Pheidole arietans Wilson 2003

Suggested synonym:
= Pheidole glyphoderma Wilson 2003

Formicidae, Hymenoptera, Insecta, Arthropoda, Animalia

worker face view

worker lateral view

major face view

major lateral view

Image is of highland Barva Transect form (see Comments). Lowland form has minor with completely smooth face.


Minor worker: head length 0.62mm, head width 0.58mm, scape length 0.58mm, Webers length 0.82mm (n=1). Head rounded to somewhat flattened behind; promesonotum evenly arched, mesonotal suture absent; propodeal spines short, sharp, upturned; sculpture on face, pronotum, and katepisternum varying from largely smooth and shining to densely punctatorugose; propodeum punctatorugose; gaster smooth and shining; dorsal pilosity abundant, long, flexuous; color dark red brown.

Major worker: head length 1.56mm, head width 1.34mm, scape length 0.62mm (n=1). Face punctatorugose beneath sweep of scapes; medial area between frontal carinae with parallel transverse rugae; vertex lobes smooth and shining; medial clypeus smooth, mat, in same plane with broad, flattened surfaces of mandibles (serving phragmotic function?); bases of frontal carinae elevated, tooth-like; hypostomal margin with no medial tooth, pair of teeth located less than one third distance to recessed teeth flanking mandibles; dorsal pilosity abundant; head with abundant suberect setae projecting from sides of head in face view.

Similar species: Pheidole rhinoceros.

See Comments for variation in this complex.


Costa Rica (type locality), Mexico (Chiapas). Costa Rica: Atlantic slope; La Selva to 1000m elevation in Braulio Carrillo; Penas Blancas Valley; Monteverde; Volcan Arenal.

Natural History

This species is found in or near primary forest. Workers recruit to baits, and may have seed caches in their nests. It nests in dead wood, dead branches, and live stems. Nests occur from ground level to the canopy.

Seven nests were observed in dead wood near ground level.

One nest was observed in a dead log, 10-20cm dia, suspended horizontally about 1m high. The log was largely solid but waterlogged throughout. The outer layers were wet and slimy with many worms. The Pheidole colony was in small chambers in a soft rotten area, about 1cm below the upper surface. The soft area was thoroughly excavated, and no more of the nest was found, so this collection probably contained most or all of the colony.

Nests may occur in live stems. Two nests were observed in internodes of Cecropia insignis saplings. In one case the colony was polydomous. Two nests were observed in live stems of Myriocarpa along streams. A nest was observed in a live branch (identity unknown) in a fresh treefall. The nest was in a discrete hollowed-out area; the stem was solid elsewhere. There were no Homoptera on the walls of the nest. There was a seed cache in the nest chamber.


This species complex shares many characters with P. rhinoceros and JTL-110 (the lowland version of rhinoceros). The minor workers are nearly indistinguishable. The majors have the clypeus flat or slightly concave, but nearly always with a trace of the medial horn that is prominent in rhinoceros and JTL-110. Is this the plesiomorphic starting condition from which the horned versions were derived, or is this a derived loss of the clypeal horn?

The following forms can be recognized, exhibiting discordant character variation. There is the potential for four or more cryptic species in Costa Rica.

Pheidole arietans s.s.: minor worker with face completely smooth and shining, nearly black; head generally smaller than other forms (need measurements); katepisternum mostly smooth and shining, with foveolation restricted to ventral and posterior margins. Major worker with clypeus flat, smooth, matte, with only a trace of medial horn; sculpture on face posterior to clypeus a short series of arcing rugae that meet medially, followed by transverse rugulae; setae on sides of head suberect; medial portion of side of pronotum, almost entire katepisternum, lateral propodeum anterior to spiracle smooth, shining, highly polished. Range: On Barva transect, La Selva to 500m elevation; In Peľas Blancas Valley, at Poco Sol and Refugio Eladio, both at 800m; on lower slope of Volcan Arenal. Field observations: Queenrite nest in dead branch of Grias (Lecythidaceae); canopy branchfall. Nest in dead 3cm diameter vertical palm trunk. Collecting in 4-day old treefall; nest in 1.7cm diameter dead stick on ground, may have fallen with treefall, only colony fragment (no queen) in isolated stick on ground. Recent branchfall; nest in center of dense wood, 3cm diameter branch. Cecropia insignis; small tree on riverbank, multiple stems sprouting from thick base; large nest low in the main stem; outposts of this colony found higher in the plant. In live stem Myriocarpum (Urticaceae). Nest in rotten wood. Also known from baits on forest floor, Winkler samples of forest floor litter.

Highland Barva Transect form: minor worker face completely foveolate; head generally larger than arietans s.s., somewhat lighter brown; katepisternum more extensively foveolate, with small shiny patch anterodorsally or almost entirely foveolate. Major worker with clypeus as in arietans s.s.; sculpture on face posterior to clypeus like arietans s.s., but anterior arcing rugae may be replaced by irregular mesh of rugae; setae on sides of head suberect, like arietans s.s.; side of pronotum with more extensive faint rugulae crossing medial area, katepisternum with larger patch of foveolation ventrally, lateral propodeum anterior to spiracle more foveolate compared to arietans s.s.. Range: On Barva transect, 600-1100m. Field observations: Colony in base of dead vertical stick along trail. Edge of treefall gap; a dead log, 10-20cm diameter, suspended horizontally about 1m high; log was largely solid but waterlogged throughout; outer layers wet and slimy with many worms; colony in small chambers in a soft rotten area, about 1cm below upper surface. Recruited to bait on surface of rotten log.

Peľas Blancas form: minor worker face faintly and irregularly rugulose foveolate (intermediate between entirely shiny face of arietans s.s. and fully foveolate face of highland Barva form and glyphoderma); head generally larger than arietans s.s., somewhat lighter brown; sculpture on side of mesosoma similar to upland Barva form. Major worker like upland Barva form, with sculpture on side of mesosoma somewhat intermediate between that form and arietans s.s.. Range: Peľas Blancas valley, only known from Refugio Eladio at 800m. Sympatric with arietans s.s. at this site. Field observations: young second growth vegetation, bananas, Piper, tall grass; colony in dead stick. In recent treefall; nest in live branch, in a discrete hollowed-out area, stem solid elsewhere; no Hemiptera in nest; a seed cache in nest. Nest in chamber in live stem Myriocarpa along stream edge. Nest in several internodes of Cecropia insignis sapling.

Pheidole glyphoderma Wilson 2003: minor worker same as highland Barva form. Major worker with clypeus more concave, sublucid instead of matte, with scattered faint longitudinal rugulae; medial face posterior to clypeus with entirely longitudinal rugulae that fan outward onto posterior lobes; setae on sides of head lanose, fully appressed; lateral pronotum with faint linear rugulae, katepisternum mostly smooth and shining, lateral propodeum anterilr to spiracle foveolate. Range: known only from Monteverde cloud forest, 1500m. Field observations: the type series was collected by P. S. Ward at a sugar bait. Two additional minor workers were collected by Lisa Schoenberg in Winkler samples. Given the abundant collecting in Monteverde, this is a rare ant.

Chiapas form: minor worker same as glyphoderma. Major worker with clypeus more concave, sublucid like glyphoderma, medial clypeal carina more developed, extending almost entire length of clypeus on most specimens (very short and nearly obsolete in one); sculpture on face posterior to clypeus like arietans s.s. on most specimens, but one specimen with entirely longitudinal rugulae like glyphoderma; setae on sides of head suberect; sculpture on side of mesosoma like glyphoderma. Range: known mainly from Nahç, a 1000m elevation site in northern Chiapas. One minor worker collection from Metzabok, a nearby 500m elevation site. Field observations: multiple collections from baits at Nahç, rarely workers in Winkler samples from Nahç, Metzabok.

Literature Cited

Wilson, E. O. 2003. Pheidole in the New World: A Dominant, Hyperdiverse Ant Genus. Harvard University Press, Cambridge, Mass

Page authors:

John T. Longino, The Evergreen State College, Olympia WA 98505 USA.

Stefan Cover, Museum of Comparative Zoology, Harvard University, Cambridge MA 02138 USA.

Last modified: 30 December 2008.

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