Octostruma pexidorsum

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

Octostruma pexidorsum P mcz-ent00511319.jpg

Octostruma pexidorsum D mcz-ent00511319.jpg

Specimen Label

The type series is from lowland Amazonian rainforest, in a Berlese sample of forest floor litter.

Identification

Longino (2013) - Face lacking transverse arcuate carina; basal five teeth of mandible acute; labrum longer than wide, sides nearly parallel, apex bluntly rounded, not bilobed; face densely punctate; ground pilosity of face, dorsal promesonotum, and first gastral tergite conspicuous, composed of thin, yellow, suberect and strongly curved setae, giving dorsum a scruffy or wooly appearance; pair of long filiform setae projecting from petiolar peduncle anterior to spiracle, shorter filiform setae projecting from sides of postpetiole and anteromedian first gastral sternite (these filiform setae otherwise only known in Octostruma iheringi).

Keys including this Species

Distribution

Distribution based on Regional Taxon Lists

Neotropical Region: Colombia (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

Known only from the worker caste.

Nomenclature

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

  • pexidorsum. Octostruma pexidorsum Longino, 2013: 47, figs. 11, 34, 44 (w.) COLOMBIA.

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

Description

Worker

HW 0.63–0.71, HL 0.59–0.65, WL 0.66–0.74, CI 107–111 (n=3). Labrum longer than wide, sides nearly parallel, apex bluntly rounded, not bilobed; mandible triangular, in profile view with mandible closed, in same plane as clypeus, apex of mandible not 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, all teeth acute; tooth 1 smaller than teeth 2–5 and 8, teeth 5–8 forming an apical fork, with 5 and 8 large, 6 and 7 small partially confluent denticles; on one specimen with spread mandibles, a minute denticle between teeth 3 and 4; 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 minutely densely punctate; clypeus with broad, shallow emargination anteriorly; clypeus and anterior face roughened and dull, grading to densely punctate on medial and posterior 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 faintly foveolate; occipital carina short, not extending anterior to occipital foramen; undersurface of head punctate. Mesosomal dorsum evenly convex in profile, promesonotal suture and metanotal groove not impressed; propodeum with distinct dorsal and posterior faces; propodeal spines well-developed, in the form of acute, laterally flattened plates, inner surface concave; a few transverse rugulae between propodeal spines; propodeal spiracle large, filling space between ventral margin of propodeal spine and metapleural lobe, extending posteriorly to form posterior margin of propodeum; entire pronotum, mesonotum, and dorsal face of propodeum densely punctate; mesopleuron and side of propodeum confluent, dorsal portion punctate, ventral portion smooth, matte; posterior face of propodeum faintly foveolate and sublucid.

Petiole in profile with peduncle differentiated from node, node with differentiated anterior face, posterodorsal face sloping to acute posterior rim; anteroventral margin with pronounced, anteriorly-directed peg-like tooth; postpetiole low, broad, crescent-shaped in dorsal view; dorsum of petiolar node and postpetiole densely punctate; first gastral tergite and sternite densely punctate, puncta of sternite larger and more widely spaced than puncta of tergite.

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 7 stiff slightly clavate setae, seta on basal lobe longest, no seta proximal to this longest seta; ground pilosity of face, dorsal promesonotum, and first gastral tergite conspicuous, composed of thin, yellow, suberect and strongly curved setae, giving dorsum a scruffy or wooly appearance; face with 2–6 long, thin, stiff, erect setae, posteromedian pair always present, other variably distributed on lateral and posterolateral margins; mesonotum and petiolar node each with a pair of stiff, weakly clavate setae; apex of mesotibia with 2 stiff setae; postpetiole with 0–2 weakly clavate setae; first gastral tergite with 8–14 erect weakly clavate setae; first gastral sternite with abundant short stiff setae; pair of long filiform setae projecting from petiolar peduncle anterior to spiracle, shorter filiform setae projecting from sides of postpetiole and anteromedian first gastral sternite; ground pilosity of first gastral tergite sparse, seta length less than distance between them.

Color dark brown.

Type Material

Holotype Specimen Labels

Holotype worker: COLOMBIA, Amazonas: 7 km N Leticia [-4.1466, -69.9343, unknown error], 75 m, 10 Feb 1972, forest, leaf litter (S. & J. Peck, B-230) Museum of Comparative Zoology, unique specimen identifier MCZ-ENT00511319. Paratype workers: same data California Academy of Sciences, MCZ-ENT00511314; National Museum of Natural History, MCZ-ENT00511315; John T. Longino Collection, MCZ-ENT00511316; MCZC, MCZ-ENT00511317, MCZ-ENT00511318, MCZ-ENT00511320, MCZ-ENT00511321, MCZ-ENT00511322.

Etymology

The name refers to the wooly ground pilosity. 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