Streblognathus peetersi

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Streblognathus peetersi
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Subfamily: Ponerinae
Tribe: Ponerini
Genus: Streblognathus
Species: S. peetersi
Binomial name
Streblognathus peetersi
Robertson, 2002

Streblognathus peetersi casent0173636 profile 1.jpg

Streblognathus peetersi casent0173636 dorsal 1.jpg

Specimen labels

Colonies are small, averaging 95 workers, and reproduction is exclusively via gamergates.


At a Glance • Gamergate  

 

Identification

Robertson (2002) - In the worker, scape, mesosoma and legs are shorter relative to width measurements in Streblognathus peetersi than in Streblognathus aethiopicus with scape index being the easiest measurement to separate the two species. The relatively longer body and appendages of S. aethiopicus, combined with its larger size results in Scape length, Meso-propodeal length and tibia length being longer in this species than in S. peetersi. There is also only a slight degree of overlap in Eye length. Mandibles in S. aethiopicus are castaneous with a black border whereas in S. peetersi they are generally black although there is a small proportion of specimens with some castaneous colouration. In the male, the subgenital plate in S. peetersi has convex lateral margins and the apex is broadly rounded, whereas in S. aethiopicus the lateral margins are concave and the apex is flat. The barbed apices of the penis valves are oriented vertically in S. peetersi whereas in S. aethiopicus, they splay outwards. In contrast, the barbs along the outer margins of the penis valves, behind the apices, are splayed outwards in S. peetersi whereas in S. aethiopicus they are oriented vertically.

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.

Keys including this Species

Distribution

Streblognathus peetersi has a wide distribution in the grassland regions of eastern South Africa, Lesotho and Swaziland. In Vernon Crookes Reserve (300m altitude), colonies were found in both coastal grasslands and patches of dense forest, where excavation was hindered by numerous roots. In Giant’s Castle (Drakensberg mountains), this species occurs at an altitude of 2200m. In Magoebaskloof (700km to the north, in Limpopo province), nests occur among dense grasses and forbs, and often have to be located by baiting foragers.


Grassland habitat of S. peetersi at 2200m in Giant’s Castle (Drakensberg mountains), at the beginning of summer. These altitudes are covered with snow during winter. Photo by Christian Peeters.
Grassland habitat of S. peetersi near Magoebaskloof (Limpopo province). Nests are hidden in the dense layer of grasses and forbs. Photo by Christian Peeters.

Distribution based on Regional Taxon Lists

Afrotropical Region: Lesotho, South Africa (type locality), Swaziland.

Distribution based on AntMaps

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Distribution based on AntWeb specimens

Check data from AntWeb

Biology

Colonies of S. peetersi have a mean of 95 workers (range 33-163), with up to 45 cocoons and 69 larvae (Cuvillier-Hot et al., 2004a). Colonies are thus more populous than Streblognathus aethiopicus (mean = 35 workers; Ware et al., 1990). Nests have 1-2 entrances commonly surrounded by a distinctive mound of pebbles brought from the immediate vicinity. Mounds vary substantially in size, with tufts of grass often growing out of well-established mounds. There is a succession of inhabited chambers along a straight vertical axis, ending at a depth of 40-50 cm; this regularly corresponded to the occurrence of a gravel layer.

The queen caste is entirely absent, with reproduction instead being performed by a single gamergate that is morphologically indistinguishable from nestmate workers but differs in its ovarian development (Cuvillier-Hot et al., 2004a), hormone levels (Brent et al., 2006), cuticular hydrocarbons (Cuvillier-Hot et al., 2005), and neurochemistry (Cuvillier-Hot & Lenoir, 2006).

A dominance hierarchy exists in a colony, with high-ranking workers (mostly young) subordinate to the alpha but dominant over low-ranking individuals. Only alpha can mate with a foreign male, and she is then the gamergate. Behavioural observations of experimentally disturbed hierarchies were integrated with physiological measures of fertility (vitellogenin titre in the haemolymph) and chemical analysis of Cuticular Hydrocarbons, which are putative fertility pheromones. This multifaceted approach revealed that alpha differentiates through aggression among high-ranking workers, but once she is established, chemical signalling is enough to maintain its monopoly. As already found in several species of ants, social wasps and termites (Peeters & Liebig 2009), egg-layers have distinct profiles of cuticular hydrocarbons compared with infertile workers. During gaster curling (see photo), an alpha worker exposes the intersegmental membranes of her posterior tergites, while biting the base of a subordinate’s antennae (Cuvillier-Hot et al., 2004a). This behaviour is strikingly similar to the ‘gaster rubbing’ performed by alpha workers in Dinoponera quadriceps. The reproductive division of labor within the colony is further maintained by the low-ranking workers, who identify alpha and aggressively prevent high-ranking workers from rising to dominance unless alpha senesces. In such instances, high-rankers aggressively compete until an alpha rises to dominance; this is usually the previous beta, or second-ranked, worker (Cuvillier-Hot et al., 2004a, 2004b).


Gaster curling is a conspicuous dominance interaction in S. peetersi, and it functions to reveal alpha's up-to-date fertility signal. Photo by Patrick Landmann.
Low-ranking workers immobilize a dominant worker for hours or days, during which it loses its high rank. Photo by Patrick Landmann.


Ovaries of a young gamergate of S. peetersi. Yolky oocytes develop in all the ovarioles, but "yellow bodies" have not accumulated yet. Photo by Christian Peeters.
Ovaries of an old gamergate of S. peetersi. Oogenesis has stopped, but the large "yellow bodies" indicate substantial egg-laying activity in the past. Photo by Christian Peeters.


Rather than just ovarian activity, Cuticular Hydrocarbons reveal the individual hormonal state that underlies reproductive activity. 'High-rankers’ that are unable to lay eggs also have a specific cuticular profile; this is consistent with an intermediate state of fertility indicated by measures of vitellogenin levels (Cuvillier-Hot et al., 2004a). This reliable information about fertility encoded on the cuticle is also the basis for the policing behaviour of low-ranking workers. When the fertility of alpha was experimentally decreased with a juvenile hormone, she remained aggressive but her levels of vitellogenin were affected). This had two important consequences: (1) one of the high-rankers began to exhibit dominance behaviors typical of the alpha rank, although her interactions were not directed at the treated alpha; (2) low-ranking workers aggressed and immobilized the treated alpha after a few days (Cuvillier-Hot et al., 2004b). In parallel, the cuticular profile of the treated alpha shifted toward that of infertile workers, and the challenger’s profile became more similar to that typical of egg-layers. In simple terms, a young high-ranker detected a change in alpha 's signal and initiated a challenge, which in turn modified her hormonal state. This led to the start of oogenesis as well as a changed cuticular signal. After a few days, the police workers were confronted with two individuals with an above average fertility signal, and they immobilized the treated alpha. This experimental uncoupling of behavioural dominance and fertility gives a better understanding of the crucial role of sterile helpers in the control of reproduction in insect societies.


Comparison of the quantities of 17 long-chained hydrocarbons (23 to 39 carbon atoms) present on the cuticle of an infertile worker and an egg-laying alpha of S. peetersi. Different cocktails of these low-volatility molecules give reliable information about fertility. From Cuvillier-Hot et al., 2004a.

Castes

Queens have been lost in Streblognathus; reproduction is carried out by gamergates (mated, egg-laying workers). Workers of S. peetersi have 10 ovarioles, more than in workers of Streblognathus aethiopicus (8 ovarioles) despite being smaller (head width measured behind the eyes : S. peetersi = 3.84 ± 0.12 mm, n = 192 ; S. aethiopicus = 4.34 ± 0.15 mm, n = 108)

There are conspicuous differences in male genitalia between the two species of Streblognathus (Robertson 2002). The large, recurved barbs on the penis valves are possibly an adaptation for preventing easy withdrawal of the genitalia after insemination. Similarly in the queenless Dinoponera quadriceps male genitalia are complex and there is behavioral evidence for a mating plug (Monnin & Peeters 1998). Males in queenless ants need to locate foreign nests with a virgin alpha worker, and a successful mate benefits from suicide, thereby preventing additional males from fathering the colony’s progeny. This also seems true in Streblognathus.


Workers of S. peetersi from Magoebaskloof, Limpopo province, South Africa. Photo by Patrick Landmann.
Male of S. peetersi from Magoebaskloof, Limpopo province, South Africa. Photo by Patrick Landmann.


Worker

Male

Nomenclature

The following information is derived from Barry Bolton's New General Catalogue, a catalogue of the world's ants.

  • peetersi. Streblognathus peetersi Robertson, 2002: 11, figs. 1, 2, 3b, 3d, 4 (w.m.) SOUTH AFRICA.

Unless otherwise noted the text for the remainder of this section is reported from the publication that includes the original description.

Description

Worker

Robertson 2002. FIGURE 1. Streblognathus peetersi worker: (a) head, dorsal view; (b) anterior view of head between opened mandibles; (c) close-up of hairs situated dorso-laterally on head, above the eyes; and (d) dorso-lateral view of declivity of propodeum, and petiole.

Matching description of genus (see Streblognathus ) but with the following specimen-specific characters.

Size. Head width 3.87 mm; Head length 3.93 mm; Eye length 0.71 mm; Scape length 3.54 mm; Pronotal width 2.54 mm; Mesonotal-propodeal length 4.66 mm; Hind tibia length 4.04 mm; Cephalic index 99; Scape index 91; Eye index 18.

Head. Mandibles glossy smooth with sparsely distributed punctures containing hairs. Fringe of black hairs on anterior surface, behind basal margin and basal tooth. Anterior clypeal margin with middle section broadly concave, projecting beyond the outer margins and meeting them in a right angle. Compound eyes situated slightly anterior to mid-length of head. Fine puncturing on head poorly defined and smeared in appearance. Golden pubescence on head sparse to absent, limited mainly to the posterior, the central region and anterior to eyes. Antennal scapes with sparse golden pubescence overlaid by sparse decumbent black hairs. Venter of head smooth to shagreenate with sparse golden pubescence and uniformly distributed short, black hairs.

Mesosoma. Propodeal dorsum in profile mainly flat but curving down slightly to the metanotal groove and terminating posteriorly in dorsally projecting short spines. Slight indentation present about one quarter of the way from the metanotal groove. No distinct lateral margins to propodeal dorsum. A hairless shallow furrow passes from the dorsal edge of the propodeal spiracle, round its posterior edge, and across to above the dorsal edge of the metapleural gland opening. Mesosoma dorsum covered in fine, ill-defined punctate sculpture, some of the punctures flowing into one another. Sides with shagreenate sculpture. Mesosoma covered in sparse, fine, golden pubescence and dorsum with scattered subdecumbent short, black, pointed hairs.

Petiole. Appearing as in description of genus. Covered in sparse golden pubescence and uniformly distributed subdecumbent short black hairs.

Gaster. As described for genus.

PARATYPE WORKERS. Matching the description of the genus and the holotype but with the following additional variation. Size. Head width 3.83-4.12 mm; Head length 3.79-4.11 mm; Eye length 0.70-0.77 mm; Scape length 3.54-3.77 mm; Pronotal width 2.82-2.70 mm; Mesonotal-propodeal length 4.55-4.93 mm; Hind tibia length 3.91-4.26 mm; Cephalic index 98-102; Scape index 89-94; Eye index 18-19 (n=6).

ALL WORKERS. Size. Head width 3.43-4.46 mm; Head length 3.60-4.50 mm; Eye length 0.62-0.85 mm; Scape length 3.28-4.12 mm; Pronotal width 2.30-2.84 mm; Mesonotal-propodeal length 4.34-5.47 mm; Hind tibia length 3.80-4.80 mm; Cephalic index 95-102; Scape index 87-98; Eye index 17-21 (n=38).

Male

PARATYPES. Size. Head width 2.26-2.42 mm; Head length 1.83-1.90 mm; Eye length 1.25-1.32 mm; Ocellus diameter 0.55-0.69 mm; Forewing length 13.46-13.81 mm; Hind tibia length 4.02-4.12 mm; Cephalic index 122-128; Eye index 53-57 (n = 6).

ALL MALES. Size. Head width 2.02-2.51 mm; Head length 1.62-1.97 mm; Eye length 1.12-1.41 mm; Ocellus diameter 0.42-0.69 mm; Forewing length 13.46-14.83 mm; Hind tibia length 3.83-4.38 mm; Cephalic index 121-130; Eye index 53-57 (n = 12).

Type Material

HOLOTYPE: worker, SOUTH AFRICA, Limpopo Province: next to Magoebaskloof Hotel, 23º53'S 29º59E, 8.iv.2001, C. Peeters, SAM-HYMC017887a (SAMC). PARATYPE: workers (unless otherwise indicated): same locality as holotype, and collector is C. Peeters unless otherwise indicated: 24.i.2000, 'MAGOE #A', SAM-HYM-C015165; 24.i.2000, 'MAGOE#B', SAM-HYM-C015166; 24.i.2000, 'ZH-2', SAM-HYM-C015168; 24.i.2000, 'ZH-3', SAM-HYM-C015169 (males); 24.i.2000, SAMHYM-C017879; 24.i.2000, 'ZH-1', SAM-HYM-C017880; v.2000, 'ZH-5', SAM-HYMC017882; v.2000, 'ZH-6a', SAM-HYM-C017883; v.2000, 'ZH-7', SAM-HYM-C017884; J de Oliveira, 9.ii.2001, 'ZH-9', SAM-HYM-C017885; J de Oliveira, 10.ii.2001, 'ZH-10', SAM-HYM-C017886; 8.iv.2001, SAM-HYM-C017887 (workers and males) South African Museum. Some duplicate specimens of above paratype material have been deposited in The Natural History Museum, London and at the California Academy of Sciences, San Francisco.

Etymology

Named after Christian Peeters who drew my attention to this new species and collected the type material. It is also named after him in recognition of the substantial contribution he has made to our understanding of ponerine reproductive biology.

References