Hypoponera opacior

Hypoponera opacior is a wide ranging and highly variable species that may actually be a species complex. It nests in rotten stumps in partial shade, grass sod, or beneath stones (Coovert 2005, Nemec et al. 2012).

Identification
Smith (1936) - P. opacior is highly variable in color, sculpture and the shape of the petiole. From other species the worker can be distinguished by its slender, graceful appearance, finely punctate body, and distinctly triangular shaped petiole (viewed in lateral profile).

This species may be part of a species complex, which is consistent with its variable morphology.

Workers and queens have a high petiole that is strongly narrowed above in lateral view. Small, reddish brown individuals can be confused with Hypoponera punctatissima, but the latter species has a smooth, shining mesopleuron, and a less elevated petiole than H. opacior. This last character state is very useful for sorting specimens, but is not easily quantified and requires a little practice in its use. Workers of H. opacior also have conspicuous punctures on the central portion of the mandible, while this area in H. punctatissima is smooth and shining. Male H. opacior have light brown wing veins (unlike H. opaciceps), and are most similar to males of Hypoponera inexorata. The latter species is considerably larger (3mm or more in length). We have occasionally found ergatoid queens (or workers with large eyes).

Distribution
Virginia south through Florida and the West Indies, west into Oregon, south through Central and South America into Chile and Argentina.

In some areas the distribution of H. opacior may be discontinuous and the populations small: in a survey of the ants of Nevada, Wheeler and Wheeler (1986) found this species only once, while in Florida it is found wherever there is cursory sampling.

Distribution based on Regional Taxon Lists
Nearctic Region: United States. Neotropical Region: Argentina, Costa Rica, Cuba, Dominican Republic, Ecuador, Greater Antilles, Guadeloupe, Haiti, Lesser Antilles, Mexico, Puerto Rico, Saint Vincent and the Grenadines, Trinidad and Tobago.



Reproduction
Fiotzik and colleagues have studied aspects of the reproductive biology of this species.

From the abstract of Fiotzik et al. (2002): Wingless (apterous) males of Hypoponera opacior sat on top of queen cocoons, inserted their genitalia into the cocoons and remained in copula with cocooned queens for up to 40 h. These males were tolerant of each other; fighting was never recorded. Our observations therefore suggest that wingless males of H. opacior ensure reproduction by copulatory mate guarding. This strategy, although time consuming, presumably reduces the likelihood of subsequent inseminations by other males. Apterous H. opacior males have only a limited amount of sperm available: histological preparations showed that, in contrast to Cardiocondyla fighter males, the testes degenerate in early adult life. Males of H. opacior have relatively few mating opportunities. Although some wingless males were reproductively active for more than 3 weeks, we observed a maximum of only six matings per male, with a mean slightly above one. Some H. opacior males used an alternative reproductive tactic of dispersal and outbreeding. We found colonies headed by single, dealate queens, which did not rear wingless sexuals but presumably reproduced through winged reproductives that mate in nuptial flights. The social structure of those colonies contrasted with nests containing wingless reproductives, which were highly polygynous and polydomous.

From the abstract of Fiotzik et al. (2010): The ant Hypoponera opacior exhibits alternative reproductive morphs of males and females associated with distinct sexual behaviours. Our long-term study reports strong seasonality in sexual production with a mating season in early and one in late summer. Winged (alate) reproductives emerge in June, swarm during the monsoon season and establish new colonies independently. In contrast, wingless worker-like (ergatoid) reproductives that appear in late August mate within their natal or adjacent nests and either do not disperse or establish new nests close by. These divergent dispersal patterns allowed us to analyse the impact of local factors on investment strategies by comparing sex allocation between and within the two reproductive events. The optimal sex ratio for ergatoid reproductives should be influenced both by competition for matings between brothers (local mate competition) and rivalry among young locally dispersing queens for workers, nest sites or food (local resource competition). The greater importance of local resource competition was demonstrated both by a male-biased sex ratio for wingless sexuals and a stronger increase in the number of males with total sexual production than for the number of queens. Microsatellite analysis revealed that inter-nest variation in relatedness asymmetry cannot explain split sex ratios in the August generation. Instead, nests with related ergatoid males raised a male-biased sex ratio contrary to the expectations under local mate competition. In conclusion, male bias in wingless H. opacior indicates that local mate competition is less strong than local resource competition among ergatoid queens over the help of workers during nest foundation.

From the abstract of Fiotzik et al. (2011): The ant Hypoponera opacior exhibits alternative reproductive morphs associated with distinct sexual behaviours. Wingless, worker-like gynes mate in fall in their mother nest. Subsequently, these polygynous nests divide and wingless queens disperse on foot accompanied by workers, which help during nest foundation. Local resource competition amongst wingless queens was indicated by male-biased sex ratios. Queens compete over access to workers, and this competition manifests in dominance interactions. We colour-marked queens and workers to study this behaviour. Marked queens were treated aggressively and frequently killed by nestmates, whilst similarly labelled workers were readopted. We hypothesise that mated wingless queens—although they resemble workers in external morphology—differ chemically and that fertile queens with divergent odours are not tolerated. Thereby, sterile H. opacior workers avoid the risk to raise offsprings of unrelated queens. However, when nests split, workers should base their decision which queen to follow not only on relatedness, but also on queen fertility. Cuticular hydrocarbon profiles of queens could serve as an honest signal for workers. Indeed, gas chromatography showed that hydrocarbon composition differs between workers and mated queens and is linked to ovary development in queens. Workers and unmated queens exhibit complex profiles of short-chained hydrocarbons, whilst fertile queens carry few, long-chained alkenes and branched alkanes. Furthermore, ovary length was correlated to the relative amounts of characteristic hydrocarbons of queens. The chemical profile was also associated with eye size—an unalterable trait in adult insects—suggesting that larval nutrition and consequently body size affects adult ovary development.

Florida
Deyrup, Davis and Cover (unpublished): This species forms small colonies in soil, rotten logs, and in accumulations of leaf litter. Is is the most abundant species of Hypoponera in Florida, and one of the most common ants in leaf litter samples.

Van Pelt (1958), reporting on this species within the Weleka Reserve in Florida: "In all respects except total length, the Welaka specimens agree with Smith's description in which he cites the length of workers of opacior as 2-2.3 mm. Workers from the Reserve measure 2.4-2.7 mm in total length.

P. t. opacior tends to prefer the higher, drier plant associations. It was taken commonly in xeric hammock; occasionally to commonly in turkey oak; occasionally in bayhead and Plummer slash pine flatwoods, in all welldrained areas except Pomello scrubby flatwoods, and in the hammocks; and rarely in the other flatwoods stations. No collections have been made from Pomello scrubby flatwoods or the seasonally flooded areas of the river swamp and marsh. Cole (1940) points out that in the Great Smoky Mountains the ant does not nest in dense wet woods, but prefers rather open areas where the soil is able to contain an appreciable amount of moisture.

The majority of the nests occupied the surface stratum. The several nests recorded from sand, moreover, did not extend more than a few inches into the sand, but were mostly under litter. The nests in the surface stratum were usually associated with debris, although some nests were found with little or no debris. Several of the nests taken in fallen logs were found under the bark against fairly hard wood. The order of preference of nests in the surface stratum is: 1) fallen logs, 2) dead stumps, 3) bases of living trees" 4) litter, 5) palmetto roots on ground, 6) under mat of palmetto roots, and 7) under mat of palmetto trunks.

Of four nests, taken from wood, the number of workers varied from 7 to 21, with an average of 13. None of these nests contained queens. Immature forms have been found in all months. Males have been observed in flight in December and February. No information has been obtained concerning females.

P. t. opacior is relatively fast moving, and characteristically evasive. Individuals, are difficult to see because they are very nearly the color of the wood or litter surrounding their nests. They seek immediately the first available crevice in which to hide.

Nests are occasionally found in the same logs and stumps as nests of Odontomachus baematoda insularis."

New Mexico
Mackay and Mackay (2002) - Habitat. Ranges from Chihuahuan Desert mixed basin bajada to short grass prairie, sagebrush into areas of pinyon-juniper, alligator bark juniper, Chihuahua pines, ponderosa pine and emery oak. Biology. These ants nest under stones, primarily in shady areas with abundant stones.

Costa Rica
Longino (1999): The form I call opacior s.s. seems to have a preference for synanthropic and seasonally dry habitats. I have scattered collections from Costa Rica, as follows:

Santa Rosa National Park: in Winkler sample collected by P. S. Ward.

Finca La Pacifica, a dry-forest riparian habitat: in Winkler sample of sifted litter from the forest floor.

Carara Biological Reserve: in Winkler sample collected by P. S. Ward.

Casa Plastico, a 600m elevation wet site on the Atlantic slope, in young second growth at the edge of a pasture: under a mossmat at the base of a tree.

A coffee farm near Heredia in the Central Valley: collected in studies of coffee farm fauna by Ivette Perfecto.

Sirena in Corcovado National Park, a lowland wet forest site: in Winkler sample of sifted litter from the forest floor.

A roadside in Monteverde, where an isolated and epiphyte-laden tree had recently fallen: a nest was under epiphytes near the ground.

La Selva Biological Station, a mature lowland wet forest site: a nest was under epiphytes on a dead branch recently fallen from the canopy.

Fila Cruces near San Vito: a nest was under a stone at the edge of a gravel road through pastures and scrubby forest.

Parque Nacional, a small landscaped park in the middle of San Jose: among a collection of stray foragers.

Genetics
From the abstract of Ruger et al. (2005): In the species Hypoponera opacior, winged and wingless males and queens co-occur and molecular markers are essential to study the reproductive success of these sexual tactics. Primers were developed for five polymorphic microsatellite loci isolated from H. opacior. Their variability was tested on 34 colonies from a population in the United States. Nine to 21 alleles per locus were found with observed heterozygosities between 0.1 and 0.7. A significantly positive Fis value suggests inbreeding in this ant with predominant intracolonial matings.

Nomenclature

 *  opacior. Ponera trigona var. opacior Forel, 1893g: 363 (w.q.) WEST INDIES. Emery, 1895c: 268 (m.); Wheeler, G.C. & Wheeler, J. 1964b: 454 (l.). Combination in Hypoponera: Taylor, 1968a: 65. Subspecies of trigona: Emery, 1906c: 116. Raised to species: Kempf, 1962b: 10.

Taxonomic Notes
There is some variation in size and color, as in many other ponerines. This variation does not seem to be correlated with any structural variation, and it is easy to set up series of specimens showing transitions between light and dark, or large and small. There is also some regional variation. Arizona specimens that we have seen are relatively large and reddish; specimens from coastal Oregon are dark brown with a yellowish tip to the gaster. It is possible that such populations represent distinct species, but it would require genetic work, or some major difference in males, to show that this is the case. It is normal for widely disjunct populations in different habitats to look a little different.

Longino (1999): Hypoponera opacior is probably a species complex; a brief perusal of collections from throughout its range reveals abundant geographic variation in details of color, pilosity, and eye size. For example, Californian material that has traditionally been identified as opacior can be separated into two or three distinct species (P. S. Ward, pers. comm.). This pattern suggests an older lineage whose current distribution is the result of vicariance rather than recent dispersal, at least over parts of its range. It is also possible that some parts of the range are the result of recent dispersal. For example, is its current distribution on many Caribbean islands a result of old vicariance or recent introductions?

Worker
Smith (1936) - Length: 2-2.3 mm. Head, excluding the mandibles, longer than broad, posterior border faintly emarginate, posterior angles rounded, sides convex. Eyes small, rounded, apparently composed of about 6 facets, placed about the anterior fourth of the sides of the head. Mandibles rather irregularly toothed. Scapes somewhat robust, not attaining the posterior border of the head; funicular segments 3-6 much broader than long, the terminal segment scarcely as long as the three preceding segments taken together. Thorax short, fairly robust, with distinct pro-mesonotal and meso-epinotal sutures; viewed in lateral profile the base of the epinotum appears to be of the same length as the declivity and meets the latter in a very distinct obtuse angle, the declivity faintly concave. Petiole when viewed in lateral profile not as robust as in the other species, very distinctly narrower (antero-posteriorly) dorsally than ventrally, strongly convex anteriorly, faintly convex posteriorly; viewed posteriorly the rounded superior border meets the lateral borders in such a way as to give an almost circular effect.

Body, due to the faint sculpturing and the nature of the pubescence rather glabrous, in some lights however somewhat subopaque. Head covered with extremely fine, dense punctures; thorax much less so; pleurae of the meso-and metathorax, especially the lower parts, finely and longitudinally striated.

Pubescence extremely fine on all parts of the body but probably most apparent on the gaster and head, pubescence scarcely hiding the general surface of the body, but in some lights giving it a pruinose effect. Pilosity poorly developed; hairs grayish, most noticeable on the dorsal surfaces of the head and gaster, on the former strikingly long, on the latter short, except toward the apex of the gaster.

Color variable; ranging from light brown to practically black, appendages lighter.

Queen
Length: 2.75-3 mm. Very similar to the worker but larger and more robust. As with the worker funicular segments 3-6 are much broader than long, and the terminal segment shorter than the preceding segments taken together. The most distinctive character is the shape of the petiole which is very narrow antero-posteriorly and clearly thinner above than at the base. The wing veins and stigma very light brown.

Type Material
Island of St. Vincent, Lesser Antilles.