Octostruma ascrobis

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

Octostruma ascrobis P casent0611295.jpg

Octostruma ascrobis D casent0611295.jpg

Specimen Label

This species inhabits lowland rainforest, from sea-level to 1000 m elevation, but is most abundant below 500 m. All specimens are from samples of sifted leaf litter and rotten wood from the forest floor. Frequencies in quantitative samples of 1 m2 litter plots range from 0–16%. Dealate queens occasionally occur in samples, and one sample contained an alate queen. Worker number in samples is always < 10. (Longino 2013)

Identification

Antennal scrobe very shallow, not distinctly margined; face with arcuate carina; frontal carinae extending back and joining termini of facial arc. Octostruma convallis is similar, but the frontal carinae and facial arc do not join. They are separated by a transverse trough, and the termini of the facial arc extend laterally beyond the termini of the frontal carinae. (Longino 2013)

Keys including this Species

Distribution

Mexico (Chiapas) to Costa Rica.

Distribution based on Regional Taxon Lists

Neotropical Region: Belize, Costa Rica, El Salvador, Guatemala (type locality), Honduras, Mexico, Nicaragua.


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.

  • ascrobis. Octostruma ascrobis Longino, 2013: 18, figs. 1H, 3G, 6C, 14I, 17, 44 (w.) GUATEMALA.

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

Description

Worker

HW 0.46–0.51, HL 0.43–0.49, WL 0.43–0.50, CI 102–107 (n=9). Labrum rectangular, formed of strap-like lateral portions joined by a thin translucent lamella; mandible strongly bowed, in profile view with mandible closed, basal portion of mandible in same plane as clypeus, apex of mandible strongly down-turned, the down-turned portion subequal in length to basal portion; with mandible fully open, dorsal face fully perpendicular to plane of clypeus and teeth projecting dorsally; mandible with 10 teeth, tooth 1 continuous with basal rim of dorsal surface, teeth 1–7, teeth 7–10 form an apical fork perpendicular to base of dorsal face, tooth 7 long and separate, tooth 8 about 1/3 length of tooth 7, teeth 9 and 10 slightly longer than 7, joined to near tip (or could be seen as single tooth bifid at the tip), tooth 9 slightly shorter than 10; dorsal surface of mandible roughened; ventral surface narrow, smooth and shiny; interior surface strongly concave, smooth and highly polished; scape flattened, with pronounced anterobasal lobe, dorsal surface shallowly foveolate anteriorly, grading to smooth and shining posteriorly; clypeus strongly emarginate anteriorly; frontal carinae sharp, narrow, divergent, joining and continuous with similar carina that arches across vertex; frontal carinae and arch delimit anterior concave surface that is shiny, with irregular median longitudinal carina and faint transverse rugulae laterally; side of head near compound eye shallowly foveolate; vertex posterior to facial arc largely smooth and highly polished, with distinct, sparse, uniformly distributed puncta; compound eye small, circular, composed of 4–5 somewhat confluent ommatidia; short carina passes obliquely between antennal socket and eye, extending a short distance posterior to eye and toward frontal carina; antennal scrobe below eye, very shallow and not delimited with carina or flange; scrobe and undersurface of head shallowly foveolate.

Promesonotum forming continuous convexity in profile; metanotal groove not impressed; propodeum a single, concave, sloping surface, not differentiated into dorsal and posterior faces; propodeal spines in the form of flattened perpendicular plates, right-angled to slightly acute, extending anteriorly as raised carinae that curve medially and join at the metanotal groove, extending ventrally as narrow lamella, thus posterodorsal face of propodeum entirely delimited by raised carinae confluent with propodeal spines; propodeal spiracle large, located immediately below propodeal spine and abutting posterior margin; pronotum sculpture smooth and shining with sparse puncta, like posterior vertex; mesonotum with feeble, mesh-like sculpture, contrasting sharply with pronotum and clearly demarcating juncture of pro- and mesonotum; posterodorsal face of propodeum shallowly foveolate; mesopleuron and side of propodeum not differentiated, matte, very faintly punctate.

Petiole in profile with peduncle not differentiated from node, anterior surface sloping evenly from petiolar foramen to node, dorsal face of node sloping posteriorly to projecting transverse cuticular rim, short concave posterior face beneath rim; anteroventral margin with small tooth or angle; dorsal face of node smooth and shining; postpetiole low, broad, crescent-shaped in dorsal view, dorsal face shallowly foveolate anteriorly, smooth and shining posteriorly, delimited anteriorly and posteriorly by thin transverse rim; first gastral tergite shallowly foveolate anteriorly, grading to smooth and shining at posterior rim; first gastral sternite foveolate anteriorly, grading to sparsely punctate with smooth and shining interspaces posteriorly.

Labrum fringed on sides and apex with soft translucent thick setae, one of these on each anterior labral lobe much longer than the others, projecting ventrally; extreme base of ventral surface of mandible with very fine, long seta that curves forward and nearly touches ventrally projecting labral seta; each larger mandibular tooth with prominent fully appressed seta; anterior margin of scape with seven stiff clavate setae; clypeus with sparse fully appressed ground pilosity, anterior face with sparser and much less visible ground pilosity, posterior face essentially lacking ground pilosity; 1–3 stout clavate setae variably distributed between apex of oblique supraocular carina and outer margin of facial carina, near juncture of frontal carina and transverse carina; mesonotum, petiole and postpetiole lacking erect setae, ground pilosity extremely short, sparse, unnoticeable; mesotibia with short, thin, sparse, decumbent ground pilosity, a single large stout clavate seta and several thin shorter erect setae at apex; mesobasitarsus with 2 pairs long, thin, erect setae; first gastral tergite with row of 4 clavate setae at posterior margin, following exposed tergites each with row of clavate setae; ground pilosity of first gastral tergite short, sparse, fully appressed, somewhat more noticeable than on mesosoma; first gastral sternite with sparse, thin but stiff erect setae over much of surface except narrow area near postpetiolar insertion.

Color red to red-orange.

Queen

HW 0.54–0.57, HL 0.52–0.53, WL 0.70–0.73, CI 102–107 (n=2). Labrum, mandible, and scape similar to worker; anterior clypeal margin very different from worker and highly distinctive, with a broad shallow semicircular concavity medially, flanked on each side by a deep semicircular notch that nearly "pinches off" an anterolateral portion of the clypeus between the notch and the large antennal socket; clypeus confluent with the vertex posteriorly, with no evidence of a clypeal suture; transverse facial carina and posterior frontal carinae feeble to absent, but marked by a semicircular row of about 9 erect spatulate setae; face anterior to row of setae flat, with faint transverse rugulae, lacking longitudinal median carina, face posterior to seta row like worker; ocelli distinct, anteromedian ocellus immediately anterior to seta row; compound eye large, multifaceted, about 12 ommatidia in longest row; outermost seta of seta row at inner margin of compound eye; antennal scrobe shallow, like worker.

Mesosoma with queen-typical alar sclerites; pronotum smooth and shining laterally and anteriorly, with faint rugulae and sparse puncta on humeri; mesoscutum and axillae irregularly longitudinally rugulose; scutellum smooth and shining medially, with a few puncta and faint rugulae laterally; anepisternum and katepisternum separated by strong sulcus, both smooth and shining; posterodorsal propodeum concave, shallowly foveolate; side of propodeum matte with faint puncta; propodeal spines pronounced, in the form of flattened perpendicular plates, right-angle to slightly acute in profile; propodeal spiracle smaller and lower compared to worker, anterior to ventral propodeal lamella.

Petiole similar to worker, but posterior rim more pronounced and projecting; postpetiole also with a posterior projecting rim, ventral margin with a short acute tooth; first gastral tergite punctate over much of surface, grading to sparser and smaller puncta at posterior border, with 6–8 erect clavate setae along posterior border, about 6 scattered over disc; first gastral sternite densely punctate, with abundant short, stiff, erect setae.

Color red.

Type Material

Holotype Specimen Labels

Holotype worker: GUATEMALA, Izabal: 5 km NW Morales, 15.51351, -88.86647, ± 26 m, 245 m, 17 May 2009, 2º lowland rainforest, ex sifted leaf litter (LLAMA, Wm-B-04-1-01) California Academy of Sciences, unique specimen identifier CASENT0611295. Paratype workers: same data except 15.51078, -88.86105, ± 50 m, 195 m (LLAMA, Wa-B-04-1-22) Museum of Comparative Zoology, CASENT0611224; National Museum of Natural History, CASENT0611225]; 15.51465, -88.86440, ± 28 m, 280 m, 18 May 2009 (LLAMA, Wm-B-04-1-09) Colección de Artrópodos, CASENT0611425; Museu de Zoologia da Universidade de Sao Paulo, CASENT0611426.

Etymology

The name refers to the lack of sharply delimited antennal scrobes. It is a dative plural noun 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

References based on Global Ant Biodiversity Informatics

  • Longino J. T. 2013. A revision of the ant genus Octostruma Forel 1912 (Hymenoptera, Formicidae). Zootaxa 3699(1): 1-61.
  • Longino J. T. L., and M. G. Branstetter. 2018. The truncated bell: an enigmatic but pervasive elevational diversity pattern in Middle American ants. Ecography 41: 1-12.
  • Longino J. et al. ADMAC project. Accessed on March 24th 2017 at https://sites.google.com/site/admacsite/