Aphaenogaster phalangium

Longino, J. T. & Cover, S. P., 2004, A revision of the Aphaenogaster phalangium complex (Hymenoptera: Formicidae: Myrmicinae)., Zootaxa 655, pp. 1-12 : 1-6

[[ Aphaenogaster phalangium complex ]]

The Aphaenogaster phalangium complex is a lineage of rainforest ants endemic to Central America. The complex is revised and interpreted as two parapatric species, A. phalangium and A. araneoides , with extensive geographic variation. Character variation is discordant within species, but in some cases appears to vary in parallel between the two species, such that both species vary the same way in montane versus lowland sites. Aphaenogaster inermis Forel 1899, A. nitidiventris Forel 1912, and A. canalis Enzmann 1947 are synonymized under A. phalangium Emery 1890; and A. brevicollis Forel 1899 is synonymized under A. araneoides Emery 1890. The complex lacks winged queens and instead has ergatoid queens with enlarged postpetiole and gaster.

Key words: Aphaenogaster phalangium , Aphaenogaster araneoides , Formicidae, geographic variation

Introduction

Forested habitats of Central America are home to ants of the Aphaenogaster phalangium complex. Within the context of the New World Aphaenogaster fauna, the workers of the phalangium complex are uniquely identified by a combination of head and propodeum characters. The portion of the head posterior to the eyes is drawn out and tapers to a strongly constricted neck, beyond which the head flares out into a pronounced collar. The propodeum lacks spines, although it may be weakly tuberculate. The workers are large, with long spindly legs.

They are a common element of many lowland rainforest sites, where solitary foragers are common on the forest floor. They are timid ants that run from any threat, and they are decidedly comical with their large heads on implausibly narrow necks. The nests are in small chambers in the ground, and colonies maintain multiple empty nests, moving among them (McGlynn et al. 2002, 2003).

At a global level the definition of the genus Aphaenogaster is problematic, and early definitions of subgenera (Emery 1915) have been relegated to synonymy (Bolton 1982). The members of the phalangium complex have been moved from taxon to taxon, spending time in Stenamma , Aphaenogaster subgenus Ischnomyrmex (now a synonym of Pheidole ) and Aphaenogaster subgenus Deromyrma (now a synonym of Aphaenogaster ). The relationship of the phalangium complex to other Aphaenogaster has not been explored, but as mentioned above, the complex forms a strongly differentiated and unmistakable lineage in Central America.

Emery (1890) named the first two species, A. phalangium and A. araneoides , both from Costa Rica. Forel subsequently described A phalangium brevicollis from Panama, A. araneoides inermis from Costa Rica and Panama, and A. araneoides nitidiventris from Costa Rica. Enzmann described A. araneoides canalis from Panama. These descriptions date from the era of typological ant naming and there has never been a synthetic taxonomic work that differentiates all the forms or addresses geographic variation. Additional collections now available reveal complex character variation that makes early taxon definitions untenable.

Here we analyze character variation and propose new hypotheses of species boundaries that propose parallel clinal variation in two species.

Character variation

The material on which we base our observations is comprised of multiple collections from a few well-sampled locations and single collections from numerous other locations. The localities for which we have multiple collections are La Selva Biological Station in the Atlantic lowlands of Costa Rica, the Penas Blancas Valley on the Atlantic slope of the Cordillera de Tilaran, Monteverde (only 10km distant from the Penas Blancas site but on the Pacific slope), Corcovado National Park in the southern Pacific lowlands of Costa Rica, and Barro Colorado Island in central Panama. These collections reveal that intrapopulational variation in obvious morphological features is always very low, but that dramatic character shifts can occur over short distances.

The most discrete character set, and the one on which we base our definitions of species, involves pilosity on the femora of workers (Fig. 1). Aphaenogaster phalangium is a "pilose" form, and A. araneoides is a "non-pilose" form. Aphaenogaster phalangium has femora with short stiff erect setae that cover all surfaces. In contrast, A araneoides has the dorsal and lateral faces of the femora with only very fine, fully appressed setae, and erect setae are confined to the ventral surface. In most cases there is also a pilosity difference on the mesepisternum. Aphaenogaster phalangium usually has ten or more erect setae scattered along the full length of the mesepisternum. In contrast, A. araneoides is usually devoid of erect setae on the mesepisternum, or with at most one or two at the ventral border. Rarely (see A. brevicollis ) there is a row of about five setae along the posterior border.

The two species exhibit a largely parapatric distribution with a narrow zone of sympatry (Fig. 2). Aphaenogaster phalangium occurs in a narrow strip along the mid-elevation Pacific slope of Costa Rica's cordilleras, fanning out into the wet Pacific lowlands of southwestern Costa Rica, and extending into Panama as far as Barro Colorado Island and the Darien. Aphaenogaster areneoides is the Atlantic slope counterpart, also occurring in the mid-elevation cordilleras of Costa Rica, but fanning out into the Atlantic lowlands, with isolated records from El Salvador and Honduras.

In the zone of sympatry collections are sparse, but one locality shows evidence of discrete sympatric species. In the Monteverde area of the Cordillera de Tilaran, one colony of A. phalangium and eight separate worker samples of A. araneoides have been collected. Elsewhere, collections of both species have been taken from closely approximated localities(Fig. 2). For example, in the Cordillera de Guanacaste, an A. phalangium worker was collected from Cerro Cacao, at 1500m, while an A. araneoides worker was collected from Maritza Biological Station, at 600m elevation and only 6 km from Cerro Cacao.

A character that we interpret as being intraspecifically variable is the sculpture on the fourth abdominal tergite. This tergite may be completely smooth and shining or variably matte and dull. For A. phalangium , specimens from the montane sites in the cordilleras of Guanacaste, Tilaran, and Talamanca have the tergite smooth and shining or matte anteriorly grading to smooth posteriorly. Specimens from the lowlands, including the southern Pacific lowlands of Costa Rica and the population on Barro Colorado Island, have the tergite completely matte. For A. araneoides , the pattern is more or less the same. Specimens from the Costa Rican cordilleras and the one collection from Honduras tend to have the tergite shiny; specimens from the Atlantic lowlands and the collection from El Salvador have the tergite matte.

The shape of the neck, although difficult to quantify, shows subtle variation (Fig. 3). In lateral view, the neck may be (1) very strongly constricted and with a strongly developed dorsal flange, or (2) relatively weakly constricted with a reduced dorsal flange. In the first instance the profile of the head looks like a bottle or flask with rounded shoulders and a separate neck; in the second it looks like a flask that tapers to the opening. Aphaenogaster phalangium always has the strongly constricted neck. Specimens of A. araneoides from the Atlantic lowlands of Costa Rica up to the Penas Blancas Valley show the weakly constricted neck, while those from Monteverde and elsewhere in the central cordilleras have the strongly constricted neck of A. phalangium . Strong character variation occurs over small spatial scales. In the Penas Blancas Valley workers have the same neck shape as workers from La Selva and the slopes of the Cordillera Volcanica Central but the shiny abdominal tergites similar to material of A. araneoides from Monteverde. In Monteverde, only 10km from Penas Blancas, not only are the abdominal tergites shiny, the neck shape is very different too, matching both A. araneoides and A. phalangium material from further north.

The propodeum may have short posterodorsal tubercles (where the propodeal spines would be in other Aphaenogaster ), or these may be completely lacking. Propodeal tubercles occur on specimens from the Atlantic slope of the Cordillera Volcanica Central south to the Rio Sixaola on the border between Costa Rica and Panama. Specimens from north and south (the Penas Blancas Valley and Changuinola, Panama, respectively) lack the tubercles.

The low number of males in collections reduces knowledge of male character variation, but some observations are warranted. We have examined males of A. phalangium from Corcovado, Quepos (a site on the Pacific coast just north of Corcovado), and Barro Colorado Island, and males of A. araneoides from La Selva, two other Atlantic lowland sites south of La Selva, and Monteverde. The leg pilosity character does not differentiate the species, because femora of both species have appressed setae on the dorsal and lateral faces (although the femoral pilosity of A. phalangium males is not as strongly appressed). The males often retain the pilosity differences on the mesepisternum. The A. phalangium males have sparse setae scattered over the katepisternum and often on the anepisternum as well. These setae vary from being thickened and easily seen to very fine, delicate, and difficult to see. In contrast, the mesepisternum of A. araneoides males is usually entirely or almost entirely devoid of setae, with at most one or two at the ventral border. However, the males of Forel's A. brevicollis (see below) have setae on the katepisternum, yet the worker leg pilosity places them in A. araneoides . The fourth abdominal tergite of all males we have examined has been entirely smooth and shiny, or with at most a small amount of shagreening near the postpetiolar insertion. The males vary intraspecifically in the size of the compound eyes (Fig. 4), and like some worker characters eye size does not seem to correlate with species boundaries. Male eyes are small in A. araneoides from Monteverde, A. phalangium from Corcovado, and A. araneoides from Changuinola. Male eyes are large and bulging in A. phalangium from Barro Colorado Island and A. araneoides from Bataan and La Selva, both Costa Rican Atlantic lowland sites. The shift from small to large eyes across the Rio Sixaola appears to correlate with the shift from the non-tuberculate to tuberculate condition in workers.

Queens

McGlynn et al. (2002) found that colonies of A. araneoides at La Selva Biological Station never contained alate queens, but instead contained one or two ergatoid queens. In our examination of museum material we have never seen an alate queen, and four nest series (two of A. araneoides collected by E. O. Wilson at La Selva, one of A. phalangium collected by G. C. Wheeler on Barro Colorado Island, and one of A. phalangium collected by Longino from Monteverde) contain single ergatoid queens. In each of these collections one adult female has an allometrically enlarged postpetiole and fourth gastral tergite (Fig. 5). Also the metanotal groove is more deeply impressed relative to workers. There are no traces of wing scars or the sclerites associated with wings. The La Selva individuals have an ocellar triangle with distinct ocelli. The Barro Colorado individual lacks ocelli but there is a small depression where the median ocellus would be. The Monteverde specimen has no trace of ocellus or depression.

Key to species (workers)

1 Dorsal and lateral faces of femora with abundant erect setae; mesepisternum with ten or more erect setae scattered over surface................................................ A. phalangium

Dorsal and usually lateral faces of femora with fully appressed setae; mesepisternum usually completely lacking erect setae, or with one or two setae near ventral border, rarely with up to five erect setae along posterior border........................... A. araneoides