Introduction
Strumigenys is a monophyletic genus of attine ants that includes over 900 species worldwide (Bolton 2013). Forty-eight described species of Strumigenys have been reported from the US (Bolton 2013), being most speciose in the Southeast where at least 43 species are known to occur. Strumigenys are predatory ants that generally feed on tiny soil arthropods (Wilson 1954).  Species are minute, cryptically colored, rarely forage openly above ground, are slow moving, and become motionless when disturbed.

Strumigenys hexamera is a tiny predatory ant that feeds on minute soil arthropods. Originally from East Asia, S. hexamera has been recently introduced to North America, apparently through human commerce. This ant is very distinctive looking with its unique dental array and setal arrangement on the head.

Taxonomic History (provided by Barry Bolton, 2013)
Brown (1958) described Epitritus hexamerus (= P. hexamera) from two sites in Japan. Bolton (1999) transferred E. hexamerus to Pyramica. Baroni Urbani and De Andrade (2007) synonymized Pyramica with Strumigenys.

Diagnosis
In the USA, Strumigenys hexamera can be distinguished from other ant species by its minute size, 6-segmented antennae, 2-segmented waist, presence of numerous large, circular, translucent setae on the head and body, and by the elongate curved mandibles, each of which terminate with a long tooth that overlaps the other mandible (when mandibles are fully  closed).

Identification
Worker: Minute (TL 2.0–2.2 mm, HL 0.50–0.53 mm, HW 0.53–0.55 mm, CI 106–108 mm, ML 0.18–0.22 mm, SL 0.28–0.31 mm, PW 0.23–0.25 mm, MeSL 0.57–0.60mm) (measurements from Bolton (2000)). Overall reddish brown in color. Head widest posteriorly, edges squared, then abruptly tapering to mandibles; entire head with fine microreticulation; face, clypeus, mandible, and scape with large, appressed, circular, somewhat translucent setae; eyes minute, located on side of head beneath antennal scrobe; antenna 6-segmented, scape short, apical flagellomere greatly enlarged forming a 2-segmented club with preapical flagellomere; mandibles elongate, in full face view only three teeth are visible including two preapical teeth and one elongate apicodorsal tooth which extends beyond the outer margin of the opposite mandible at closure. Entire mesosoma with fine reticulation, dorsum with large, appressed, circular, somewhat translucent setae; promesonotum flattened, slightly raised above propodeum; propodeum with a pair of small spines directed rearward; propodeal declivity with a narrow spongiform lamella. Waist 2-segmented, with dense microreticulation and with large, appressed, circular, somewhat translucent setae present dorsally; petiole pedunculate, node rounded rectangular in dorsal view, a heavy curtain of spongiform tissue present ventrally; postpetiole rounded rectangular, about twice as wide as petiole (in dorsal view), with a few scattered erect, apically forked setae present dorsally, and with thick spongiform tissue present posteriorly and ventrally. Gaster shiny, with only a few scattered appressed pubescent setae; with scattered erect, apically forked setae present dorsally; sting present.

Queen: Small, slightly larger than worker. Overall reddish brown in color. Head widest posteriorly, edges squared, then abruptly tapering to mandibles; entire head with fine microreticulation; face, clypeus, mandible, and scape with large, appressed, circular, somewhat translucent setae; eyes large, multifaceted, located on side of head beneath antennal scrobe; three ocelli present; antenna 6-segmented, scape short, apical flagellomere greatly enlarged forming a 2-segmented club with preapical flagellomere; mandibles elongate, in full face view only three teeth are visible including two preapical teeth and one elongate apicodorsal tooth which extends beyond the outer margin of the opposite mandible at closure. Entire mesosoma with fine reticulation, dorsum with large, appressed, circular, somewhat translucent setae; pronotum visible dorsally as a "U" shape; pronotum, mesoscutum, and mesoscutellum forming a slightly convex arc, elevated above propodeum; propodeum with a pair of short, triangular spines directed rearward; propodeal declivity with a narrow spongiform lamella. Wings, if present, lacking pigmented veins, short triangular stigma present, with fringe of setae along wing edges apically and on ventral sides of wings to about midway toward base. Waist 2-segmented, with dense microreticulation and with large, appressed, circular, somewhat translucent setae present dorsally; petiole pedunculate, node rounded rectangular in dorsal view, with a narrow curtain of spongiform tissue present ventrally; postpetiole rounded rectangular, about twice as wide as petiole (in dorsal view), with a few scattered erect, apically forked setae present dorsally, and with thick spongiform tissue present posteriorly and ventrally. Gaster shiny, with longitudinal striae present at anterior portions of first tergite; scattered appressed pubescent setae present and scattered erect, apically forked setae present dorsally; sting present.

Biology
Unlike many of our other exotic ant species in the US, S. hexamera thrives in natural wooded habitats. MEM collections of this species have primarily from rich mesic hardwood or mixed pine hardwood forests, often in areas with hilly terrain. Strumigenys hexamera is a specialized ambush predator of small long-bodied soil arthropods, such as Diplura, Chilopoda, and Collembola (Masuko 1984). Masuko (2009) reported that “Diplura composed 60% of the prey in the field material.” This prey preference is unusual compared with most other species of Strumigenys whose biologies have been studied with prey selection often being Collembola (Masuko 2009). Foraging S. hexamera workers hunt for their prey in small crevices in the soil. Upon encountering prey, S. hexamera moves to a crouching position, pulls its antennae back into recessed antennal scrobes lining the side of the head, closes its mandibles, and remains motionless. The ant may remain still for >20 minutes while it waits for the prey to advance and crawl on top of her head. Strumigenys hexamera is uniquely equipped for this situation, having a flattened head and slightly upturned mandibles, each of which terminates with a sharp apical tooth, allowing the ant to strike at prey passing overhead.  When the prey is in the correct position, the ant opens its mandibles and suddenly snaps them shut, impaling the prey with the apical teeth (Masuko 1984). Strumigenys hexamera has been reported to coat its body with soil and other detritus using its forelegs, a behavior that may camouflage the ant's odor from the prey (Masuko 1984).

Distribution
Native Range: Japan (AntWiki.org).

Nearctic: United States (AntWeb.org).
Oriental: Taiwan (AntWeb.org).
Palearctic: Japan, South Korea (AntWeb.org and AntWiki.org).

U.S. Distribution: AL, FL, LA, MS (AntWeb.org and MEM).      

Southeastern U.S. Distribution: AL, FL, LA, MS (AntWeb.org and MEM).

Acknowledgments
Thanks to Ryan J. Whitehouse for help with measuring specimens, comments on descriptions, photography of some specimens, and proofreading. 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 (2001-2014) and USDA-ARS Areawide Management Invasive Ants 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

Baroni Urbani, C. & De Andrade, M.L. 2007. The ant tribe Dacetini: limits and constituent genera, with descriptions of new species. Annali del Museo Civico di Storia Naturale “G. Doria” 99: 1-191.

Bolton, B.  1999.  Ant genera of the tribe Dacetonini (Hymenoptera: Formicidae).  Jour. Nat. Hist. 33: 1639-1689.

Bolton, B. 2013.  Bolton World Catalog Ants. Available online: http://www.antweb.org/world.jsp. Accessed 16 April 2013.

Brown, W. L. 1958 A new Japanese species of the dacetine ant genus Epitritus. Mushi 31: 69-72.

Masuko, K. 1984. Studies on the predatory biology of Oriental dacetine ants (Hymenoptera: Formicidae). I. Some Japanese species of Strumigenys, Pentastruma, and Epitritus, and a Malaysian Labidogenys, with special reference to hunting tactics in short-mandibulate forms. Insectes Sociaux 31:429–451.

Masuko, K. 2009. Studies on the predatory biology of Oriental dacetine ants (Hymenoptera: Formicidae). III. Predation on gamasid mites by Pyramica mazu with a supplementary note on P. hexamerus. Journal of the Kansas Entomological Society 82: 109-113.

Wilson, E. O. 1953. The ecology of some North American dacetine ants. Annals of the Entomological Society of America  46: 479-497.

Links

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