Pheidole roosevelti group

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Pheidole roosevelti-group, As defined by Sarnat (2008).

Species

Worker Diagnosis

The group is diagnosable from all other Pheidole by the unusual posterior mesonotal process and corresponding concave mesonotal declivity exhibited by the minor worker, in combination with the lack of pronotal spines or processes. Within the group, there exist distinctive modifications of the propodeal spines and acute margination of the posterior region of the head.

Description

Worker

MAJOR Head, in full face view, subquadrate in shape, broadest at about 2/3 length, posterior margin triangularly emarginate; in profile vertex weakly concave. Eyes in full face view convex, situated in anterior 1/3 of head, breaking outline of head; in profile distance between eye and mandibular insertion approximately twice length of eye. Mandibles triangular, masticatory margin bidentate both apically and basally on younger individuals (older individuals often edentate). Hypostoma with one median tooth, two inner teeth and two outer teeth. Frontal lobes well developed, overhanging antennal insertions. Frontal carinae distinct and elevated, overhanging antennal scrobes, terminating at about 2/3 length of head before reaching posterolateral lobes. Antennal scrobes weakly impressed, bounded mesally by frontal carinae and laterally by longitudinal carinae. Clypeus distinctly concave between frontal lobes, anterior margin concave medially. Antennae 12-merous. Antennal scapes reaching approximately half the distance from antennal insertions to corners of posterolateral lobes, slender and arcuate basally, thickened at distal third. Funicular segments 2–8 about twice as long as broad. Antennal club 3-segmented and slender, distinctly shorter than remainder of funiculus. Promesonotum forming a high dome. Mesonotum with a posterior process that often overhangs anterior portion of propodeal dorsum. Mesonotal declivity concave. Pronotal humeri indistinct or obtusely angulate. Metanotal groove impressed. Propodeal spines extending approximately to level of mesonotal process, often bifurcate or bent apically so that they project posteriorly at an oblique angle; in dorsal view moderately divergent. Petiolar peduncle elongate and thick. Petiole broadly cuneate, dorsum of node excised. Postpetiole, in dorsal view, with lateral projections, much broader than long.

Sculpture varying strongly among species. Mandibles weakly striate with scattered piligerous pits. Scapes striate. Metapleuron rugose. Erect long, fine and yellow hairs abundant on all body surfaces. Body dark reddish-brown with lighter appendages; color of head sometimes redder than rest of body.

MINOR Head ovate, subcordate or subquadrate with sides weakly to strongly convex; posterior margin, in full face view, roundly convex, truncate or weakly bilobed; in profile, posterior margin weakly to strongly dorsoventrally pinched, dorsum and venter meeting at an obtuse to acute angle. Eyes in full face view convex, in front of midline, break outline of head. Mandibles triangular, masticatory margin with 7–8 teeth. Frontal carinae short and weak, either terminating near eye level or integrating with other face sculpture. Clypeus with anterior margin convex laterally and flat to weakly concave medially. Antenna 12-merous. Antennal scapes distinctly surpassing posterolateral corners, slender and weakly arcuate basally, thickened at distal third. Funicular segments 2–8 approximately twice as long as broad, 3-segmented club slender, as long as remainder of funiculus. Mesonotum with a posterior process that often overhangs anterior portion of propodeal dorsum. Mesonotal declivity concave. Pronotal humeri inconspicuous. Metanotal groove occasionally impressed. Propodeal spines, in profile, obtaining height approximately level with mesonotal process, often bifurcate or bent apically so that they project posteriorly at an oblique angle; in dorsal view moderately divergent. Petiolar peduncle elongate and thin. Petiole cuneate, dorsum of node flat. Postpetiole, in profile, subtriangular with rounded dorsum.

Sculpture varying strongly among species. Mandibles weakly striate with scattered piligerous pits. Scapes smooth and shining. Erect long, fine and yellow hairs abundant on all body surfaces. Body dark reddish-brown to light reddish brown with lighter appendages and occasionally lighter waist and gaster.

Queen

Head with sides subparallel or diverging posteriorly, subquadrate to subtriangular in shape, posterior margin moderately to strongly concave. Eyes, in full face view, large convex, situated in front of mid-line, breaking outline of head. Mandibles triangular, masticatory margin bidentate both apically and basally. Frontal lobes weakly overhanging antennal insertions. Frontal carinae distinct, diverging, overhanging antennal scrobes, terminating at about 2/3 length of head before reaching posterolateral lobes. Antennal scrobes weakly impressed, bounded mesally by frontal carinae and laterally by longitudinal carinae. Clypeus distinctly concave between frontal lobes, anterior margin convex laterally and concave medially. Antennal scapes reaching approximately half the distance from antennal insertions to corners of posterolateral lobes, slender and arcuate basally, thickened at distal third. Antenna 12-merous. Funicular segments 2–8 about twice as long as broad. Antennal club 3-segmented and slender, shorter than remainder of funiculus. Mesoscutum either small without obscuring pronotum in dorsal view, or large and obscuring pronotum in dorsal view. Scutellum produced as a weakly elevated circular to subtriangular plate. Metanotum transversely striate. Propodeal spines often bent apically so that they project posteriorly at an oblique angle; in dorsal view moderately divergent. Petiolar peduncle elongate and thick. Petiole broadly cuneate, dorsum of node weakly concave to strongly excised. Postpetiole, in dorsal view, with lateral projections, much broader than long.

Sculpture varying strongly among species. Mandibles weakly striate with scattered piligerous pits. Scapes striate. Metapleuron rugose. First gastral tergite and sternite sculptured basally. Erect long, fine and yellow hairs abundant on all body surfaces. Body reddish-brown with lighter appendages.

Notes

The Pheidole roosevelti-group (Formicidae: Myrmicinae), as recognized here, consists of seven ant species endemic to the montane forests of the Fiji Islands. Although Fiji supports an unusually rich number of endemic ant radiations (Mann, 1921; Sarnat, 2006; Ward & Wetterer, 2006; Wilson, 1959a; Wilson, 1961), few among them compare to the spectacular morphological modifications achieved by the Pheidole roosevelti-group. Despite a remarkable morphology, unusual ecology and enigmatic evolutionary history, this distinct group of Pheidole species has received little attention since its initial discovery nearly a century ago.

In addition to their unique appearance, species of the P. roosevelti-group are characterized by a distinctive natural history. All species prefer the cooler, undisturbed wet forests of higher elevation mountains – a habitat where Fijian ant diversity becomes substantially diminished. Although ranges are often small, with some species known only from the moss forests of single mountain summits, the ants of the P. roosevelti-group are surprisingly abundant where populations persist. Where they do occur, minor workers patrol the ground and lower vegetation with long striding legs and sweeping antennae. Although most of the foraging is observed to be solitary, the species will recruit strongly to desirable food resources. While the minor caste is often ubiquitous in the vicinity of the nest site, the major caste is rarely seen above ground and is usually captured only by excavation of the nest or by patient baiting. Even if workers cannot be found foraging, the conspicuous vertical turret entrances of their nests, which stand 2-5 cm above the bare soil, serve as evidence of their presence.

Until the last five years, the only records of the group were from Fiji’s largest and oldest island, Viti Levu, and the nearby island of Ovalau (Ward & Wetterer, 2006). Records of the group are now known from the highest peaks of all of the archipelago’s main islands (Viti Levu, Vanua Levu, Taveuni, Kadavu) in addition to those of the smaller islands of Koro and Ovalau. Unlike the diversity of Lordomyrma Emery in Fiji, which is concentrated on Viti Levu (Sarnat, 2006), the diversity of the P. roosevelti-group is more evenly dispersed across the archipelago, with each of the larger islands supporting a distinct fauna. A salient question for this unique group is from which region, and from what lineage, it is derived. While the modified spines bear superficial resemblance to species previously assigned to the erstwhile Old World subgenus Pheidole (Pheidolacanthinus) F. Smith, the species of the P. roosevelti-group all bear consistent morphological distinctions that separate them from any other Pheidole species. A detailed phylogenetic study of the P. roosevelti-group holds the potential to illuminate biogeographic patterns and processes both within the Fijian archipelago and between Fiji and other regions of the Pacific.

The Pheidole roosevelti-group is known only from six islands belonging to the Fijian archipelago. While future expeditions to other higher elevation islands within the archipelago may produce new records of the group, preliminary examination of museum material from the Pacific region suggests the group is endemic to Fiji.

With two of the P. roosevelti-group species known only from their type localities (Pheidole bula, Pheidole pegasus), and a third (Pheidole uncagena) known only from two localities, there is a distinct possibility that additional species remain undiscovered. Tempering this possibility is the narrow habitat range preferred by the group. For example, all of the three species known from two or fewer localities are restricted to the highest mountain peaks of their respective islands. Pheidole bula is known only from the summit Mt. Tomanivi (the highest mountain of Viti Levu), P. pegasus is known only from the summit of Mt. Delaikoro (the second highest mountain of Vanua Levu), and the only reliable records of P. uncagena are from Mt. Delaikoro and the summit of Devo Peak (the highest mountain of Taveuni). Fortunately, where they do occur, species of the P. roosevelti-group are relatively conspicuous, owing to the high numbers of their ground foragers, their strong recruitment to baits, their distinctive turret nest entrances built into bare soil, and the general paucity of other sympatric ant species. Therefore, a determined collector can be relatively certain whether or not a given locality harbors one or more species of the P. roosevelti-group.

The most likely candidate island within Fiji to support an unknown P. roosevelti-group population is Gau, with its intact high elevation interior forests. Gau remains the only island with significant elevation and suitable habitat that the author has not surveyed.

References