Petalomyrmex phylax

An obligate ant-plant mutualist that is associated with the coastal-rainforest understory tree Leonardoxa africana africana.

Identification
WORKER: Monomorphic: head cordate; antenna nine-segmented, without distinct apical club; ocelli near top of head. FEMALE: Antenna ten-segmented; head cordate, as broad as to broader than long; frontal carinae separated by more than their length; openings of propodeal spiracle and metapleural gland circular, latter with a few long guard hairs. MALE: Antenna ten-segmented; eye large, occupying most of side of head; ocelli normal; frontal carinae obsolete; pygostyles absent; mandible six-toothed.

This monomorphic genus is closely allied to Aphomomyrmex. The strongly cordate head of the worker will separate Petalomyrmex from Aphomomyrmex and related genera. Petalomrymex workers and females differ from those of Aphomomyrmex by the reduced palpal segmentation and the worker caste is monomorphic rather than polymorphic. The strong dorso-ventral flattening of the mesosoma of the Petalomyrmex female and male is unusual and distinctive; it is reminiscent of the bizarre females of the Neotropical subgenus Myrmostenus of Camponotus and of certain species of Myrmelachista. (Snelling 1979)

Distribution based on Regional Taxon Lists
Afrotropical Region: Cameroun.

Biology
This ant has been the subject of numerous studies (Guame 1997, Guame et al. 1997, Guame and Mckey 1999) aimed at understanding the relationships between Petalomyrmex phylax, its mutualist host plant Leonardoxa africana africana and the ant Cataulacus mckeyi.

Petalomyrmex phylax is able to reduce herbivory on its host plants, which in turn provides nectar and swollen, hollow internodes for ant nesting. Workers are active on the plants 24 hours a day, being particularly attentive to the plant's vulnerable young leaves. The ants are most active on the nectar-producing older leaves during the early afternoon when these leaves are producing nectar. The percentage of plants occupied by this ant species can be as high as %75. Leonardoxa africana africana produces a prostoma (a small unlignified portion of the tip of the domatia where ants create an entrance hole) with dimensions that are large enough to allow for the entry of queens of its mutualist ant partner but prevents other, larger ant species from exploiting the domatia.

Some populations of this species contain only single-queened colonies while others have colonies where supernumerary queens are accepted (as per Dalecky et al. 2002, citing Guame 1997).

Genetics
Genetic polymorphism in two populations of this species were studied by Dalecky et al. 2002. The results showed there was high within-population variation in many of the microsatellite loci studied.

Nomenclature

 *  phylax. Petalomyrmex phylax Snelling, R.R. 1979b: 5, figs. 1-7, (w.q.m.) CAMEROUN. Wheeler, G.C. & Wheeler, J. 1980: 544 (l.). See also: McKey, 1984: 81.

Worker
Holotype. HL 0.55 mm; HW 0.56 mm; CI 1 02; SL 0.38 mm; SI 68; EL 0.13 mm; OI 23; PW 0.36 mm; WL 0. 68 mm; TL 2.36 mm.

Head broadest near upper margin of eyes, strongly cordate in frontal view, width at mandibular base 0.61 x HW. Eye flattened, in frontal view not interrupting lateral head margin; OMD 1 4 x EL. Scape stout, not attaining occipital corner; flagellum gradually swollen, flagellomeres 2-5 broader than long, 6-7 about as long as broad, 8 almost twice longer than broad.

Mesosoma robust, PW 0.53 x WL. Pronotum, from above, about 1.75 wider than long; posterior margin concave. Mesonotum not fused to pronotum, a little wider than long; weakly convex in profile, sloping from pronotal margin to metanotum; mesometanotal suture impressed dorsally, absent on pleura. Metanotum, from above, short, broader posteriorly, sides slightly raised; metanotal-propodeal suture impressed dorsally and on upper third of side; metanotal spiracle absent. Propodeum, from above, broadened posteriorly; spiracular atrium fully visible, large; in profile, basal face convex and rounded into oblique, much longer posterior face, spiracle a little below juncture of faces.

Scale of petiole thick in profile, margins nearly parallel below level of spiracle, sharply convergent above to acute apex; spiracle at about midpoint; in posterior view, margins parallel below spiracle, convergent above, crest narrow and broadly emarginate; from above, about twice wider than long.

Gaster rather broad, a little longer than mesosoma in dorsal view.

Integument smooth and shiny, without conspicuous coarse punctures except on clypeus and mandible.

Body and appendages with abundant erect hairs of very varied length, longest hairs on occiput and mesosomal dorsum exceeding EL. No obvious appressed pubescence.

Reddish yellow, occiput and gaster brownish; mandibular teeth dark reddish.

Paratypes: HL 0.47-0.55 mm; HW 0.49-0.59 mm; CI 100-109; SL 0.33-0.38 mm; SI 64-72; EL 0. 12-0. 14 mm; OI 23-28; PW 0.31-0.37 mm; WL 0.59-0.71 mm; TL 1.78-2.46 mm (54 measured).

Queen
Paratypes. HL 0.90-1.08 mm; HW 0.90-1.08 mm; CI 99-107; SL 0.31-0.37 mm; SI 56-60; EL 0.58-0.62 mm; OI 32-36; PW 1.03-1.23 mm; WL 1.81-2.10 mm; Wing length 4.6-6.0 mm (10 measured).

Head shape similar to that of worker, but eyes more bulging and interrupting side margin of head in frontal view; occipital margin, in frontal view, broadly concave; ocelli slightly below occipital margin, interocellar distance about twice diameter of anterior ocellus, ocellocular distance almost four times diameter of anterior ocellus. Head, in profile, strongly flattened, about 2.7 x longer than thick. OMD 0.55-0. 68 times EL.

Mesosoma greatly flattened, angled about 45° at anterior margin of scutellum; thickness at metanotum about 0.36 x length from anterior margin of tegula to apex of metasternal lobe. Propodeal spiracle on posterior face, atrium large, circular.

Scale of petiole, in profile, low, thick, spiracle near summit; crest broadly, shallowly concave in posterior view; from above, about twice wider than long.

Gaster about as long as combined head and mesosoma, elongate-ovoid, three basal terga almost parallel-sided in dorsal view.

Pilosity about as described for worker, but hairs more abundant on cephalic dorsum and with conspicuous long hairs on gena (short, sparse hairs on gena of worker).

Color as in worker. Wings medium brown, veins and stigma dark brown.

Male
Allotype. HL 0.62 mm; HW 0.65 mm; CI 106; SL 0.44 mm; SI 66; EL 0.32 mm; OI 52; PW 0.85 mm; WL 1.45 mm; Wing length 4.2 mm.

Head somewhat flattened, eye large and strongly bulging in frontal view, OMD 0.2 x EL. Head margins strongly convergent above eye level, abruptly rounded onto nearly flat occiput. Interocellar distance and ocellocular distance about twice diameter of anterior ocellus.

Mesosoma flattened and angled about as in female, thickness at metanotum about 0.47 x length from anterior margin of tegula to apex of metasternal lobe.

Integument smooth and shiny, without conspicuous punctures.

Pilosity about as in female and worker, but cephalic hairs relatively a little shorter.

Color yellowish; occipital area and margins of gastric terga brownish. Wing light brownish, stigma medium brown, veins yellowish brown.

Paratypes. HL 0.59 mm; HW 0.62 mm; CI 104; SL 0.45 mm; SI 73; EL 0.32 mm; OI 54; PW 0.83-0.85 mm; SL 1.29-1.39 mm; Wing length 4. 2 mm (2 measured).

Type Material
Holotype worker, allotype male, 394 worker, 19 female, 4 male paratypes: CAMEROON: near Lac Tissongo, Douala-Edea Reserve (lat. 3°29'N, long. 9°50'E), about 5 km S of Sanaga River, about 15 km E of Mouanko, 15 July 1976 (D. McKey). Holotype, allotype and most paratypes in ; paratypes also in, , and collections of D. McKey and G.C. & J. Wheeler.

Etymology
Phylax (Gr., guardian), appropriate to this ant which protects leaves of the host plant, Leonardoxa africana.

References based on Global Ant Biodiversity Informatics

 * IZIKO South Africa Museum Collection