Streblognathus

This southern African genus has two species, one in the southern karoo of the Eastern Cape and the other in the eastern grasslands.

Identification
Robertson (2002) -The shape of the petiolar node in the worker, with its sharp-edged dorsal margin and concave posterior face is unique among ants. The shape of the anterior margin of the clypeus is similar to that of Dinoponera in that in both genera there are lateral projections with a concave margin in between. However, the projections are acute and spine-like in Dinoponera whereas in Streblognathus they are merely obtuse points or short and tuberculate.

Schmidt and Shattuck (2014) - Diagnostic morphological apomorphies of Streblognathus workers include their subtriangular mandibles, paired teeth on the anterior clypeal margin, small paired propodeal teeth, broad cuticular flange posterior to the metapleural gland orifice, and tall fin-like petiole. This combination of characters does not occur in any other ponerine genus, and indeed the shape of the petiole is unique among ants. Subtriangular mandibles occur in several other ponerine genera, paired clypeal teeth occur in Dinoponera, propodeal spines or teeth occur in a handful of other genera, and a cuticular flange behind the metapleural gland orifice occurs in Paltothyreus, but none of these genera has all the apomorphies of Streblognathus in combination, and none of them has a similar petiole. Streblognathus workers are the largest of any African ponerine, exceeded globally only by those of Dinoponera.

There are two species of Streblognathus

Streblognathus aethiopicus - Scape both absolutely and relatively longer (Scape length 4.32-5.11 mm; Scape index 102-113); Eye length 0.86-1.03 mm; Mesonotal-propodeal length 5.56-6.32 mm; hind tibia length 4.80-5.54 mm

Streblognathus peetersi - Scape both absolutely and relatively shorter (Scape length 3.28-4.12 mm; Scape index 87-98); Eye length 0.62-0.85 mm; Mesonotal-propodeal length 4.34-5.47 mm; Hind tibia length 3.80-4.80 mm.

Species richness
Species richness by country based on regional taxon lists (countries with darker colours are more species-rich). View Data



Biology
Robertson (2002) - Distribution of the genus is limited to grassland and southern karoo regions of southern Africa, and it has the largest worker ants in Africa, measuring up to 2.5 cm long. Like many large southern African ponerine ants, Streblognathus has lost the queen caste and instead, reproduction is undertaken by a single, mated worker, termed a gamergate (Ware et al.  1990; Peeters 1991). Mandibular glands of workers contain three types of pyrazines which, in a study based on two colonies, were found to occur in smaller quantities and in different proportions in the gamergate compared with the mated workers (Jones et al. 1998). Colony size of S. aethiopicus is moderately small ranging from 9 to 51 workers (Ware et al. 1990). Workers stridulate, apparently for alarm purposes, by moving the presclerite of the second gastral segment, which has ridges on it, against the posterior edge of the first gastral segment (Lewis 1896, Ware 1994).

Schmidt and Shattuck (2014) - Relatively little is known about most aspects of Streblognathus ecology. They occur in arid thorn scrub and grasslands in extreme southeastern Africa, and apparently are specialist predators of tenebrionid beetles (Brown, 2000; Robertson, 2002). Workers stridulate when disturbed (Ware, 1994) and are able to differentiate nestmates from non-nestmates (Schlüns et al., 1996).

In contrast, a fair bit is known about the reproductive and social behavior of the genus. Colonies are small, with usually around 100 workers or fewer (mean = 35 for Streblognathus aethiopicus; mean = 95 for Streblognathus peetersi; Ware et al., 1990; Cuvillier-Hot et al., 2005). The queen caste is entirely absent, with reproduction instead being performed by a single mated egg-laying worker. This "gamergate" has the alpha rank in a dominance hierarchy, but is morphologically indistinguishable from the other workers except in its ovarian development (Ware et al., 1990), hormone levels (Brent et al., 2006), neurochemistry (Cuvillier-Hot & Lenoir, 2006), cuticular hydrocarbons (Cuvillier-Hot et al., 2005), and relative proportions of mandibular gland secretions (Jones et al., 1998). Within a colony, workers are behaviorally differentiated into foragers, nest workers and the sole reproductive gamergate (Ware et al., 1990).

A dominance hierarchy exists among the workers in a colony, with high-ranking workers subordinate to the alpha but dominant over the low-ranking individuals. Gamergates inhibit reproduction by subordinate workers through chemical signaling (Cuvillier-Hot et al., 2004b). The reproductive division of labor within the colony is further maintained by the low-ranking workers, who can recognize the alpha worker's level of fertility and aggressively prevent subordinate workers from ascending to dominance unless the alpha senesces (i.e. a drop in fertility). In such instances, high-ranking workers (often young) aggressively compete until a new alpha differentiates; this is usually the previous beta, or second ranked, worker (Cuvillier-Hot et al., 2004a, 2004b).



Castes
There is marked size variation in the workers of S. peetersi, and this has a geographical basis from north to south (Robertson 2002).

Nomenclature

 *  STREBLOGNATHUS [Ponerinae: Ponerini]
 * Streblognathus Mayr, 1862: 716. Type-species: Ponera aethiopica, by monotypy.

Worker
Robertson (2002) - Large, black, monomorphic ants. Head: Mandibles elongate with broad angle (about 138º) between apical and basal margin. Apical margin with three teeth followed by three denticles and with a tooth at the basal angle. Inner margin of apical tooth sinuate. Basal margin of mandible with an obtuse angle at about one third of its length from the base. The margin proximal to this angle lies adjacent to the clypeus when mandibles are closed. Mandibles glossy smooth or slightly shagreenate with sparsely distributed punctures containing hairs. Fringe of black or golden hairs inserted on dorsal surface just behind the distal two thirds of the basal margin and the basal tooth, and the hairs extending over the margin into the triangular space between the closed mandibles and the clypeal margin. Also a line of golden hairs along the external margin, and one behind the inner side of the apical margin. Maxillary and labial palps both four segmented. Margin of labrum deeply bisected and fringed with golden hairs. Anterior clypeal margin with middle section straight or concave, projecting beyond side margins and meeting them in a right angle or in a tooth-like projection. In anterior view, the region between the clypeal teeth is crescent-shaped, shiny, and overhangs the labrum. Posterior median portion of clypeus broadly inserted between the frontal lobes and indented medially. Clypeus with sparse, fine golden pubescence and smooth to shagreenate with sparsely distributed punctures containing short, black, pointed subdecumbent hairs that point anteriorly. Frontal triangle prominent between frontal carinae and inlaid into surface of head. Frontal lobes in dorsal view half obscuring the antennal sockets. Frontal carinae short, fading out just behind the level of the anterior margins of the compound eyes. Compound eyes in dorsal view situated at, or slightly anterior to, midlength of head and do not break the lateral margins. Head, excluding mandibles, rounded rectangular, usually a little longer than broad with a flat posterior margin and sides only slightly convex. Head covered in fine punctate sculpture, overlaid by fine, sparsely distributed golden pubescence, and covered by short, black, pointed, subdecumbent, curved hairs. Punctures poorly defined and unevenly distributed. Antennal scapes bowed, covered in sparse, short, golden or black subdecumbent or decumbent hairs and sparse, fine, golden pubescence. Neck between scape and condylar bulb (radicle) curved strongly downwards. Ventral surface of head smooth to shagreenate with variable densities of golden pubescence and with more-or-less evenly distributed anteriorly pointing, subdecumbent curved hairs.

Mesosoma. In dorsal view, promesonotal suture sharply defined, strongly curved backwards. Metanotal groove slightly impressed, defined in lateral view by the down- wardly curving mesonotum meeting the flat propodeal dorsum, although in some speci- mens the propodeal dorsum curves down slightly at the metanotal groove. In lateral view, propodeum with elongate, flat dorsum and a nearly vertical declivity. Dorsum often with a slight indentation about a quarter of the way from the metanotal groove. Lateral margins of dorsum sometimes distinct posteriorly. At the junction of the dorsum and declivity is a pair of upwardly projecting teeth which are continuous with the sharply raised lateral margins of the declivity. Propodeal spiracle slit-shaped. Large metapleural gland opening, fringed with long golden hairs. In many individuals there is a hairless shallow furrow pass- ing from the dorsal edge of the propodeal spiracle, round its posterior edge, and across to above the dorsal edge of the metapleural gland opening. This furrow is reduced to varying degrees across individuals. Mesosoma covered in fine, ill-defined punctate sculpture, smeared to striolate in parts, or appearing shagreenate. Mesosoma covered in sparse, fine, golden pubescence and dorsum with scattered subdecumbent short, black, pointed hairs.

Legs. Middle and hind tibiae each with a pair of pectinate spurs, the posterior spur longer and more pectinate than the anterior. Tarsal claws simple. All legs covered in short subdecumbent hairs.

Petiole. Petiolar node with about a third of its height above the level of the propodeal dorsum. In lateral view, anterior and dorsal faces form a single steep convexity that terminates in an acute point where it meets the vertical, concave posterior margin. Dorsum laterally compressed to form an acute, sharp-edged ridge. In dorsal view, petiole node longer than wide and with the posterior point of the dorsum overhanging the poste- rior face. Subpetiolar process in lateral view with more-or-less horizontal ventral margin meeting posterior margin at an acute but narrowly rounded angle. Petiole unsculptured except for hair pits, covered in sparse, golden pubescence and sometimes with a few strongly curved subdecumbent black hairs.

Gaster. Constriction between first and second gastral segments hardly discernable. Gaster smooth, shiny except for hair pits, and covered in sparse to moderately dense golden pubescence, and sparse black hairs that are bent near their tip. Sting well developed.

Male
Robertson (2002) - Head. Dominated by the compound eyes which take up 69-75% of the length of the head and which are hemispherical in dorsal view. Ocelli large, hemispherical. Width of median ocellus about the same as the shortest distance between the lateral ocelli. Occipital carina a blade-like ridge. Antennal sockets situated at midlength on the head; diameter of sockets greater than the shortest distance between them and also greater than the distance from outer margin of socket to margin of compound eye. Torulus built up into raised rim round antennal socket. Antennae 13-segmented, elongate, about 83% of total length (from head to tip of gaster). Scape short, equal to or slightly shorter than diameter of median ocellus. Pedicel length slightly shorter than half the length of scape. Flagellum decreases in diameter and length of individual segments from base to tip. Antennae cov- ered in fine, white pubescence. Anterior margin of clypeus built out as a narrow lamella that is broader at the sides than in the middle so that the median anterior margin is concave. Tentorial pits deeply impressed. Large frontal triangle present above the clypeus and between the antennal sockets. Head smooth with scattered hair pits except for shagreenate sculpture between the antennal sockets and on the frontal triangle and clypeus. Short, sparse golden hairs distributed over head; densest on frontal triangle and clypeus. Vertex reddish brown through to dark brown; golden brown forwards from the anterior margin of the median ocellus.

Mouthparts. Mandibles short, rounded, without teeth, not meeting when closed. Labrum bilobed and small, narrower than the distance between the closed mandi- bles. Maxillary palpi 4-5 segmented, the apical segment being a fusion segment in 4-seg- mented palpi. One individual had one palp 4-segmented and the other 5-segmented. Labial palpi 3-segmented.

Mesosoma. Pronotum small, vertical, without a dorsal surface; displaced by the large mesoscutum. Median line on mesoscutum fades out at about half its length. Parapsidal lines present, notauli absent. Scutellum dome-like, dorsellum strongly convex in lateral view and slightly overlapping the top of the propodeum. . Propodeum gently convex in lat- eral view and with spiracle slit shaped. Wings as illustrated.

Petiole. In lateral view, node more-or-less an equilateral triangle with nar- rowly rounded dorsum. In dorsal view, petiole longer than wide; spiracles on projecting tubercles; shallow ridge along the median line of node. Subpetiolar process with an acute, backwardly projecting spine. Petiole covered in backwardly projecting, slender, pale hairs.

Gaster. Smooth with fine, sparse, golden pubescence. Fifth gastral sternite with a pair of broad lateral ridges with a concavity in between them; long, curved golden hairs along posterior margin. Backwardly projecting golden hairs present on the lateral ridges and shorter less dense hairs in the concavity in between. Subgenital plate narrowing posteriorly and broadly rounded at the apex. Sixth gastral tergite terminates in an acute point

Genitalia. Parameres in cross-section with a sharp-edged dorsal margin and a broadly convex ventral margin. In lateral view, ventral margin of paramere is convex and dorsal margin more-or-less straight. In dorsal view, dorsal margin of paramere is bowed outwards. Exterior and apex of parameres with golden hairs. Volsellae pincer-like: digitus curved outwards and inner margin of cuspidus convex, with a narrow gap between them at their closest point. In lateral view, digitus arches downwards whereas cuspidus is more-or- less straight with a broadly rounded apex. Inner surfaces of both digitus and cuspidus cov- ered with small tubercles. Golden, curved hairs on ventral surface of volsella and inner surface of digitus and cuspidus. Ventral margins of penis valves with large, acute, sharp, backwardly projecting barbs, oriented vertically or outwards, depending on species. Apex of valves splayed outwards or inwards (depending on species) and broadly rounded with backwardly projecting barbs on ventral margin. Dorsal outer edges of penis valves joined by a large trough-like lamella that is V-shaped in cross-section, the whole complex forming the aedeagus.

Larva
Schmidt and Shattuck (2014) - The larvae of Streblognathus aethiopicus were described by Wheeler & Wheeler (1989), though given their collection locality they were probably actually larvae of Streblognathus peetersi (described later by Robertson, 2002).