Large or medium-sized black or castaneous ants, with shining surface, sparsely punctate.
Worker monomorphic, with large, rectangular and rather flat head, with small, flat, anteriorly situated eyes. Clypeus very short, its anterior border straight in the middle, emarginate on each side at the mandibular insertion, apparently not extending back between the frontal carinae, the latter overhanging the clypeus and forming with the front an elevated lobe, longitudinally sulcate in the middle. Mandibles long, linear, feebly curved, with a deep narrow furrow running nearly their full length on the dorsal surface, their tips blunt, the inner margin armed with a strong tooth at the basal third and another obtuse tooth, sometimes indistinct, between the latter and the tip. Antennae 12-jointed, the funiculi somewhat thickened towards their tips, the first joint shorter than the second. Thorax large and depressed, premesonotal suture distinct, mesoepinotal suture obsolete, epinotal declivity marginate on the sides. Petiole with a laterally compressed node, with the anterior and posterior surfaces vertical in profile, the dorsal surface horizontal. Constriction between the postpetiole and gaster pronounced, with well-developed stridulatory surface. Gaster short, formed largely by the first segment. Median spurs of middle and hind tibiae large and pectinated, lateral spurs lacking.
Female winged, apterous or ergatomorphic, larger than the worker but otherwise similar. Eyes and ocelli small. Anterior wings with a discoidal cell, two cubital cells and the radial cell closed.
Male about the size of the worker. Frontal carinae short, erect, closely approximated, bringing the insertions of the antennae close together. Antennal funiculi filiform, their first joint very short; scapes stout, shorter than the second funicular joint. Mandibles small, linear, parallel-sided, edentate, with rounded tips. Mesonotum with distinct Mayrian furrows; scutellum longitudinally grooved in the middle. Genitalia retracted; pygidium terminating in a blunt or truncated point. Wings short.
Map 13. Distribution of the genus Plectroctena .
This singular genus is confined to the Ethiopian Region (Map 13). Arnold has observed the habits of the type species, P. mandibularis , in South Africa. "The entrances to the nest are generally indicated by large heaps of earth. The chambers are placed deep below the surface, seldom less than 2 feet, and the number of individuals seldom exceeds 50. It is a sluggish and timid ant, the workers foraging singly. The food includes termites, but consists chiefly of millipeds and beetles." Another South African species described by Arnold as P. subterranea is castaneous red, measures only 7.5 to 10 mm., and has exceedingly small eyes. It, in all probability, belongs to a different genus. In the generic key it runs down to Myopias and is provisionally referred to that genus.
The character of the females in the four described species of Plectroctena has not been adequately ascertained. Winged females of P. minor and subterranea are known, but no winged females of mandibularis . According to Arnold, this species has ergatoid females differing "from the worker chiefly in size, but the head and abdomen are proportionally wider and longer. The longitudinal impression on the pronotum is shallower, while that of the dorsum of the epinotum is deeper and wider. In a nest of three dozen or so individuals, not more than two or three of these forms are to be found, and usually only one." It seems that Forel saw one of these ergatoid females and described it as a subspecies (major) of mandibularis . There is, however, still another type of female, at least in P. minor, of which I describe a specimen below, with ocelli and slightly larger eyes than the worker and with the thorax essentially like that of the winged female, but without the slightest indications of ever having borne wings.
Fig. 10. Plectroctena cristata Emery . Worker, a. head from above; b, thorax and abdomen in profile.
Head subquadrate; antennae shorter than the head and thorax; the flagellum clavate, inserted very forward on the face near its anterior margin on each side of a central protuberance, which is advanced over and conceals the clypeus; the anterior margin of the latter widely emarginate; eyes small and ovate, not prominent, situated very forward on the sides of the head near the base of the mandibles; mandibles elongate, very slightly bent and crossing each other, their inner margin with a blunt tooth towards their base; the labial palpi 4-jointed; the maxillary palpi 3- jointed. Thorax oblong; the calcar at the apex of all the tibiae, pectinate within. Abdomen oblong, with a deep strangulation between the first and second segments; the node subglobose.
This subgenus of Ponera differs in such remarkable peculiarities, and will not assimilate with any divisional group yet proposed, that it must stand isolated for the present: the form of the mandibles appears to indicate some peculiar economy; the small advanced eyes are also remarkable: the characters, being drawn from a worker ant, will doubtless require hereafter some additions and modifications. The beautiful pectination of the tibial spurs is common to several of the Poneridae, but is sometimes only to be observed on the anterior legs, as in Ectatomma .
Plectroctena is an Afrotropical genus of ants, with most species occurring in the rainforest zones of West and Central Africa.[2] Some species are cryptic or subterranean foragers,[2] while others forage in open grassland terrain. The workers forage singly[3] or in groups of 2 to 3.[4] They nest in the earth at varying depths, or in collapsed logs. They prey mainly on millipedes, including their young or eggs.[2]
A colony of P. lygaria (a small species of the mandibularis-group) may number in excess of 300 adults. An excavated colony in the Ivory Coast consisted of 277 workers, 8 alate queens, and 42 alate males. Consequently 15% of their number was allocated to reproductives at the specific time.[2] The colony size of P. mandibularis however, seldom exceeds 50 individuals.[2][3]
In a colony of P. lygaria, the nest chambers are located at shallow depth, in moist soil under dense leaf litter. The deepest chamber may be located about 7 cm below the surface, with each chamber up to 1 cm in height.[2] The chambers are specialized to house either brood or prey items. Nests of the widespread species P. mandibularis however, are composed of chambers typically located 2 feet or more below the surface, with entrances that are usually marked by large piles of earth.[3][2]
It is believed that millipede eggs may at times constitute the exclusive diet of P. lygaria,[2] while newly emerged millipede young may be an additional food source for the smaller Plectroctena species. Larger species like P. conjugata, P. mandibularis and P. minor specialize on adult millipedes.[2]
P. mandibularis Plectroctena is an Afrotropical genus of ants, with most species occurring in the rainforest zones of West and Central Africa. Some species are cryptic or subterranean foragers, while others forage in open grassland terrain. The workers forage singly or in groups of 2 to 3. They nest in the earth at varying depths, or in collapsed logs. They prey mainly on millipedes, including their young or eggs.
