but colonies of Plectroctena lygaria, Plectroctena mandibularis and Plectroctena minor are reported to have about 300 or fewer workers (Bolton et al., 1979; Déjean et al., 2001, 2002; Wilkins et al., 2006).

Most Plectroctena species have winged queens, but at least four species have ergatoid queens (Bolton, 1974), and at least one of these (Plectroctena mandibularis) is facultatively polygynous (Wilkins et al., 2006). In laboratory conditions, ergatoid queens of P. mandibularis successfully captured prey and were able to rear brood without the assistance of workers, suggesting that colony foundation in this species is semiclaustral (Villet, 1991a; confirmed in natural conditions by Villet, 1999), in contrast to most ants with ergatoid queens. Mating behavior by P. mandibularis is also unusual in that virgin females leave the nest and apparently call for males using a pheromone; in most ponerines with ergatoid queens, mating occurs in the natal nest of the queen (Villet, 1999). In P. mandibularis, ergatoid queens apparently inhibit reproduction by workers but orphaned workers of P. mandibularis will begin laying eggs and can successfully rear male brood (Peeters & Crewe, 1988).

Plectroctena are primarily specialist predators of millipedes or millipede eggs, but they also prey to a lesser extent on termites and other arthropods, including other ants (Arnold, 1915; Fletcher, 1973; Bolton et al., 1979; Lévieux, 1983; Peeters & Crewe, 1988; Schatz et al., 2001; Bolton & Brown, 2002; Déjean et al., 2002). Workers typically forage individually but may hunt in small groups (Bolton, 1974; Peeters & Crewe, 1988), and sometimes recruit nestmates to help with large prey (see below). Foraging behavior has been extensively studied in Plectroctena minor, which specializes to a large degree on millipedes. In cafeteria experiments, P. minor workers overwhelmingly preferred millipedes, but also accepted centipedes, termites, isopods, grasshoppers, and beetle larvae (Suzzoni et al., 2000; Schatz et al., 2001). Queens foraging shortly after colony foundation, on the other hand, ignored large millipedes and preferred smaller, more easily captured prey such as isopods or termites (Déjean & Suzzoni, 1991; Suzzoni et al., 2000). The presence of millipedes in the diet of a P. minor colony is required for it to produce reproductive females and significantly enhances the production of workers, but is not required for production of male brood (Suzzoni et al., 2000).

Déjean & Suzzoni (1991) and Déjean et al. (2001) studied the capture of millipedes by P. minor. Workers of this species use their linear mandibles, paralyzing sting, and nestmate recruitment to capture and retrieve millipedes of a wide range of sizes, including very large individuals. Workers demonstrate significant flexibility in their foraging behavior, depending on the size and location of their prey. Their mandibles are able to grasp millipedes under 4 mm in diameter, which are stung repeatedly until paralyzed. Larger millipedes pose more of a problem and require creative strategies for capture and retrieval, including use of mandibular snapping (see below) and recruitment of two to five nestmates via use of a chemical trail. Large millipedes are either cut up or collectively transported whole, while smaller prey are retrieved by single workers. Individual P. minor foragers are able to retrieve millipedes weighing more than 100 times their own weight, the largest ratio of prey to worker weight known for any ant (Déjean et al., 2001). Like Plectroctena minor, foragers of Plectroctena mandibularis recruit nestmates to assist in prey retrieval (Fletcher, 1973), and also lay chemical trails from the pygidial gland for individual orientation and recruitment during nest emigrations (Villet et al., 1984; Wilkins et al., 2006).

Plectroctena workers are able to snap their mandibles to stun or kill enemies or prey (Déjean and Suzzoni, 1991; Déjean et al. 2001, 2002). This behavior is unique among ponerines but also occurs in the ambyloponine genus Mystrium and some termites (Gronenberg et al., 1998). The forceful snapping of the mandibles is used in territorial aggression, defense, and prey capture. In the study of Déjean et al. (2002), P. minor foragers almost always snapped their mandibles when confronted with termite soldiers (which are potentially dangerous) or large prey, while smaller prey were usually captured without snapping. Déjean et al. (2002) suggest that the snapping mechanism is an adaptation to hunting in tight spaces, though it is also an effective weapon against other ants and is readily employed when other ponerines (especially other Plectroctena) are encountered in the vicinity of the nest.

Nomenclature

 *  PLECTROCTENA [Ponerinae: Ponerini]
 * Plectroctena Smith, F. 1858b: 101. Type-species: Plectroctena mandibularis, by monotypy.
 * Plectroctena senior synonym of Cacopone: Bolton, 1974c: 313.
 * CACOPONE [junior synonym of Plectroctena]
 * Cacopone Santschi, 1914d: 325. Type-species: Cacopone hastifer, by monotypy.
 * Cacopone junior synonym of Plectroctena: Bolton, 1974c: 313.

Description
Schmidt and Shattuck (2014):

Worker
Medium to very large (TL 5.6–23.5 mm; Bolton, 1974) ants with the standard characters of Ponerini. Mandibles linear, crossing each other apically when closed, edentate or with one or two teeth, and with a dorsal longitudinal groove and a basal groove. Clypeus excavated anteromedially and with a lateral excavation near each mandibular articulation. Frontal lobes greatly expanded, closely approximated, and overhanging the anterior clypeal margin. Eyes small to absent, located far anterior on the sides of the head. Mesopleuron divided by a transverse groove, the anepisternum apparently fused to the mesonotum and metapleuron. Metanotal groove usually absent, occasionally vestigial. Propodeum broad dorsally, the posterolateral margins expanded into lamellae. Propodeal dorsum rarely with a weak longitudinal groove. Propodeal spiracles round. Metapleural gland orifice opening laterally. Meso- and metafemora with a dorsal longitudinal groove. Metatibial spur formula (1p). Anteroventral articulatory surface of petiole long and broad, with a narrow median groove. Petiole nodiform. Gaster with a strong constriction between pre- and postsclerites of A4. Head and body shining, punctate, with striations on the sides of the mesosoma, minimal pilosity, and no pubescence. Color red to black.

Queen
Usually alate, but ergatoid in some species. Alate queens are similar to workers but slightly larger, with larger eyes and with ocelli. Ergatoids are similar but at most have only vestigial ocelli (Bolton, 1974; Bolton & Brown, 2002).

Male
See description in Bolton (1974); also discussed in Bolton & Brown (2002).

Larva
Described for P. mandibularis by Wheeler & Wheeler (1989).