Acropyga epedana

This subterranean (hypogaeic) ant is the only species in its genus that occurs in the nearctic region. A. epedana is dependent on a mealybug species (Rhizoecus colombiensis) for their survival. The ants feed on the sugar-rich feces of the mealybugs, which in turn feed on root-sap. It is presumed A. epedana also feed mealybugs to their larvae. Workers in a laboratory colony refused a variety a food items that ranged from dead insects to sugary substances (e.g. honey, sugar water). This ant’s relationship and dependence on R. colombiensis means they are obligate coccidiphiles.

Identification
Diagnosis (Snelling 1973): Mandible slender, cutting margin oblique and quadridentate; eye minute, consisting of one or two facets; antennae 10-segmented, scape surpassing occipital margin, terminal segment longer than preceding four segments; pubescence and hairs abundant on all body surfaces.

Diagnosis (LaPolla 2004): Worker: antennae 10-11 segmented; scape reaches posterolateral comers; mandible with three distinct teeth, occasionally a smaller fourth tooth found along inner mandibular margin; mesosoma with abundant, generally appressed hairs. Queen: as in worker with modification expected for caste. Male: antennae 12 segmented; mandibles with two distinct teeth, occasionally a smaller tooth is present at basal angle; rectangular shaped parameres; cuspi and digiti meet apically forming an oval-shaped space between them in ventral view. Compare with species of the goeldii complex.

A member of the decedens species-group, this species is very similar in general appearance to Acropyga goeldii and Acropyga palaga. It may represent a northern population of A. goeldii and thus may be over-split as recognized here (see discussions of A. goeldii and A. palaga for further analysis of problems with some species of goeldii complex). However, differences in the penis valves of the three species suggest they are distinct species. The main difference is in the ventral extension of the penis valve. In A. epedana the ventral extension is much narrower at the base and comes more to a point than in the broadly rounded base of A. goeldii, and lacks distinct ventral curve from the caudal end of the penis valve through the ventral extension found in A. palaga. The head of A. epedana workers are generally not as wide or long as is found in the other two species. Finally, A. epedana workers do not possess very many erect hairs on the mesosomal dorsum, unlike A. goeldii and A. palaga. Instead the dorsum is largely covered in a dense layer of appressed hairs.

Distribution
USA. Arizona. (LaPolla et al. 2002): Though A. epedana is presently known from 5 sites in southeastern Arizona, it probably ranges south into Mexico through the Sierra Madre Occidental mountain range. A. epedana appears to only occur at higher elevations (where annual rainfall is greater) between 1676 m - 1052 m, in open oak/juniper woodland habitat.

Distribution based on Regional Taxon Lists
Nearctic Region: United States.

Habitat
Known from oak/juniper woodlands with scattered grasses as the dominant ground cover.

Foraging/Diet
Workers are not known to forage above ground. (LaPolla et al. 2002): Laboratory observations of a few captured colonies and what is known about other species in this genus suggest A. ependana is an obligate coccidiphile. Workers in laboratory nests refused food items that ranged from dead insects to sugary substances (e.g. honey, sugar water).

Queen Number
Most colonies have been found with multiple queens but the number of nest samples has been small. Polygyny is believed to be the norm for this species, based on these observations and a scattering of records of other species in this genus (see LaPolla et al. 2002 for references).

Colony Size
A complete nest excavated in July from the Chirachua Mts had 234 workers, one queen, 48 mealybugs and 100 males. (Smith et al 2007)

Nesting Biology
(LaPolla et al. 2002): Subterranean soil nests with no exposed ground openings or indication of their presence. Chambers may be located under rocks that are exposed to full sunlight but the full extent of the nest is more expansive. Diffuse tunnels and loosely differentiated chambers are located from 10 - 30 cm underground, some of which intersect roots. Queens, workers, brood and mealybugs are scattered throughout the nest. The nesting biology is similar to A. paramaribensis (now known as Acropyga exsanguis) (Bunzli 1935).

The vertical depth of the colony is likely dependent upon the dryness of the soil. During dry times of the year colonies may be deeper below ground, and quite difficult to find. Colony activities are likely to be closer to the ground surface during the time of the year when mating flights occur and the ground becomes moister.

Reproduction
Mating flights presumably occur sometime from late July through mid-August, occurring when the ground is moister due to seasonal rains. Newly produced queens leave their natal nest for their mating-flight carrying a mealybug in their mandibles. If the queen is able to mate and begin a new nest this single mealybug will serve as the foundress of the ant colony’s mealybug “herd.” The coccid carrying behavior practiced by the queen is known as “trophophoresy.”

A published account (Smith et al. 2007) of the mating flight of this species provides a interesting account of this aspect of the biology of this genus:

A mating aggregation was observed in a shallow ditch alongside a primitive asphalt road, bordered by short grass, just 200 m west of the Southwestern Research Station (1645 m N31°53.028’ W109°12.378’), near Portal, Arizona. Beyond the ditch was a steep incline without vegetation and the ditch and road received direct sunlight for most of the day. The surrounding habitat was dominated by an open oak-pine-juniper forest.

The mating flight occurred 2 days after the first heavy summer monsoon rains (26 July 2005), although there was very little precipitation on the day of the flight. The aggregation was first seen at ~16:30h. Most of the males were hovering or swarming above small clumps of grass and rocks. Females entered the aggregation, after which multiple males simultaneously attempted to mate with them. Males outnumbered females by ~10:1. After mating most females immediately shed their wings and searched for suitable nest sites; only a few females flew from the aggregation. All females carried a mealybug in their mandibles. On 6 August 2005 at the same site, a single alate female carrying a mealybug was found actively mating, but no swarm could be found and only a single male was present.

Behavior
This ant’s relationship and dependence on 'R. colombiensis' means they are likely to be obligate coccidiphiles.

Workers are extremely photophobic.

Workers meeting within nest tunnels often exhibit a body jerking behavior that is similar to what Weber (1957, 1972) described as jigging.

Other Ants
Hypoponera inexorata

(Smith et al. 2007): Queens have been found underneath rocks with A. epedana. This association is presumed to be fortuitous. Queens of H. inexorata are likely attempting to found new colonies and the two species are potentially indifferent, at least at this early stage of colony founding, to one another.

Other Insects
Mealybug Rhizoecus colombiensis

(Williams and LaPolla 2004): Workers appear to feed exclusively on the sugar-rich feces of this mealybug, which feeds on root-sap. It is presumed R. colombiensis are also culled and eaten for their protein content, being feed directly to larvae and perhaps to the queens.

Morphology
Individuals have thin cuticles, which makes them highly susceptible to dessication when they are removed from their nest.

Nomenclature

 *  epedana. Acropyga epedana Snelling, R.R. 1973a: 7, fig. 1 (w.) U.S.A. LaPolla, Cover & Mueller, 2002: 369 (q.m.). See also: LaPolla, 2004a: 46.

Worker
Head, in frontal view, as wide as long, Cl l00; widest at about eye level, sides nearly straight; occiput slightly concave in middle; in lateral view, thickest above eye level, occiput compressed, eyes at lower one-fourth of side. Mandibles slender, cutting margin strongly oblique, with three large triangular teeth and a minute tooth at basal angle. Apical margin of clypeus evenly slightly convex; clypeus in profile with apical third abruptly sloping to margin. Scape reach ing a little beyond occipital margin, SI 74; median flagellar segments distinctly broader than long, apical segment large, longer than preceding four segments combined.

Thorax short and robust, PW 0.67 x WL. Pronotal face declivitous; meso- metanotum flattened, evenly curved from pronotum to propodeum. Propodeum entirely oblique, without distinct basal face, surface flat. Legs stout; tarsal segments flattened. Petiolar scale, in profile, compressed, fully erect, crest thin; in frontal view,crest very slightly concave. Integument smooth and shiny on head and thorax; head with abundant setigerous punctures; coarser and more conspicuous on lower half; gaster shiny, integument roughened and with obscure coarse piligerous punctures.

Head with abundant appressed and subappressed short, coarse pubescence on sides and beneath; front and occiput with much pubescence fully erect; with scattered long erect hairs, numerous on clypeus. Thorax and propodeum with sparse subappressed to fully erect coarse pubescence and scattered long, erect coarse hairs. Gaster with abundant pubescence, subappressed to erect on ter gites, more closely appressed laterally, appressed on venter; with numerous long, fully erect hairs. Scape pubescence abundant, reclinate to suberect, coarse. Legs similar, coarse hairs abundant on tibiae; tarsal segments with coarse seta-like hairs and a coarse seta on apical corners of segments.

Color uniformly brownish yellow, mandibles transparent, cutting margin piceous.

Measurements: (Holotype) HL 0.50; HW 0.50; SL O.37; PW 0.03; WL 0.53; TL 1.75 mm.

LaPolla (2004): Measurements (n=10): TL: 1.83-2.28; HW: 0.481-0.539; HL: 0.482-0.534; SL: 0.343-0.394; ML: 0.592-0.612; GL: 0.731-1.12; CI: 94.01-106.1; SI: 66.04-77.05.

Queen
(LaPolla et. al, 2002) - Head: yellow to brownish-yellow, darker along segmental margins; slightly broader than long and covered with a fine pubescence; apex slightly darker with three prominent ocelli; eyes large, positioned towards the anterior margin at lower 1/4 of side; eyes break lateral margins when viewed in full frontal view; antennae 11 segmented and incrassate; scape reaches to slightly passes posterior margin; posterior margin entire, with scattered, short, erect hairs; clypeus wide, medially slightly convex with several (usually around 12 or fewer) long, erect hairs, the longest hair placed medially; mandible infuscated with three distinct teeth, occasionally a smaller tooth is also present at basal angle; a large gap exists between anterior clypeal margin and inner mandibular margin. Thorax: yellow to brownish-yellow, darker along segmental margins; pronotum short and collar-like; anterior of mesonotum rounded, dorsum flat, covered in short, suberect hairs; scutellum distinct, roughly oval, slightly lighter than mesonotum, with fewer suberect hairs covering dorsum; propodeum with fewer short, suberect hairs than other parts of the thorax; declivity at ca. 45° angle. Gaster: petiole erect and short, triangular in lateral view with a rounded apex that is occasionally concave; gaster yellow to brownish-yellow darker along segmental margins; covered in a fine pubescence, with a few scattered suberect hairs present.

Measurements (LaPolla et. al, 2002) - (n= 6). TL: 3.03-3.17HW: .592-.612 HL: .571-.602SL: .443-.485 EL: .126-.147 EW: .105-.113 THL: .854-.956 GL: 1.51-1.71 CI: 100.3-111.0 SI: 74.580.7 EI: 21.3- 24.0

Male
(LaPolla et. al, 2002) - Head: brownish-yellow and covered with fine pubescence; apex darker brownish-yellow, with three prominent ocelli; eyes large, in frontal view bulging out from outline of head; antennae 12 segmented; scapes surpass posterior margin, by about the length of the first two funicular segments; clypeus with several long hairs on anterior margin, though clypeus generally less pilose than the rest of the head; mandibles with two distinct teeth, an apical tooth and a basal tooth, a smaller tooth is occasionally present at basal angle; a large gap exists between anterior clypeal margin and inner mandibular margin. Thorax: yellow to brownish-yellow, darker along segmental margins; pronotum short and collarlike; anterior of mesonotum rounded, dorsum flat, covered in short, suberect hairs; scutellum distinct, roughly oval, slightly lighter than mesonotum, with fewer suberect hairs covering dorsum; propodeum with fewer short, suberect hairs than other parts of the thorax; declivity at ca. 45° angle. Gaster: petiole erect and short, triangular in lateral view with a rounded apex; gaster brownish-yellow with more brown dorsally; covered in a fine pubescence, with a few scattered suberect hairs present. Genitalia: parameres long, rectangular, curving towards each other apically; anteriorally parameres without hairs, the posterior ends with scattered erect hairs; vol sellar lobes distinct, with many erect hairs; cuspi cylindrical, with short, peg-like teeth at the apex; digiti elongate with apical end roughly anvil-shaped; dorsal tip of digiti with short, peg-like teeth where they meet the cuspi, forming an oval-shaped space between them in ventral view; ventral side of penis valves without teeth; apical tips flattened and laterally expanded.

Measurements (LaPolla et. al, 2002) - (n= 3) TL: 2.00-2.13 HW: .419-.425 HL: .410-.424 SL:.330-.374 EL: .132-.144 EW: .108-.114 THL: .726-.775 GL: 2.00-2.13 C1:100.5-102.4 SI: 78.88-8.0 E1: 31.0-34.0

Type Material
Holotype and three paratypes.

Type Locality
3.1 mi. W of summit of Montezuma Pass, 5800 ft. elev., Huachuca Mts., Coshise Co., Arizona, 14-VIII-1969 (R.R. Snelling, No 69-307)

Etymology
Morphology. "Gr., epedanos, weak or feeble, so-called because of the thin, collapsible integument."

Additional References

 * Bünzli, G. H. 1935. Untersuchungen über coccidophile Ameisen aus den Kaffeefeldern von Surinam. Mitteilungen der Schweizerischen Entomologischen Gessellschaft. p. 453-593.
 * LaPolla, J. S., S. P. Cover, and U. G. Mueller. 2002. Natural history of the mealybug-tending ant, Acropyga epedana, with descriptions of the male and queen castes. Transactions of the American Entomological Society 128:367-376.
 * Smith, C. R., J. Oettler, A. Kay, and C. Deans. 2007. First recorded mating flight of the hypogeic ant, Acropyga epedana, with its obligate mutualist mealybug, Rhizoecus colombiensis. Journal of Insect Science 7.
 * Snelling, R. R. 1973. Two ant genera new to the United States (Hymenoptera: Formicidae). Contr. Sci. (Los Angel.) 236:1-8.[PDF]
 * Weber, N.A. 1972. Gardening ants, the attines. Philadelphia American Philosophical Society, Philadelphia, Pa, 146 pp.
 * Weber, N.A. 1957. Fungus-growing ants and their fungi. Ecology 38: 480-494.
 * Williams DJ, LaPolla JS. 2004. The subterranean mealybug,Rhizoecus colombiensis (Hambleton)(Hem.,Pseudococcidae), described originally from Colombia, now found associated with the ant Acropyga epedana Snelling (Hym., Formicidae) in Arizona, U.S.A. Entomologist’s Monthly Magazine 140: 106