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Pseudomyrmex gracilis (Fabricius) 1804
"graceful twig ant", "elongate twig ant", or "Mexican twig ant"

by Joe A. Macgown, uploaded on 23 July 2009, updated on 29 April 2016

Pseudomyrmex gracilis, full face view of worker (FL, Sarasoto Co.) (photo by James Lewis and Joe MacGown)
Pseudomyrmex gracilis, lateral view of worker (FL, Sarasoto Co.) (photo by James Lewis and Joe MacGown)
Pseudomyrmex gracilis, dorsal view of worker (FL, Sarasoto Co.) (photo by James Lewis and Joe MacGown)
Pseudomyrmex gracilis, full face view of queen (LA, Iberia Par.) (photo by James Lewis and Joe MacGown)
Pseudomyrmex gracilis, lateral view of queen (LA, Iberia Par.) (photo by James Lewis and Joe MacGown)
Pseudomyrmex gracilis, dorsal view of queen (LA, Iberia Par. (photo by James Lewis and Joe MacGown)
Pseudomyrmex gracilis, full face view of male (LA, Iberia Par.) (photo by Ryan Whitehouse and Joe MacGown
Pseudomyrmex gracilis, lateral view of male (LA, Iberia Par.) (photo by Ryan Whitehouse and Joe MacGown
Pseudomyrmex gracilis, dorsal view of male (LA, Iberia Par.) (photo by Ryan Whitehouse and Joe MacGown

Pseudomyrmex gracilis, full face view of a worker
(Photo by Joe A. MacGown)

Pseudomyrmex gracilis, lateral view of a worker
(Photo by Joe A. MacGown)

Most of the species in the genus Pseudomyrmex make their nest in dead twigs or stems of woody plants, hollow stems of grasses, and various other natural or manmade cavities. A few of these species are uniquely adapted for living in live plant domatia, with the swollen-thorn acacias being a good example of this.

This genus is distinctive and can easily be distinguished from other US genera by the following characteristics: body elongate with postpetiole; eyes extremely large, more than 1.5 times as long as wide; antennal scapes short; 12-segmented antenna; relatively short mandibles with 7-10 teeth; sting present. 
Pseudomyrmex gracilis is large orange, black, to bicolored orange and black species thought to be native to Mexico, but also found from Argentina and Brazil through Texas, parts of southeastern US, and the Caribbean. Its status as an invasive species in the US is unclear, but due to its apparent spread in the US and its painful sting, this species is worth monitoring.

Taxonomic History (provided by Barry Bolton, 2013)
Formica gracilis Fabricius (1804); Combination in Leptalea: Erichson (1839); in Pseudomyrma, Roger, (1862); in Pseudomyrmex (Clavanoda), Enzmann (1944); in Pseudomyrmex, Kusnezov (1953). Senior synonym of Pseudomyrmex longinoda, Brown (1949); of Pseudomyrmex variabilis, Ward (1989); of Pseudomyrmex bicolorPseudomyrmex canescensPseudomyrmex dimidiata, Pseudomyrmex glabriventrisPseudomyrmex mexicanaPseudomyrmex pilosulaPseudomyrmex sericata, Pseudomyrmex velifera,Pseudomyrmex volatilis and material of the unavailable name Pseudomyrmex guayaquilensis referred here, Ward (1993); of Pseudomyrmex atrinoda, Wild (2007).

Worker: HL 1.5-1.74mm, HW 1.54-1.72mm, SL 0.73-0.84mm, EL 0.89-1.05mm, MeSL 2.08-2.42mm (n=5) (MEM specimens). Large ant (TL 8-10mm) with a wasp-like appearance. Color ranging from concolorous black to conclorous orange with many bicolored combinations of orange and black in between. The mandibles, antennae and legs are often lighter in color when compared to the head, body, and gaster. Head roughly square with dense, appressed pubescence and some erect setae; eyes large (REL 0.54-0.62) and situated laterally; three small ocellis present; mandibles with two apical teeth followed by minute dentition with a total of 7-10 teeth total; clypeus short and wide; antennal insertion points set closely together; antennae 12-segmented with the scape curved outwards. Mesosoma with silvery-white, dense pubescence and erect setae on the dorsal surface; distinct declivity at the metanotal groove. Waist elongate and two-segmented; nodes relatively low. Gaster with dense, appressed pubescence and a mixture of erect setae that becomes more prevalent near the apex; well developed sting present.

Queen: HL 1.77-1.92mm, HW 1.58-1.74mm, SL 0.75-0.82mm, EL 1.05-1.10mm, MeSL 2.52-3.04mm (n=5) (MEM specimens). Large wasp-like ant that ranges from conclorous black to bicolored orange and black most often with a black head and gaster. Head roughly rectangular, slightly longer than wide compared to the worker; covered with dense, silvery setae and some darker erect setae; elongate, well developed eyes situated laterally along the midline of the head; three small ocelli present; mandible with two apical teeth followed by minute dentition; clypeus short and wide; 12-segmented antennae with the scape slightly curved outwards; often with a slight medial depression running from the antennal insertion point to the middle ocelli. Mesosoma with a mixture of appressed and erect setae; enlarged with four wings or wing scars; mesonotum dorsally flat and often with a dark spot in the bicolored specimens. Waist is elongate and two-segmented; nodes with erect setae; the dorsal-lateral spiracle on the anterior side of the first segment distinct and tubercle-like. Gaster elongate with a mixture of appressed and erect setae with more erect setae present at the apex; well developed sting present.

Male: HL 1.32-1.38mm, HW 1.42-1.60mm, SL 0.23-0.25mm, EL 0.80-0.90mm, MeSL 2.56-2.72mm (n=5) (MEM specimens). Bicolored brown and orangish to brown and pale-yellow with the dorsum of the mesosoma, gaster, apex of head, and antennae brown. Head almost circular in shape a mixture of erect and fine, appressed setae; eyes well developed and situated laterally at the midpoint of the head; three distinct ocelli present; mandibles with one apical tooth followed by minute dentition; 12-segmented antennae with a short scape; minute tubercle located between the antennal insertion points. Mesosoma densely covered with slivery-white, erect and appressed setae; four wings present. Waist elongate, thin and two segmented; nodes very low, almost continuous with the gaster. Gaster narrow and elongate with appressed and erect setae.
Workers of the graceful twig ant is easily recognized by its elongate, wasp-like appearance, large size (TL 8-10 mm, HW 1.39-2.07), orange and black to entirely blackish or entirely organgish-brown coloration, large eyes (REL 0.54–0.60) ocelli, elongate two-segmented waist, and a well developed sting. Mandible with 7 -10 teeth. Body with numerous fine, silvery-white setae.

Biology and Economic Importance
Pseudomyrmex gracilis is an arboreal ant found across Central and South America as well as the southern United States. It can be found nesting in hollow twigs, branches, stems, and grasses, but also in crevices in buildings (Wetterer 2010). Pseudomyrmex gracilis has also been reported to opportunistically nest in the thorns of ant-acacias (Wetterer & Wetterer 2003, Wheeler, 1942), but do not provide much defense for the trees according to Wetterer (2010). According to Ward (1993), P. gracilis may be found in a wide variety of habitats such as mangrove forests, thorn scrub, and rainforests. However, it may also be common in disturbed habitats such as old fields, roadsides, and secondary forest. Colony size is typically small with one queen per colony.

Pseudomyrmex gracilis workers and queens may inflict painful stings when disturbed or agitated. Stings usually occur after a worker falls from foliage onto human skin, but also when workers are defending their colony. According to Wheeler and Wheeler (1956), P. gracilis readily defends it host plant or colony by swarming and stinging intruders. Also, Klotz et al. (1995) reported finding nests of this species in wooden door frames of houses. The fact that this ant has a high likelihood of coming in contact with humans and its painful sting makes it a species worth monitoring.

Native Range: Tropics and subtropics of the Neotropics (Wetterer 2010).

Australian: Hawaii (
Nearctic: United States (
Neotropical: Argentina, Barbados, Belize, Bolivia, Brazil, Colombia, Costa Rica, Dominican Republic, Ecuador, El Salvador, French Guiana, Greater Antilles, Guadeloupe, Guatemala, Guyana, Haiti, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru, Suriname, Trinidad and Tobago, Uruguay, Venezuela (

U.S. Distribution: AL, CA, FL, GA, HI, LA, MS, SC, TX (MEM records; Wetterer 2010; Doug Booher, Pers. Comm; 
Southeastern U.S. Distribution: AL, FL, GA, LA, MS, SC (MEM).

Pseudomyrmex gracilis is a widespread New World species, ranging from southern United States to Argentina and Uruguay. It is present as an introduced species in Hawaii, Florida and the southern United States.

Funding for the ant work being done by the MEM in Alabama and Mississippi is from several sources including the National Institute of Food and Agriculture, United States Department of Agriculture, under Project No. MIS-012040, the Mississippi Agricultural and Forestry Experiment Station at Mississippi State University, with support from State Project MIS-311080, NSF Grants BSR-9024810 and DFB-9200856, the Tombigbee National Forest (U.S. Forest Service), the Noxubee Wildlife Refuge, Mississippi Natural Heritage Program Research Grant, USDA Forest Service Agreement No. 08-99-07-CCS-010, the William H. Cross Expedition Fund, and primarily by the USDA-ARS Areawide Management of Imported Fire Ant Project. Additionally, special cooperation has been provided by State Parks, National Forests, National Wildlife Refuges, the Natchez Trace Parkway, and from various private landowners in both Alabama and Mississippi.

Literature Cited
Bolton, B. 2013.  Bolton World Catalog Ants. Available online: Accessed 25 Novemberl 2013.

Erichson, W. F. 1839. Bericht über die Leistungen im Gebiete der Naturgeschichte während des Jahres 1838. IX. Insecten. Archiv für Naturgeschichte 5(2): 281-375.

Enzmann, E. V. 1944. Systematic notes on the genus Pseudomyrma. Psyche (Cambridge) 51: 59-103.

Fabricius, J. C. 1804. Systema Piezatorum secundum ordines, genera, species, adjectis synonymis, locis, observationibus, descriptionibus. Brunswick: C. Reichard, xiv + 15-439 + 30 pp.

Kempf, W. W. 1961. Estudos sôbre Pseudomyrmex. III. (Hymenoptera: Formicidae). Studia Entomologica 4: 369-408. 

Kusnezov, N. 1953. La fauna mirmecológica de Bolivia. Folia Universitaria. Cochabamba 6: 211-229. 

Roger, J. 1862c Synonymische Bemerkungen. 1. Ueber Formiciden. Berliner Entomologische Zeitschrift 6: 283-297. 

Ward, P. S. 1989. Systematic studies on pseudomyrmecine ants: revision of the Pseudomyrmex oculatus and P. subtilissimus species groups, with taxonomic comments on other species. Quaestiones Entomologicae 25: 393-468.

Ward, P. S. 1993. Systematic studies on Pseudomyrmex acacia-ants (Hymenoptera: Formicidae: Pseudomyrmecinae). Journal of Hymenoptera Research 2: 117-168. 

Wetterer, J.K., and A.L. Wetterer. 2003. Ants (Hymenoptera: Formicidae) on non-native Neotropical ant-acacias (Fables: Fabaceae) in Florida. Florida Entomologist 86: 460-463.

Wetterer, J. K. 2010. Worldwide spread of the graceful twig ant, Pseudomyrmex gracilis (Hymenoptera: Formicidae). Florida Entomologist 93: 535-540.

Wheeler, W.M. 1942. Studies of Neotropical ant-plants and their ants. Bulletin of the Museum of Comparative Zoology 90: 1-262.

Wheeler, G.C., J. Wheeler. 1956. The ant larvae of the subfamily Pseudomyrmicinae (Hymenoptera: Formicidae). Annals od the Entomological society of America 49: 374-398.

Wild, A. L. 2007. A catalogue of the ants of Paraguay (Hymenoptera: Formicidae). Zootaxa 1622: 1-55.


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