Octostruma gymnosoma

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Octostruma gymnosoma
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Subfamily: Myrmicinae
Tribe: Attini
Genus: Octostruma
Species: O. gymnosoma
Binomial name
Octostruma gymnosoma
Longino, 2013

Octostruma gymnosoma P jtlc000015356.jpg

Octostruma gymnosoma D jtlc000015356.jpg

Specimen Label

Octostruma gymnosoma is known only from the three type workers, all from one Winkler sample of sifted litter and rotten wood from the forest floor. The sample was taken in mesophyll cloud forest in the Sierra Madre de Chiapas, at 1520 m elevation. (Longino 2013)

Identification

Face lacking transverse arcuate carina; basal five teeth of mandible acute; apex of labrum bilobed; face and mesosomal dorsum lacking erect setae. This is the largest Central American species. It is very similar to the allopatric Octostruma schusteri, from Volcán Atitlán in Guatemala. The two differ only in the presence of spatulate setae on the face of O. schusteri. (Longino 2013)

Keys including this Species

Distribution

Distribution based on Regional Taxon Lists

Neotropical Region: Mexico (type locality).


Distribution based on AntMaps

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Distribution based on AntWeb specimens

Check data from AntWeb

Biology

Longino (2013) - Brown and Kempf (1960) summarized the biology of basicerotines as follows: The basicerotines all come from tropical or subtropical areas, and predominantly from mesic habitats, particularly rain forest, where they live primarily in the upper layers of the soil and in the soil cover, including large and small pieces of rotten wood. They are fairly common in soil cover berlesates. Nests have been found in snail shells, and in the peaty masses gathered about epiphytic ferns above the ground level. So far as is known, colonies are small, consisting of one or more dealate—or rarely ergatoid—females, and a few workers. Judging from the structure of the workers and females, one would suppose that they were predaceous on small arthropods...

Besides this summary, the behavior of three basicerotine species has been studied. Wilson (1956) observed a small captive colony of Eurhopalothrix biroi, a New Guinea species. Workers moved slowly and captured a variety of small, soft-bodied prey, including spiders, symphylans, entomobryid Collembola, campodeids, and hemipteran nymphs. Wilson and Brown (1984) observed a captive colony of Eurhopalothrix heliscata, a species from Singapore. The colony contained over 400 workers, multiple alate and dealate queens, several adult males, and brood. Foraging workers acted "rather like miniature ferrets," readily wedging themselves into small crevices. They foraged solitarily, attacking a variety of prey but mostly termites. They used their sharply-toothed mandibles to abruptly snap onto appendages of prey, maintaining purchase and slowly reaching around with the gaster to sting the prey. The strongly sclerotized labrum was also employed to press against the clamped appendage. The behavioral repertoire was limited. There did not appear to be trophallaxis, as workers and larvae fed directly from prey in the brood chambers. Nor did there appear to be any form of alarm communication. While there was generally an increase in the number of foragers when clusters of prey were presented, there was no evidence of any pheromone-based recruitment. Workers were non-aggressive and responded to disturbance by tucking the appendages and becoming immobile, often for minutes at a time. Wilson and Hölldobler (1986) studied captive colonies of Basiceros manni from Costa Rica and observed behavior not substantially different from E. heliscata. Foraging workers of many basicerotines are often encrusted with a firmly bonded layer of soil, which is thought to function as camouflage, enhancing crypsis (Hölldobler & Wilson, 1986).

Knowledge of the basic natural history of these ants has hardly progressed since the observations of Wilson, Brown, and Hölldobler. More specimens are now available for examination due to quantitative litter sampling, enhancing knowledge of basicerotine diversity and distribution, but discovering nests remains exceedingly difficult. Quantitative samples of 1 m2 litter plots reveals that small basicerotines can be very frequent, occurring in over 50% of samples in some cases, but never in large numbers. Individual samples usually contain fewer than ten workers, and workers are often accompanied by dealate queens. These results suggest that colonies, at least among New World species, are usually small, with tens of workers.

Less than half of the species of Octostruma have their queens described. Ergatoid queens are known from some species. Males are known from collections for some species but none have been described. The mating biology of these ants and how common ergatoid queens are across the genus and within colonies is not known.

Castes

Nomenclature

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

  • gymnosoma. Octostruma gymnosoma Longino, 2013: 33, figs. 1B, 3B, 5M, 25, 43 (w.) MEXICO.

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

Description

Worker

HW 0.88–0.98, HL 0.76–0.82, WL 0.91–1.07, CI 115–120 (n=3). Labrum triangular, about as long as wide, strap-like lateral portions converging from base to near apex, joined by thin translucent cuticle medially but leaving distinctly bilobed apex, with short median notch; mandible triangular, in profile view with mandible closed, in same plane as clypeus, apex of mandible not strongly down-turned; with mandible fully open, dorsal face remains in same plane as clypeus; mandible with 8 teeth, tooth 1 continuous with basal rim of dorsal surface, teeth 1–5 acute, similar in shape, teeth 5–8 forming an apical fork, with 5 and 8 large, 6 and 7 small partially confluent denticles; dorsal surface of mandible roughened; ventral surface flat and parallel to clypeus apically, twisting basally to nearly perpendicular orientation basally, smooth and shining; interior surface concave, smooth and shining; scape flattened, with pronounced anterobasal lobe, dorsal surface very faintly sculptured; clypeus with broad, shallow emargination anteriorly; clypeus and face matte, with very faint sculpture, nearly smooth (difficult to determine sculptural detail; three known specimens have a thin layer of wax-like material on face); frontal carinae faint, nearly obsolete; antennal socket deep, dorsal rim of socket continuous with pronounced dorsal margin of antennal scrobe; antennal scrobe deep, strongly delimited dorsally, posteriorly, and ventrally with sharply defined thin cuticular rim; compound eye small, circular, composed of about 7 ommatidia; distinct carina extends from ventral margin of antennal socket across floor of scrobe to compound eye; scrobe floor matte, faintly foveolate anteriorly, grading to smooth posteriorly; occipital carina distinct, extending anteriorly on ventral surface of head nearly to hypostoma; postgenal suture visible as dark line on undersurface of head; undersurface sculpture similar to face.

Promesonotum moderately convex in profile, promesonotal suture moderately impressed, medial mesonotum with shallow longitudinal impression; metanotal groove moderately impressed, inconspicuous in profile view, stronger in dorsal view, with coarse longitudinal rugae in depression; propodeum with distinct dorsal and posterior faces; dorsal face flat, sloping; propodeal spines well-developed, in the form of acute flat translucent perpendicular plates, extending ventrally as thin carinae; a few thin carinulae extend between propodeal spines, separating dorsal and posterior faces of propodeum; propodeal spiracle small, diameter much less than width of base of propodeal spine, slightly projecting on low tubercle, located below propodeal spine and anterior to posterior margin of propodeum; all surfaces of mesosoma matte except posterior face of propodeum, which is shiny; dorsum of pronotum and lateral mesonotum irregularly rugose, medial impression of mesonotum smooth, dorsal face of propodeum faintly punctate, posterior face of propodeum smooth, lateral pronotum very faintly punctate; meso- metapleuron and side of propodeum confluent, smooth.

Petiole in profile with peduncle differentiated from node, node with differentiated anterior face; node triangular, with posterodorsal face long, sloping, medially impressed; anteroventral margin with acute tooth; postpetiole low, broad, crescent-shaped in dorsal view; dorsa of petiolar node and postpetiole faintly sculptured like frons; first gastral tergite covered with very faint, dense, confluent puncta, appearing shagreened; first gastral sternite anteriorly smooth, shiny to faintly shagreened, posteriorly punctate, interspaces subequal in width to puncta, smooth and shining.

Anterior labral lobe with radiating tuft of soft, thick, translucent, capitate setae of unequal length projecting from apex; each larger mandibular tooth with fully appressed seta running length of tooth; anterior margin of scape with about 10 spatulate setae; clypeus and face with fine, sparse fully appressed ground pilosity; face, mesosomal dorsum, petiole, postpetiole, and first gastral tergite lacking erect setae; mesotibia with moderately abundant ground pilosity, about 5 short spatulate setae at apex; second and subsequent gastral tergites with usual complement of clavate setae; first gastral sternite with abundant short clavate setae on posterior half to two thirds, anterior portion devoid of setae.

Color dark brown.

Holotype Specimen Labels

Type Material

Holotype worker: Mexico, Chiapas: 2km SE Custepec,15.72099, -92.95054, ±200 m, 1520 m, mesophil forest, ex sifted leaf litter, 17 May 2008 (LLAMA, Wm-A-02-1) California Academy of Sciences, unique specimen identifier JTLC000015356. Paratype workers: same data Colección Entomológica de El Colegio de la Frontera Sur, CASENT0609703; John T. Longino Collection, CASENT0627366.

Etymology

The name refers to the general lack of spatulate setae on the dorsal surfaces. It is a noun in apposition and thus invariant.

References

  • Longino, J.T. 2013. A revision of the ant genus Octostruma Forel 1912 (Hymenoptera, Formicidae). Zootaxa 3699, 1-61. doi:10.11646/zootaxa.3699.1.1