Odontomachus brunneus

In the US, O. brunneus occurs in a wide variety of natural habitats including flatwoods, mesic forests, pine savannas, swamp forests, oak-pine scrub, upland scrub, sandhills, bayheads, edges of seasonal ponds, and elevated tussocks. Nests of O. brunneus have been found in leaf litter, rotting logs, at tree bases, and in open to partially covered sandy areas. Nest architecture has been explored and discussed by Cerquera and Tschinkel (2010). Workers occasionally forage during the day, but are more active at night. Upon colony disturbance, workers are not aggressive, but instead quickly retreat or vacate the nest. This is in sharp contrast to the aggressive defensive stinging behavior of Odontomachus haematodus. Alates have been collected from May through December. This broad time frame for alate activity contrasts sharply with the early summer activity of O. haematodus. (MacGown et al., 2014)

Identification
Deyrup and Trager (1985) - Worker: gaster densely covered with fine, almost contiguous appressed hairs; striations of pronotum not transverse posteriorly; petiolar node faintly rugose at extreme base only; inner side of hind femur at base finely pubescent; color piceous in n. Florida, body (except gaster) frequently lighter in s. Florida. Within the Southeast, there is some variation in workers, but apparently based on habitat rather than geographic variation. Specimens from dry habitats are often smaller and paler than specimens from wet habitats.

Male: each ocellus as wide as the ocello-ocular space, ocelli on a conspicuous turret: petiole smooth; head and body entirely yellowish orange.

Identification Keys including this Taxon

 * Key to US Odontomachus species
 * Key to Odontomachus of the New World

Distribution
MacGown et al. (2014) - Odontomachus brunneus appears to be restricted to the southeastern US. Previous records of O. brunneus from the Caribbean, and Central and South America (Brown 1976) all appear to be of Odontomachus ruginodis. The situation was clarified by Deyrup et al. (1985) who recognized the distinction between O. ruginodis and O. brunneus and revived the former from synonymy with the latter. Brown’s statement (1976) that O. brunneus is well adapted to “marginal habitats is consistent with the ecology of O. ruginodis, whereas O. brunneus in the Southeast is generally found in undisturbed natural habitats".

Deyrup and Cover (2004) - This species apparently occurs throughout Florida, although there are no records from the three westernmost counties. It is also known from southern Georgia and Alabama. We have seen specimens from low coastal areas in Alabama, and there is no obvious reason why it should not occur in coastal Mississippi, Louisiana and Texas, although it is not reported from any of those states. A report of brunneus from Cuba (Fontenla 1997) might refer to some other species, as we have seen specimens that would key to brunneus from the Dominican Republic, but are probably closer to Odontomachus insularis Guerin. Its distribution in Central and South America is unclear, since this species was combined with Odontomachus ruginodis in Brown’s revision of the genus (1976).

Distribution based on Regional Taxon Lists
Nearctic Region: United States. Neotropical Region: Brazil, Costa Rica, Cuba, Ecuador, Mexico, Trinidad and Tobago.

Biology
In Florida Deyrup and Trager (1985) found that brunneus occurred in wet areas such as hammocks and ditches. Males were found to fly in the spring.

Deyrup and Cover (2004) - Odontomachus brunneus occurs in both well-drained and poorly drained habitats; nests may be in soil or in rotten wood. This species was studied by Van Pelt (1958) at the Welaka Reserve (now Welaka State Forest) in Putnam Co., Florida. Van Pelt found many colonies, which occurred in all the terrestrial habitats in the area, including flatwoods, mesic forest, swamp forest, upland scrub and sandhill. Nests were in various microhabitats, including deep leaf litter, fallen logs, at the bases of trees, and open or sparsely covered sandy areas. At the Archbold Biological Station, brunneus occurs in moist habitats, including flatwoods, bayheads, the edges of seasonal ponds, and elevated tussocks or fallen pines within seasonal ponds. It has not been found in the more elevated upland areas of the Station, which are occupied by Odontomachus relictus. This distribution gives the impression that there is some competitive displacement based on differential adaptation to moisture conditions, but the evidence remains circumstantial. It may be relevant that in parts of its range devoid of relictus, where brunneus occurs in dry, upland areas individuals never achieve the large size and dark color seen in some specimens from wet areas. Nobody knows, however, whether the smaller, paler individuals represent stressed individuals in suboptimal conditions, or whether they represent an adaptive phenotypic response in a robust population.

Workers of brunneus sometimes emerge to forage on cloudy days, but are generally nocturnal. The formidable jaws of brunneus are not used as assertively as one might expect, and there is fragmentary evidence that brunneus is sensitive to chemical defenses. Prey are approached tentatively, and the ant recoils immediately after striking the prey (Brown 1976). There may be a delay before the prey is picked up and carried away; Brown (1976) suggested that these ants react to chemical defenses, which are allowed to dissipate before the prey is retrieved. Alex Wild, while a student at the Archbold Biological Station, twice observed brunneus retreating hastily when confronted by aroused workers of Dorymyrmex bureni (Trager) (unpublished natural history notes on file at the Archbold Biological Station). Dorymyrmex bureni is much smaller than O. brunneus, but can release large quantities of defensive chemicals that are pungent to the human nose. Van Pelt (1958) reported accumulations of brunneus head capsules in the nests of Formica archboldi Smith, and suggested the possibility that brunneus is a regular part of the diet of F. archboldi. If this is the case, it is more likely that the brunneus are subdued by chemical means than by mandible-to-mandible combat.

Fox et al. (2017) - This species was found to have three larval instars. Laboratory colonies were found to attach first and second instar larvae to the walls and ceilings of their artificial nest chambers. These individuals were always oriented with their dorsal side against the surface being suspended from their "doorknob" protuberances. Third instar larvae were only observed on the floors of the nest chamber, dorsal side oriented down, and arranged individually, not pilled together.

Cerquera and Tschinkel (2010) - O. brunneus excavates simple nests, each consisting of a single, vertical shaft connecting more or less horizontal, simple chambers. Nests contained between 11 and 177 workers, from 2 to 17 chambers, and 28 to 340 cm2 of chamber floor space and reached a maximum depth of 18 to 184 cm. All components of nest size increased simultaneously during nest enlargement, number of chambers, mean chamber size, and nest depth, making the nest shape (proportions) relatively size-independent. Regardless of nest size, all nests had approximately 2 cm2 of chamber floor space per worker. Chambers were closer together near the top and the bottom of the nest than in the middle, and total chamber area was greater near the bottom. Colonies occasionally incorporated cavities made by other animals into their nests.

Castes
Associating the various brunneus-like forms with their males in Central and South America and the West Indies would be an interesting and useful project for local myrmecologists, and might easily yield distributional surprises or new species (Deyrup and Cover 2004).

Nomenclature

 *  brunneus. Atta brunnea Patton, 1894: 618 (w.) U.S.A. Deyrup, Trager & Carlin, 1985: 191 (m.). Combination in Odontomachus: Emery, 1895c: 268. Junior synonym of insularis: Emery, 1895c: 268. Revived from synonymy and senior synonym of fuscus Stitz, 1925: Brown, 1976a: 103. See also: Brown, 1976a: 139.
 * fuscus. Odontomachus haematodes var. fuscus Stitz, 1925: 115, fig. 2 (w.) CUBA. [Unresolved junior primary homonym of fuscus Stitz, 1916, above. Note that the proposed replacement name repetita Baroni Urbani, 1971b: 361 is unavailable.] Junior synonym of brunneus: Brown, 1976a: 103.

Larva
Described by Fox et al. (2017).

Determination Clarifications
Southeastern records of insularis in the Formicidae section of the Catalog of the Hymenoptera (D. R. Smith 1979) refer to brunneus. Although the catalog appeared several years after Brown’s revision, the cut-off date for changes in the Formicidae section was mid-1975. Smith’s treatment of North American Odontomachus differs from those of Creighton (1950) and M. R. Smith (1951) in elevating to species level three subspecies of Odontomachus haematodus (Linnaeus). This was backed by no taxonomic references, and certainly was not intended to compete with the earlier, but unavailable, revision by Brown. (Deyrup and Cover 2004).