Subfamily MYRMICINAE
Tribe DACETINI

Strumigenys hexamera (Brown) 1958

by Joe A. MacGown, last updated 25 November 2013

Strumigenys hexamera, full face view of a worker
(photo by Joe A. MacGown)
Strumigenys hexamera, full face view of a worker
(photo by Joe A. MacGown)
Strumigenys hexamera, full face view of a worker
(SEM by Joe A. MacGown and Richard L. Brown)
Strumigenys hexamera, mandibles of a worker
(SEM by Joe A. MacGown and Richard L. Brown)
Strumigenys hexamera, closeup of scale-like setae
(SEM by Joe A. MacGown and Richard L. Brown)
Strumigenys hexamera, spongiform tissue on waist of a worker
(SEM by Joe A. MacGown and Richard L. Brown)

Introduction
Strumigenys is a monophyletic genus of dacetine ants that includes over 900 species worldwide (Bolton 2013). Forty-eight described species of Strumigenys have been reported from the US (Bolton 2013), but this genus is most speciose in the Southeast where at least 43 species are known to occur. In the US, members of the genus Strumigenys can easily be distinguished from other genera by their minute size; 4-6 segmented antennae; elongate, snapping mandibles; unique and often "bizarre" pilosity, and presence of "spongiform lobes" beneath the petiole and postpetiole (Bolton 1999). Dacetines are predatory ants that generally feed on tiny soil arthropods (Wilson 1953).  Most dacetines are small, 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, P. 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.

Identification
TL 2.0–2.2, HL 0.50–0.53, HW 0.53–0.55, CI 106–108, ML 0.18–0.22, SL 0.28–0.31, PW 0.23–0.25, WL 0.57–0.60 (measurements and description from Bolton (2000)). Strumigenys hexamera is a minute (~2.0 mm in total length), brown species with elongate mandibles; in full face view only 3 teeth are visible, 2 preapical teeth and one very long apicodorsal tooth which extends over the tooth on the opposite mandible and beyond the outer margin of the opposite mandible. The entire head, scape, and even the mandibles are covered with large circular shaped, somewhat translucent setae. Spongiform tissue present beneath the petiole and postpetiole.

Based on its unique mandibular structure and scale-like setae on the head and body, S. hexamera is unlikely to be confused with other dacetines in the southeastern US.

Biology and Economic Importance
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 dacetines whose biologies have been studied, and which apparently prey primarily on 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 min 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 also 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).

Strumigenys hexamera is now well established in the southeastern US, especially in Mississippi where we have collected it throughout the state in over 17 counties. Compared to other alien species, S. hexamera appears to do well in natural wooded habitats. We typically have collected this species in rich mesic hardwood or mixed pine hardwood forests, often in areas with hilly terrain.

Distribution
Worldwide Distribution: Japan, South Korea, and Taiwan.

US Distribution: Alabama, Florida, Louisiana, 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.