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 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.

Eguchi et al. (2006) - 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.

Oliveira & Feitosa (2019) - Probolomyrmex can be easily recognized from the other genera of Proceratiinae mainly by the shape of the gaster. The second gastral sternite is not reduced, so the gaster is tubular and not bent antero-ventrally as in Discothyrea and Proceratium.

Afrotropical species
Images of Afrotropical species

Distribution
Oliveira & Feitosa (2019) - In the Neotropical region it is found from northern Argentina to southern Mexico, mainly in the leaf-litter of rainforests. In the Indo-Pacific Region Probolomyrmex is divided into two species-groups (Eguchi et al. 2006), Probolomyrmex greavesi group and Probolomyrmex longinodus group. Both species-groups occur from India east to Australia, with the greavesi group extending on to the Solomon Islands. In Australia, species can be found in habitats ranging from rainforests, Eucalyptus woodlands to spinifex grasslands (Shattuck et al. 2012). In the Afrotropical region the genus is well distributed in sub-Saharan forests (Hita Garcia et al. 2013). In Madagascar, species can be found in several environments, such as tropical dry forest, littoral rainforest, lowland rainforest, and montane rainforest (Hita Garcia & Fisher, 2014).

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.

Some species of Probolomyrmex present the postero-ventral lobe of the petiole subquadrate and lack a prora. On the other hand, species with a rounded postero-ventral lobe of the petiole often possess a prora. This could suggest a relationship between these structures, possibly involving a distinct bending mechanism of the gaster when ants are capturing (stinging) or transporting prey. The presence of a fitting mechanism between the prora and subpetiolar process could improve precision and better allow the sting to reach the mandibular region where the prey would be grasped. Thus, when bending the gaster forwards, and engaging the prora in the subpetiolar process, the postero-ventral lobe of the petiole could serve as a support, when they are rounded; in species with a subquadrate subpetiolar process, it would be an obstacle for that fitting to occur, therefore the prora is absent. In addition, if this fitting mechanism really occurs, it is possible that it causes friction between these two structures, which could explain the small differences in the degree of development of the prora and the subpetiolar process observed in individuals of the same species, such as in Probolomyrmex kelleri. This fitting mechanism between prora and subpetiolar process may occur in other genera in which these structures are present, as in Gnamptogenys, Prionopelta, Pseudoponera, and Thaumatomyrmex. In these groups, and even in the remaining proceratiine genera, the morphology of the abdomen seems to be closely related to alimentary habits (Hölldobler & Wilson, 1990). However, more detailed observations should be made to confirm this condition. Little is known about the feeding habits of Probolomyrmex, except for observations by Ito (1998) on the Oriental species Probolomyrmex dammermani feeding on polyxenid millipedes. Different kinds of potential prey were offered to colonies of Probolomyrmex boliviensis kept in artificial conditions, but none were accepted (Taylor, 1965).

Ito (1998) - 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.

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.

Worker
Oliveira & Feitosa (2019) - (after Taylor, 1965; Agosti, 1994; Eguchi et al. 2006; Keller, 2011; and this study): Small monomorphic ants. Body color from pale yellow to reddish brown. Integument opaque and covered by extremely fine pubescence, small punctures or foveae; foveae generally deeper and more visible on first segment of gaster. Head foveated, interval between foveae covered by micropunctures. Antennal scapes densely punctate, with sparse small foveae. Dorsum of mesosoma and metasoma with incomplete (open posteriorly) foveae; latero-dorsal region of pronotum with micropunctures and sparse foveae.

Outer surface of mandibles with thick and short setae. Antennal funiculi with abundant pubescence and relatively long appressed hairs set in longitudinal grooves, space between hairs filled by short hairs.

Head longer than wide, with broadly convex sides. Sclerites of the anterior frons and the clypeus are highly fused and extend anteromedially beyond the clypeo-labral articulation, forming a shelflike platform that projects anteriorly covering mandibles in full-face view. Antennae with 12 segments; antennal insertions fully exposed, positioned on frontoclypeal shelflike projection and separated by narrow vertically raised carina, which is formed by the fusion of the frontal carinae. Mandibles small and triangular, hidden in frontal view by frontoclypeal shelflike projection; each mandible has a well-developed apical tooth followed by series of denticles; palpal formula 4,2; three basal maxillary palpomeres equal in size, and apical one longer; labial palpomeres equal in size. Eyes absent, except for holotype of Probolomyrmex brevirostris Forel, one worker of Probolomyrmex dentinodis, and two workers of Probolomyrmex kelleri.

Mesosoma slender and long in profile, with dorsum flat to weakly convex, without visible promesonotal and metanotal sutures, which are represented only by weak ventro-lateral superficial lines. Propleuron inflated, projecting ventrally. Propodeal declivity usually posteriorly emarginated on each side by low and obtuse carina, which can present an apical tooth. All tibiae with single pectinate spur; tarsal claws simple, without internal teeth. Petiolar node narrow and strongly elevated, with uniform antero-dorsal curve in profile; posterior face high and usually concave in lateral view. Subpetiolar process present. First and second segments of gaster separated by constriction. First segment of gaster with tergite and sternite fused laterally forming tubular structure. Sting well-developed.

Taylor (1965) - 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
Oliveira & Feitosa (2019) – (after Taylor, 1965; Agosti, 1994; Eguchi, et al. 2006; and this study): Size, color, sculpturing, structure of head, appendages, and metasoma as in workers. Additionally, queens present well-developed eyes, three ocelli similar in size and associated to dark spots. Pronotum subtriangular in lateral view. Mesoscutum and scutellum shield-shaped; transcutal suture straight; notauli absent; parapsidal lines vestigial. Mesopleural sulcus diagonal, rendering katepisternum triangular. Metanotum convex. Flight sclerites developed in most species. Wings long and narrow, venation greatly reduced. Forewings with Sc+R and M+Cu forming submedian cell; veins 2r-rs, Cu, cu-a e A not extending to external border of wing. Posterior wings with short R-Rs and A veins; three submedian hamuli present.

Male
Oliveira & Feitosa (2019) – (after Eguchi et al. 2006; Yoshimura & Fisher, 2009; and this study): Head subglobose, frontoclypeal shelflike projection not as strongly projecting as in workers and queens. Antennae with 13 segments; scapes relatively long. Eyes well developed, occupying half of head lateral margins; three ocelli present. Metepisternum separated from propodeum by a strong suture. Wings as in female

Taylor (1965) - The only known male of Probolomyrmex is a paratype 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
Taylor (1965) - 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
Taylor (1965) - 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.