Streblognathus Mayr

Schmidt, C. A. & Shattuck, S. O., 2014, The Higher Classification of the Ant Subfamily Ponerinae (Hymenoptera: Formicidae), with a Review of Ponerine Ecology and Behavior, Zootaxa 3817 (1), pp. 1-242 : 135-137

Streblognathus Mayr

Fig. 26

Streblognathus Mayr, 1862: 716 (as genus in Ponerinae [Poneridae]). Type-species: Ponera aethiopica Smith, F., 1858: 91 ; by monotypy.

Streblognathus is a small genus (two described species) restricted to southern Africa. It is notable for the large size of its workers and for its reproduction by gamergates.

Diagnosis. 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 .

Synoptic description. Worker. Very large (TL 25 mm; Robertson, 2002) ants with the standard characters of Ponerini . Mandibles subtriangular, with relatively short masticatory margins and a weak basal groove. Anterior margin of clypeus straight or with a broad concavity, bounded on each side by a short tooth. Frontal lobes widely separated anteriorly by an extension of the clypeus. Eyes of moderate size, located just anterior of head midline and somewhat medially. Metanotal groove very shallowly impressed. Propodeal dorsum narrowed, with a shallow longitudinal depression and a pair of small teeth at the posterodorsal margin. Propodeal spiracle slit-shaped. Metapleural gland orifice with a broad shallow cuticular flange posteriorly. Metatibial spur formula (1s, 1p). Petiole fin-shaped, in profile with a convex anterior face and a concave posterior face, tapering to a sharp point dorsally, with sharp lateral and anterior margins on the dorsal third; petiole taller than the mesosoma and gaster. Gaster squat, without a girdling constriction between pre- and postsclerites of A4. Stridulitrum present on pretergite of A4. Head and body sparsely punctate, with generally sparse pilosity except for scattered short decumbent black hairs on the head and pronotum; pubescence of moderate density. Color black.

Queen. Unknown and apparently absent.

Male. See description in Robertson (2002).

Larva. The larvae of S. “ aethiopicus ” were described by Wheeler & Wheeler (1989), though given their collection locality they were probably actually larvae of S. peetersi (described later by Robertson, 2002).

Geographic distribution. Streblognathus is restricted to Lesotho, Swaziland and South Africa ( Robertson, 2002).

Ecology and behavior. 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 S. aethiopicus ; mean = 95 for S. peetersi ; Ware et al., 1990; Peeters, 1993). The queen caste is entirely absent, with reproduction instead being performed by a single mated gamergate worker. This gamergate, or “alpha” worker, is morphologically indistinguishable from the other workers but differs from them 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 identify the alpha worker and aggressively prevent sub-alpha workers from ascending to dominance unless the alpha senesces. In such instances, high-ranking workers aggressively compete until a single alpha ascends to dominance; this is usually the previous “beta,” or second ranked, worker ( Cuvillier-Hot et al., 2004a, 2004b).

Phylogenetic and taxonomic considerations. Streblognathus was erected by Mayr (1862) to house the species Ponera aethiopica F. Smith. All subsequent authors have maintained distinct generic status for this taxon. Robertson (2002) revised the genus and divided S. aethiopicus into two species. We continue to treat Streblognathus as a distinct genus on both morphological and molecular grounds. Schmidt's (2013) molecular phylogeny of the Ponerinae places Streblognathus solidly within the Odontomachus group, though its sister group is unresolved.

Carpenter (1930), in describing the fossil ponerine genus Archiponera , argued for a close relationship between Streblognathus and Dinoponera , even going so far as to suggest that they be considered a supergenus. He apparently based this hypothesized relationship on the relatively medial placement of the eyes, the presence of paired teeth on the anterior margin of the clypeus, and presumably their large size; to this list of similarities could be added the loss of a queen caste. This purported relationship has been repeated in much of the subsequent literature on Streblognathus and Dinoponera (e.g., Haskins, 1970; Haskins & Zahl, 1971), but is clearly false. The similarities between Streblognathus and Dinoponera are apparently the result of convergence, as morphological and molecular evidence otherwise argue against a sister relationship between these genera. Archiponera , which Carpenter believed was close to both Dinoponera and Streblognathus , has uncertain affinities but is unlikely to actually represent an ancestor or sister group to either of these extant genera (see further discussion under Dinoponera ). The true sister group of Streblognathus is still unresolved.

Figures

FIGURE 26. Worker caste of Streblognathus peetersi: lateral and dorsal view of body and full-face view of head (CASENT0173636, April Nobile and www.antweb.org); world distribution of Streblognathus.