Strumigenys hexamera

Strumigenys hexamera is a small 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. It is rare in Hong Kong with only two records, both from secondary forests at elevations of 142 and 160 m (Tang et al., 2019). It seems to have small monogynous colonies of about 35 individuals (Terayama et al. 2014).

Identification
A member of the argiola complex in the Strumigenys argiola group. 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.

Within the United States, the unique mandibular structure and scale-like setae on the head and body will separate S. hexamera from other members of the genus.

Distribution
A rare, introduced species known from two sites in central Florida. The sites were in mesic forest. This is our only known example of an ant apparently introduced into Florida from Japan. Pest status: none. First published Florida record: Deyrup 1988; earlier specimens: 1987. (Deyrup, Davis & Cover, 2000.)

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. Joe MacGown has typically collected this species in rich mesic hardwood or mixed pine hardwood forests, often in areas with hilly terrain.

Distribution based on Regional Taxon Lists
Nearctic Region: United States. Oriental Region: Taiwan. Palaearctic Region: Japan, Republic of Korea.

Biology
Strumigenys hexamera is subterranean, rather than living in leaf litter. It 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, see Strumigenys behavior).

Tang et al. (2019): The record of S. hexamera in Hong Kong represents the first record of this species for continental China. This species is known as an introduced species in Southeast USA (Alabama, Florida, Louisiana, and Mississippi), and was reported as introduced within the Ogasawara Islands (Shindo 1979). Here we tentatively classify this species as introduced to Hong Kong in light of its tramp characteristics, including its known thelytokous reproductive strategy (Masuko 2013), and the lack of previous collections in Hong Kong or other parts of mainland China. However, for this species, as for many tramp species across Asia, further study is needed to determine their exact origin and the extent of their native range.

Also see Hunting Tactics in Short-Mandibulate Strumigenys for more about this ant's hunting behavior.

Nomenclature

 *  hexamera. Epitritus hexamerus Brown, 1958d: 70, figs. 1-3 (w.q.) JAPAN. Combination in Pyramica: Bolton, 1999: 1672; in Strumigenys: Baroni Urbani & De Andrade, 2007: 121. See also: Bolton, 2000: 388.

Worker
Bolton (2000) - TL 2.0-2.2, HL 0.50-0.53, HW 0.53-0.55, CI 106-108, ML 0.18-0.22, MI 36-42, SL 0.23-0.25, SI 46-47, PW 0.28-0.31, AL 0.57-0.60 (3 measured).

Apicodorsal tooth of mandible elongate and spiniform. Mandible with 2 preapical teeth, both located in distal half of mandible and the proximal preapical tooth somewhat longer than the distal. Dorsal surfaces of mandibles and scapes with orbicular hairs. Hairs on leading edge of scape curved toward apex of scape. Cephalic dorsum with orbicular hairs more or less evenly distributed; there is no hairless patch on each side mesad of the lateral margin in the area where the margins are most strongly divergent. CI > 100. Mesonotum in profile bulging posteriorly over base of propodeum, overhanging metanotal groove.

Queen
TL 2.8, HL 0.60, ML 0.20, WL 0.76, scape L 0.27, funiculus L 0.49 mm. CI 102, MI 34. Eyes moderate in size, well underneath on dorsal scrobe borders. Alitrunk robust, humeri very weakly subangulate, without tubercles. Mesonotum gently convex; scutellum rounded behind and not strongly projecting. Propodeal teeth extremely small, with infradental lamellae. Parapsidal furrows fine but distinct. Petiolar node shorter and broader than in worker, the anterior face more concave and the two carinae more distinct, forming a miniature peak on each side above. Mesonotum with orbicular squamiform hairs like those of head. As in the worker, there are no erect specialized hairs on head or alitrunk.

Type Material
Bolton (2000) - Holotype worker and paratype worker, JAPAN: Kyushu, Provo Chikugo, Korasan, 25.ix.1955 (S. Miyamoto); paratype queen (dealate), JAPAN: Honshu, Numazu City, Sige-toge, 22.v.1949, 250 m., under stone (K. Okano) [examined].

Brown (1958) reported the head of the paratype queen has been lost.