Poneracantha triangularis

A millipede feeder that is found in humid forests. The USA record (Deyrup, et. al. 1989:93) is undoubtedly a recent introduction and they have apparently found prey in the local species of millipedes, as they are well established. (Lattke 1995)

Poneracantha triangularis is a specialized predator on millipedes. In the laboratory, P. triangularis is especially fond of the invasive Asian millipede Oxidus gracilis (Koch), which is abundant on the Statesboro campus. Like many millipedes, O. gracilis defends itself against predators with a foul-smelling cocktail of toxic chemicals, including hydrogen cyanide (Taira et al. 2003). These defenses, however, are apparently ineffective against P. triangularis, which has been recorded attacking O. gracilis in the field and can survive for hours in cyanide killing jars that killed other ants within a few minutes (Lattke 1990). Poneracantha triangularis workers emit an odor similar to that of O. gracilis (Wang et al. 2021), suggesting that this ant can not only tolerate the millipede’s chemical weapons but perhaps also co-opt them for its own protection. Although P. triangularis is an invasive species (i.e., introduced by humans to a new area, from which it has subsequently spread), its apparent scarcity and specialized diet suggest that it is not likely to become a serious pest, as MacGown and Wetterer (2012) and Wang et al. (2021) have noted. (Harvey, 2022)

Identification
A member of the rastrata complex (in the rastrata subgroup of the rastrata species group). Promesonotal suture weakly impressed; node dorsum with transverse costulae and subquadrate costulae and subquadrate subpetiolar process; first gastric sternum with transverse costulae; metacoxal tooth long and thin. Piceous body. (Lattke 1995)

Distribution
Introduced into Alabama and Florida (Dade and Escambia counties) from Central or South America. It is not as yet common there, but seems well established. First published Florida record: Deyrup et al. 1989; earlier specimens: 1985. (Deyrup, Davis & Cover, 2000.)

Distribution based on Regional Taxon Lists
Nearctic Region: United States. Neotropical Region: Argentina, Bolivia, Brazil, Colombia, Costa Rica, Ecuador, French Guiana, Peru, Uruguay.

Biology
This species was studied in Venezuela by Lattke (1990). He found nests in fallen logs and branches, inhabited by 80-120 individuals. In the four nests examined he found only the remains of millipedes, in the form of disarticulated segmental rings, surrounding the area of the nest. Larvae were found with their heads thrust into the body of a dead millipede. Many millipedes secrete powerful defensive secretions, including cyanide. Lattke found that both adults and larvae of P. triangularis are resistant to cyanide, surviving for three hours in a potassium cyanide killing jar that killed other ants in less than five minutes. Even the P. triangularis were dead in twelve hours.

A colony from Homestead, Florida, had many millipede fragments in the nest (Gary Umphrey 1987, pers. comm.; Deyrup, Davis & Cover, 2000).

The following notes are from Wang et al. (2021):

Poneracantha triangularis records in the southeastern United States are uncommon, but come from a wide range of habitats, from relatively intact forest to highly disturbed urban sites. It is known distribution in Texas, Louisiana, Mississippi, Alabama, Florida, Georgia and South Carolina. Our northernmost site records come from 34.1°N in South Carolina. However, based on its South American range extending to 38.1°S, P. triangularis may have the potential to spread further north in the US, perhaps as far as Richmond, Virginia (37.5°N) and Lexington, Kentucky (38.0°N) (MacGown & Wetterer 2012). A large portion of records of P. triangularis in the southeastern US come from specimens and photographs of alate queens and males. This may indicate that this species is more common than generally appreciated, but simply overlooked.

In both its native and exotic range, P. triangularis preys on millipedes (Lattke 1990, 1995, Deyrup et al. 2000). Millipedes are often particularly common in disturbed environments, so potential prey for P. triangularis is plentiful. The greenhouse millipede, O. gracilis, a cosmopolitan species originally from Asia, is now very common in the southeastern US. Millipedes produce a wide range of defensive chemicals that protect them from most potential predators. Although O. gracilis produces several highly toxic defensive chemicals, including hydrogen cyanide (Taira et al. 2003), P. triangularis has overcome these defensives. Poneracantha triangularis may even be able to use these chemicals to make themselves toxic to potential predators.

There have been no studies examining potential impacts of P. triangularis. Even if this species were having a localized impact on millipede populations, this might be difficult to detect. Nonetheless, it seems unlikely that P. triangularis will ever become a significant pest.

Nomenclature

 * . Ectatomma (Gnamptogenys) triangulare Mayr, 1887: 544 (q.) URUGUAY.
 * Type-material: holotype queen.
 * Type-locality: Uruguay: (no further data) (C. Berg).
 * Type-depository: NHMW.
 * Emery, 1906c: 113 (w.).
 * Combination in E. (Parectatomma): Emery, 1911d: 44;
 * combination in Gnamptogenys: Brown, 1958g: 230;
 * combination in Poneracantha: Camacho, Franco, Branstetter, et al. 2022: 11.
 * Status as species: Dalla Torre, 1893: 26; Emery, 1896g: 46 (in key); Emery, 1906c: 113; Emery, 1911d: 44; Bruch, 1914: 213; Santschi, 1916e: 366; Gallardo, 1918b: 39; Luederwaldt, 1918: 34; Borgmeier, 1923: 59; Kusnezov, 1953b: 336; Brown, 1958g: 229, 323; Kusnezov, 1969: 35 (in key); Kempf, 1972a: 116; Zolessi, et al. 1988: 2; Bolton, 1995b: 211; Lattke, 1995: 190; Deyrup, 2003: 45; Lattke, et al. 2004: 349; MacGown & Forster, 2005: 68; Lattke, et al. 2007: 263 (in key); Lattke, et al. 2008: 100; MacGown & Wetterer, 2011: 1; Feitosa, 2015c: 98; Guénard & Economo, 2015: 226; Deyrup, 2017: 21; Feitosa & Prada-Achiardi, 2019: 673; Camacho, et al. 2020: 461 (in key); Camacho, Franco, Branstetter, et al. 2022: 11.
 * Senior synonym of aculeaticoxae: Lattke, 1995: 190; Camacho, Franco, Branstetter, et al. 2022: 11.
 * Senior synonym of richteri: Brown, 1958g: 230, 323; Kempf, 1972a: 116; Bolton, 1995b: 211; Lattke, 1995: 190; Camacho, Franco, Branstetter, et al. 2022: 11.
 * Distribution: Argentina, Bolivia, Brazil, Colombia, Costa Rica, Ecuador, French Guiana, Guyana, Panama, Peru, Suriname, Uruguay, U.S.A., Venezuela.
 * aculeaticoxae. Ectatomma (Parectatomma) aculeaticoxae Santschi, 1921g: 82 (w.m.) FRENCH GUIANA.
 * Type-material: 2 syntype workers, 1 syntype male.
 * Type-locality: French Guiana: Haute Carsevenne, 1898 (F. Geay).
 * Type-depositories: MNHN, NHMB.
 * Wheeler, G.C. & Wheeler, J. 1971b: 1201 (l.).
 * Combination in E. (Poneracantha): Santschi, 1929h: 476;
 * combination in Gnamptogenys: Brown, 1958g: 227.
 * Status as species: Santschi, 1929h: 476; Brown, 1958g: 227, 300; Kempf, 1961b: 491; Kempf, 1972a: 111; Deyrup, et al. 1989: 93; Lattke, 1990b: 5; Brandão, 1991: 345; Bolton, 1995b: 208; Deyrup, et al. 2000: 295 (error).
 * Junior synonym of triangularis: Lattke, 1995: 190; Camacho, Franco, Branstetter, et al. 2022: 11.
 * richteri. Ectatomma (Parectatomma) triangulare r. richteri Forel, 1913l: 203 (w.) ARGENTINA (Buenos Aires).
 * Type-material: syntype workers (number not stated).
 * Type-localities: Argentina: Buenos Aires, Belgrano (Richter), Buenos Aires, Rojas (Weiser).
 * Type-depositories: MHNG, NHMB.
 * Forel, 1914d: 265 (q.).
 * Subspecies of triangularis: Forel, 1914d: 265; Bruch, 1914: 213; Gallardo, 1918b: 41.
 * Junior synonym of triangularis: Brown, 1958g: 230, 323; Kempf, 1972a: 116; Bolton, 1995b: 210; Lattke, 1995: 190; Camacho, Franco, Branstetter, et al. 2022: 11.

Lattke (1995) - The number of transverse costulae on the petiolar dorsum can vary from 8 to 14, and those on the pronotum from 13 to 23. Specimens from Argentina tend to have a higher count but there is no gap separating the values. The length of the coxal teeth is variable and bears no relation to the number of petiolar costulae. Propodeal teeth also show variation form a low mound to the usual low, sharp teeth. Occasional specimens can have up to 4 transverse costulae on the anterior pronotal face, and rarely longitudinal costulae on the petiolar node. Other traits used by Santschi to separate Gnamptogenys aculeaticoxae, such as degree of impression of the promesonotal suture, gastric constriction and gauge of hairs, length vs. width of petiolar node and the mandibular costulation all show continuous variation that is best described as infraspecific.

References based on Global Ant Biodiversity Informatics

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