Octostruma stenognatha

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Octostruma stenognatha
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Subfamily: Myrmicinae
Tribe: Attini
Genus: Octostruma
Species: O. stenognatha
Binomial name
Octostruma stenognatha
Brown & Kempf, 1960

Octostruma stenognatha P casent0905907.jpg

Octostruma stenognatha D casent0905907.jpg

Specimen Label

The type was collected "in soil cover berlesate from very moist forest".

Identification

Octostruma stenognatha, Octostruma batesi, and Octostruma betschi are all similar in size and head shape and possibly form a clade. Although geographic coverage is poor, the known specimens form an allopatric or parapatric replacement series in South America. (Longino 2013)

Keys including this Species

Distribution

Distribution based on Regional Taxon Lists

Neotropical Region: Brazil (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.

  • stenognatha. Octostruma stenognatha Brown & Kempf, 1960: 196, figs. 25, 28 (w.q.) BRAZIL.

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

Description

Worker

Holotype. TL 2.2, HL 0.52, HW 0.52 (CI 100), WL 0.56 mm. Paratype workers: TL 2.0-2.3, HL 0.49-0.56, HW 0.49-0.56 (CI 94-103), WL 0.52-0.60 mm, based on 33 workers from at least 14 different nest series from 4 different localities in all parts of the known range. Paratypes, ergatoid females (or gynecoid workers): TL 2.6-2.8, HL 0.58-0.60, HW 0.60-0.63 (CI 100-106), WL 0.65-0.71 mm, based on 4 examples.

The holotype, paratypes and ergatoids are very similar to the corresponding castes of Octostruma balzani, but the head averages narrower and differs slightly but significantly in form. Sides of head in the region of the eyes less strongly converging and longer; distance between anteocular and postocular angles normally exceeding distance between postocular and occipital angles (usually the postocular angle — occipital angle distance is slightly greater in balzani). Dorsum of head gently convex, with a shallow median impression centered on the vertex (dorsum not divided into convex occipital and flat or concave frontal surfaces as it is in balzani).

Mandibles narrower than in balzani, with nearly or quite straight outer margins; basal tooth subtriangular, pointed, its basal border sloping sharply away to the basal margin of the mandible in such a way as to leave a gap in front of the clypeus at full closure (though the gap may be hard to see if the labrum is extended beneath it). Masticatory margin with 7 triangular teeth plus 1-3 minute intercalary denticles. Basal lobe of scape more broadly rounded (less produced) than in balzani.

Alitrunk and remainder of body formed much as in balzani, but petiolar node not so strongly compressed anteroposteriorly, its dorsum only slightly broader than long and about half as wide as postpetiole.

Ground pilosity even less well-developed than in the average balzani worker. Erect clavate-spatulate hairs more abundant and longer than in average balzani workers, but not exceeding the maximum number for that species; usually 16-18 erect hairs on cephalic dorsum, of which the hair nearest the eye is placed slightly in front of the eye (more mesad of eye in balzani). The postpetiole carries 4 hairs along the posterior margin, of which the outer pair is placed on the posterolateral angles. Alitrunk with 1 or 2 pairs, petiolar node with a single pair.

Color and sculpture as in balzani, except that the interpunctural spaces on the gastric dorsum seem to be smoother and more shining than the average for balzani.

The ergatoids have slightly coarser sculpture and more erect hairs than the worker has; eyes much larger than in worker; ocelli small, forming an obtuse triangle; petiolar node more transverse.

Queen

Dealate paratype: TL 2.7, HL 0.58, HW 0.58 (CI 100), WL 0.78 mm. With true pterothorax and other modifications of its caste; erect hairs more abundant than in ergatoid or worker.

Paratype Specimen Labels

Type Material

Longino (2013) - Holotype worker: Brazil, São Paulo, Agudos, 23 Jan 1955, in soil cover berlesate from very moist forest (W. W. Kempf, No. 1334) Museu de Zoologia da Universidade de Sao Paulo (not examined). Paratype workers, worker intercastes: same data as holotype; São Paulo, Cantareira, 1 Mar 1959 (Kempf & Santos); Rio de Janeiro, Itatiaia (J. F. Zikán, Coll. Borgmeier); Santa Catarina, Nova Teutonia, 27° 11' S, 52° 23' W, Aug 1952 (F. Plaumann) Museum of Comparative Zoology] (examined). Paratype queen not examined.

References