Probolomyrmex

Pantropically distributed but rare wherever they do occur (Taylor, 1965a; Agosti, 1994; O'Keefe & Agosti, 1997; Ito, 1998; Fisher, 2007). A scarcity of material was a challenge for the first global taxonomic revision. It included just 57 specimens belonging to nine species (Taylor, 1965a). Since Taylor's (1965a) revision further new species have been described (e.g. Agosti, 1994; O'Keefe & Agosti, 1997; Eguchi et al., 2006; Fisher, 2007; Bolton, 2012). These rarely encountered ants are cryptic foragers in leaf litter and presumably also forage below the ground surface. Nests are small, complete colonies containing from a few up to about 20 workers, and are found in soil under rocks, in leaf litter or in rotten wood. Most species in this genus have normal, fully winged queens, one species (Probolomyrmex guanacastensis) is known to have ergatoid queens (O'Keefe and Agosti (1997)). An Asian species seems to be a specialised predator of polyxenid millipedes (Ito, 1998), though it remains unclear if the other genus members prefer the same diet.

Identification
The members of the genus share the following characteristics in the worker: body hairless but with extremely fine pubescence, micro-punctate, overlain by scattered foveolae; antennal socket located on shelf-like frontoclypeal region that overhangs the mandibles; mandible small, elongate-triangular, with acute apical tooth followed by a series of small denticles; outer surface of mandible with several thick and short setae; promesonotal suture and metanotal groove absent; abdominal tergite IV not vaulted and sternite IV not reduced different from the members of the tribe Proceratiini) (see also Taylor, 1965; Bolton, 2003). The antenna is 12- segmented in the worker and queen, and 13-segmented in the male. The mesosoma is unreduced in the queen. (Eguchi et al. 2006)

The frontal lobes are reduced to a narrow, sharp ridge between the antennal sockets and the insertion point of the antennae are clearly visible when viewed from the front. The eyes are absent. The lack of eyes, combined with the exposed antennal insertion points, will allow identification of these ants.

Afrotropical species
Images of Afrotropical species

Biology
Taylor (1965) - Very little is known concerning the biology of Probolomyrmex. The few available ecological details indicate that most of the extra-Australian collections were made in rain-forest, or in islands of native forest in plantations. Nests in such situations are apparently located in leafmould or fragments of rotting wood on the forest floor. A shift in ecological preferences may have taken place in the evolution of the Australian Probolomyrmex greavesi; both collections of this species were made in drier forest types (open Eucalyptus woodland and an exotic Pinus plantation), in which the nests were located in the soil under rocks.

Some features of the social biology of Probolomyrmex angusticeps are known and described. These are based on the only known observations of a live colony of Probolomyrmex; unfortunately it is impossible to estimate whether certain features, particularly the peculiar aspects of larval and pupal life and such details as colony size and composition, are normal for the genus. Direct positive feeding records are not available, although the holotype worker of Probolomyrmex brevirostris was taken in a termite nest, where it may have been seeking prey. It is noteworthy that several other ponerine genera (Discothyrea and Proceratium), which have similar oral and anterior head structure to that of Probolomyrmex, are evidently obligatory arthropod egg predators (Brown, 1957). All known sexual forms of Probolomyrmex are of the normal winged type, so that colony proliferation probably includes a mating flight, as is usual in ants.

Oliviera and Feitosa (2019) - Traditional collection techniques for ants have shown little efficiency in finding Probolomyrmex specimens. This is due, in part, to the cryptic habit of the species, the low population densities of the colonies, and the solitary foraging of workers (Kikuchi & Tsuji, 2005; Ito, 1998). O’Keefe & Agosti (1997) collected only two specimens of Probolomyrmex in more than 2000 pitfall samplings between 1995 and 1996 in Barro Colorado, Panama. Hita Garcia & Fisher (2014) examined only 41 specimens from more than 6000 leaf-litter samples, 4000 pitfall traps and 9000 additional hand collecting events between 1992 and 2011, in Madagascar. Shattuck et al. (2012), in the revision of the Australian and Melanesian species of the genus, observed only 39 specimens available in collections for those regions. Given the difficulty of collecting specimens, Probolomyrmex is relatively under-represented in entomological collections.

Nests of Probolomyrmex have been observed for at least three species in the Neotropics and Asia. They can be established in rotten logs, empty snail shells, and natural cavities in soil and leaf-litter, comprising about 20 workers (Taylor, 1965; Ito, 1998; Kikuchi & Tsuji, 2005). Colonies can be found in a wide diversity of environments including xeric habitats, dry forests, grasslands, lowland and montane rainforests, occurring from 180m to 2150m (Shattuck et al. 2012; Fisher, 2007). Little is known about the feeding habits of Probolomyrmex, but some observations suggest that they may be specialist predators of polyxenid millipedes as observed for the ponerine ant Thaumatomyrmex (Ito, 1998).

Most species for which the reproductive caste is known have winged queens and males, suggesting that mating occurs in a nuptial flight. However, an ergatoid queen is reported for at least one species, Probolomyrmex guanacastensis (O’Keefe & Agosti, 1997).

Nomenclature

 *  PROBOLOMYRMEX [Proceratiinae: Probolomyrmecini)
 * Probolomyrmex Mayr, 1901b: 2. Type-species: Probolomyrmex filiformis, by monotypy.
 * Probolomyrmex senior synonym of Escherichia: Taylor, 1965d: 346.
 * ESCHERICHIA [junior synonym of Probolomyrmex]
 * Escherichia Forel, 1910c: 245. Type-species: Escherichia brevirostris, by monotypy.
 * Escherichia junior synonym of Probolomyrmex: Taylor, 1965d: 346.

Description
Taylor (1965):

Worker
Known for all species except the South American Probolomyrmex boliviensis.

Small sized monomorphic ponerine ants. Head longer than broad, its maximum width less than 0.5 mm. Clypeus and anterior part of frons produced forwards as a narrow subrectangular shelf bearing the exposed and closely approximated antennal insertions, which are separated by a thin, vertical lamella formed by fusion of the frontal carinae. Mandibles small, elongate-triangular, obscured in facial view by frontoclypeal process; each with an acute apical tooth followed by a series of small denticles, the anterior one of which may be enlarged. Labrum transverse, its anterior border with a deep median cleft. Palpal formula, maxillary 4: labial 2. The 3 basal maxillary palpomeres about subequal in size (1-1.5 times longer than broad), the apical one longer (3-5 times longer than broad). Labial palpomeres subequal in length, about 2.5-4 times longer than broad. Eyes lacking, except in the unique holotype of Probolomyrmex brevirostris, in which they are well developed, with about 14 facets. Antennae 12-segmented; apical portion of scape with the flexor surface more or less concave in cross-section, receiving the folded funiculus; the latter slightly incrassate but without a distinct segmental club, its second joint sometimes strongly transverse, apical joint about as long as the 3 preceding together.

Body and legs slender. Mesosomal sutures virtually lacking, represented only by weak ventrolateral traces. Propleura inflated, projecting ventrally. All tibiae with a single pectinate spur; pretarsal claws simple, lacking a median tooth. Declivitous face of propodeum margined on each side by a low obtuse carina, which is usually bluntly dentate above. Petiolar node narrow, strongly arched above, higher behind than in front, with an evenly curved anterodorsal profile and an almost vertical posterior face. The latter usually quite strongly concave in side view and enclosed laterally and dorsally by a low carina. A moderate constriction between first and second post-petiolar segments. Second post-petiolar segment (abdominal IV) with its tergite and sternite fused laterally to form a tubular structure (as usual in ponerine ants). Sting well developed.

Sculpturation with 2 basic components: dense fine shagreening and associated large scattered punctures, latter often weakly incised and rarely lacking. Pilosity very reduced, limited to a few minute bristles on underside of frontoclypeal shelf, some long stout hairs on mandibles and a few fine ones about openings of metapleural glands. Pubescence very fine and short, essentially absent in some species, moderately abundant in others. Colour pale yellowish- or reddish-brown.

Because of the extreme structural reduction of Probolomyrmex, taxonomic discrimination of the species is almost entirely dependent on characters of dimensions and proportions, especially those of the head, antennae and node, and sculptural details.

Brown (1975) - Size small; TL only 2.4-4.2 mm.

Antennal insertions very close together and situated far forward on a platelike extension of the anterior part of the head, consisting of the shortened clypeus fused with the surrounds of the antennal insertions and the completely eclipsed frontal area. Separating the antennal insertions is a thin, rounded vertical plate representing the raised and fused frontal lobes. The clypeofrontal structure covers the small curved mandibles, which are modified-triangular, each with a short masticatory border carrying an acute apical tooth and a few small, serially crowded teeth or denticles basad. No basidorsal mandibular groove. Palpal formula, so far as known, 4, 2.

Compound eyes usually absent in worker; present and rather small in one species, and even in this case, the specimen could be an ergatoid queen. Ocelli absent.

Antennal scapes fail to reach posterior border of head by a considerable margin.

Trunk completely fused into one structure; without dorsal sutures.

Middle and hind tibiae each with a single spur; tarsal claws simple.

Full adult color yellowish brown to red brown.

Queen
This caste is known for only four Probolomyrmex species: Probolomyrmex parvus, Probolomyrmex greavesi, Probolomyrmex angusticeps and Probolomyrmex boliviensis. The general habitus is very standard, with interspecific differences parallel to those of the workers, which are known for all these species except boliviensis.

Size about as in conspecific workers. Structure and proportions of head capsule, frontoclypeal process, mandibles, labrum, labio-maxillary complex, oral palpi, antennae, legs, petiole and gaster almost exactly as in workers; the scapes proportionately a little shorter and the gaster slightly more voluminous. Compound eyes and ocelli well developed. Mesosoma structurally unreduced. Pronotum large; pro pleura as in worker. Mesoscutum lacking notauli; parapsidal lines fine but distinct. Profile of mesonotum not indented at trans-scutal suture, which is finely incised. Scutellum shield-shaped, its anterior border straight, dorsal outline (viewed from side) evenly convex. Metanotum moderately convex, not produced into a point like that of the male. Suturation lacking between metepistemum and propodeum; general form of latter as in worker.

Wings (known for two species only) long and narrow, with very reduced venation. Fore wing with a single closed (median) cell. Hind wing with a single longitudinal vein (probably radius + subcosta) and 3 subapical hamuli and with no trace of an anal lobe. Pilosity, pubescence, sculpturation and colour as in conspecific workers.

Brown (1975) - Much like worker in size and form, but winged when virgin. Ocelli present and fairly well developed. Flight sclerites developed. Venation greatly reduced; in fore wing, consisting only of R + Sc, Stigma, M + CuA, and basal vein (with closed CuA continuing free to hind margin) forming a single closed (“median”) cell, and a floating radiapex (= 2r continued into the apical free abscissa of Rs, which does not reach margin). In the narrow hind wing, only a single vein persists (R) in the basal half of the wing near the anterior margin, and there are 3 submedian hamuli; anal lobe absent. Compound eyes moderately well developed.

Color as in worker.

Male
The only known male of Probolomyrmex is a para type of the Australian Probolomyrmex greavesi, which is described below.

Head subglobose, frontoclypeal region and frontal carinae produced anteriorly as in the female castes. Antennae 13-segmented; scapes relatively long, reaching back to the anterior ocellus; funiculus slightly incrassate. Mandibles large, triangular, with a single strong apical tooth; masticatory border rounding evenly into posterior one. Palpal formula, maxillary 4: labial 2; proportions of palpomeres as in worker.

Pronotum well developed. Mesonotum lacking notauli; parapsidal lines distinct. Scutellum moderately convex. Metanotum produced into an obtusely pointed median dorsal tooth. Metepisternum separated from propodeum by a strong suture, and itself divided obliquely into anepistemal and katepisternal areas. Legs each with a single pectinate tibial spur. Pretarsal claws simple. Wing structure and venation as in female. Petiole rounded above, with a low, simple subpetiolar process. Constriction between postpetiole and gaster barely marked. Second post-petiolar segment with its tergite and sternite fused laterally to form a tubular structure, which is slightly arched ventrally. Pygidium (tergite VIII) without a terminal spine, its apex broadly rounded. Cerci lacking. Subgenital plate (stemite IX) short, its apical margin transverse, with a very obtuse median point. Genital capsule simple. Basal ring entire; gonoforceps fairly narrowly digitate; volsellae well developed, cuspal heads somewhat expanded, and digitae simple; penis valves triangular, narrowed apically, with ventral edge finely serrate, teeth directed basally.

Brown (1975) - Size nearly that of corresponding workers and queens.

Head subglobular, including large eyes, slightly broader than long. Mandibles short, subtriangular, opposable, mostly covered by the projecting clypeofrontal plate with its fused vertical frontal lobes. Antennal insertions on the clypeofrontal plate, at the very anterior margin of the head. The plate itself is not as strongly projecting as in the workers and queens.

Notauli lacking. Metanotum forms a blunt median tooth.

Wings as in queen. Middle and hind legs each with a single tibial spur. Tarsal claws simple.

Petiolar node rounded above, and rounded into posterior face without margin, teeth, or angles.

Genitalia, according to Taylor, with all primitive parts. Pygidium with rounded apex. Cerci absent (or vestigial?).

Larva
Described from two cuticles of final instar larvae, which originally contained pharate pupae.

Body straight, elongate-subelliptical, with 13 differentiated somites, separated by rather indistinct intersegmental lines. Head anteroventral, almost orthocephalic. Prothorax not narrowed to form a neck. Abdomen stout, diameter greatest at its third and fourth segments. Leg vestiges present on all thoracic segments. Spiracles small, apparently lacking on prothorax and last 2 abdominal segments. Terminal somite forming a stout, blunt, posteroventrally directed tail; also with a median posterodorsal suspensory process; a low cone-shaped structure articulated to the terminal somite by a narrow neck (in life the flat base of this cone serves to attach the larva to the ceiling or walls of the nest). Anus ventral, at anterior base of tail. Sides of body longitudinally crinkled (this may not be a feature of the larval cuticle prior to pupal formation). Body beset with numerous low mammiform tubercles, 12 each on the thoracic and first 8 abdominal segments; arranged in 12longitudinal rows: 2 mid-dorsal, 2 mid-ventral, a midlateral pair on either side, and single dorsolateral and ventrolateral series. The tubercles form a single transverse row on each somite, except the prothorax, where the ventrolaterals are displaced anteriorly, and the mesothorax, where the mid-ventrals are displaced slightly forwards to accommodate the leg vestiges. Mid-ventral prothoracic tubercles displaced laterally by the leg vestiges and a large median welt, which lies across anteroventral part of segment and is apparently not homologous with the tubercles. Each tubercle bears a single median nipple-like papilla, except the prothoracic ventrolaterals, which each carry 2 papillae, the anterior one with a pair of minute bristle-like sensilla. Tubercles and papillae vary in size and shape. Integument, apart from the surfaces of the tubercles, densely papilligerous; papillae 0.003-0.005 mm. high, arranged generally in transverse rows. Pilosity completely lacking.

Cranium large, subcircular in anterior view, slightly concave behind. Head naked, except for a few sensilla and some minute hairs. Antennae a pair of low flat subcircular elevations, each with 3 sensilla. Mouthparts only moderately prominent. Labrum small, semicircular, breadth at base slightly more than twice length; apical border entire, with a few small sensilla; posterior surface densely spinulose, the spinules arranged in arcuate rows. Mandibles long, narrow, moderately sclerotised, not greatly expanded at base; apex slightly curved posteriorly and drawn into a strong mesally inclined tooth, with 2 much smaller teeth on its inner border. Maxillae hemispherical. Palpi not peg-like, each represented by a group of 3 sensilla. Galea closely adjacent to palpal sensilla, a relatively very small finger-like structure with a slender apical process. Labrum prominent. Palpi reduced similarly to those of maxillae, each represented by a group of 4 sensilla. Opening of sericteria small, slit-like. Hypopharynx spinulose, the spinules arranged in many short arcuate rows.

The Probolomyrmex larva is distinguished from those of all other known ponerine ants by the shape of the body and the unique posterodorsal suspensory organ, which is analogous (but clearly not homologous) with the dorsal “doorknob” tubercles found in some genera of the tribe Ponerini (see G. C. and J. Wheeler, 1952, 1964).

Pupa
This stage is known only for Probolomyrmex angusticeps, the pupae of which are unusual in that they lack cocoons. A very few other ponerine ants, including some species of Amblyopone, Discothyrea and Ponera, share this same negative characteristic. It is not a universal character in any of these genera and may not be in Probolomyrmex.