Formicidae, Hymenoptera, Insecta, Arthropoda, Animalia
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Identification
Wet to moist forested sites in Costa Rica are usually inhabited by a population of a large Aphaenogaster. I have never found a site with two or more discrete forms; there do not appear to be multiple sympatric species. However, there is geographic variation in a number of relatively conspicuous characters. A mosaic of parapatric forms can be recognized as follows:
La Selva Biological Station and Braulio Carrillo area: worker neck long and tapering behind eyes, both in dorsal view and lateral view; worker gastric dorsum shagreened, opaque; male eyes strongly convex, bulging.
Penas Blancas Valley and adjacent Rio San Lorenzo Valley: worker neck long and tapering behind eyes, both in dorsal view and lateral view; worker gastric dorsum polished and shining; male eyes strongly convex, bulging.
Upper western slopes of Cordillera de Tilaran and Cordillera de Guanacaste: worker neck more convex, less tapering behind eyes; worker gastric dorsum polished and shining; male eyes less strongly convex, not strongly bulging.
San Vito area, Osa Peninsula, Quepos area: worker neck more convex, less tapering behind eyes; worker gastric dorsum shagreened, opaque; male eyes less strongly convex, not strongly bulging. The San Vito specimens are larger and red brown. The lowland specimens are smaller and orange brown.
Figure showing contrasting worker head shape (reduced, original).
Figure contrasting sculpture on worker gaster (original).
Figure Contrasting male faces (original).
Range
Costa Rica, Panama. Widespread in wet to moist forest areas of Costa Rica.
Natural History
Nests are subterranean with an inconspicuous entrance at the soil surface. Workers are typically observed as solitary foragers on soil and litter surfaces.
Twice I have observed the following phenomenon, once in Corcovado National Park, and once at La Selva Biological Station. During night collecting, I shined a light on an araneoides nest entrance. Workers immediately rushed out of the entrance, each carrying a larva. They ran a few cm from the nest entrance and stopped, then slowly returned to the nest. The exodus from the nest was so rapid that the workers must have been near the nest entrance, already with larvae. It was as though they were waiting at the entrance, ready for a panic evacuation, and set on a hair trigger, such that the smallest stimulus would send them out.
Taxonomy
The oldest names in this complex are Emery's araneoides and phalangium (Emery 1890). At this point I propose to use the name araneoides for the entire complex. However, future study may recommend splitting it into multiple species. For this reason, I outline what I know of the existing names in the complex.
The araneoides worker was described as having an opaque gaster, and the illustration shows a tapered neck (Emery 1890). The phalangium worker was described as having a shiny gaster, and the illustration shows a more convex neck. A syntype male of phalangium is also described and illustrated. Emery's paper states the type localities for both araneoides and phalangium as "Alajuela, Jimenez". I examined the syntypes at MCSN in 1990. Under araneoides were (1) a pin with one worker, labeled "Alajuela", and a "Typus" label; and (2) a pin with two workers and an "Alajuela" label. Under phalangium were (1) a pin with one worker, labeled "Costa Rica, Alfaro", and with a "Typus" label; (2) a pin with two workers and a "Costa Rica, Alfaro" label, (3) a pin with one worker and a "Alajuela, Alf." label; (4) a pin with a male labeled "Jimenez"; and (5) two pins with material collected in 1895, one from "Monte Redondo" and one from "Miravalles". The worker material under araneoides matched the form with tapered neck and opaque gaster. The worker material under phalangium matched the form with convex neck and shiny gaster. I do not remember whether the male had bulging eyes or not. Alajuela is in the Central Valley of Costa Rica; Jimenez was an Alfaro collecting site near modern day Guapiles, in the Atlantic lowlands. I suggest that there was a labeling error, and the two araneoides workers were from Jimenez. If the male that Emery described is correctly labeled, then it probably belongs with the araneoides workers, not the phalangium workers. If we accept as lectotypes the workers that Emery labeled "Typus", the Atlantic lowland form is araneoides, and the northern Pacific slope form is phalangium.
araneoides and phalangium are both in the same publication, so it is the revisor's perogative to choose which has priority. I recommend araneoides because of the proximity of the type locality of araneoides to La Selva Biological Station. If future work results in the splitting of araneoides into multiple species, the name araneoides will remain stable for studies originating at La Selva.
Forel (1899) named two varietal forms. phalangium var. brevicollis was differentiated from phalangium by having the head shorter behind the eyes, and less narrowed, and more abruptly narrowing to a very short neck before the flanged collar. It was also described as being larger and more robust than phalangium. There was no mention of the sculpture on the gaster. He also briefly described a male. The types were from Panama, Volcan de Chiriqui (Champion). araneoides var. inermis was differentiated from araneoides by the absence of teeth on the propodeum, by the head less narrowed behind and thus more like phalangium, by smaller size, and by some color differences. There was no mention of gastric sculpture. A male was briefly described. Syntype workers were from "Costa Rica" (Tonduz), workers and male from Panama, Bugaba (Champion). Forel (1912) described araneoides var. nitidiventris. It was differented from araneoides by the gaster being shiny (subopaque only at base) and by the sides of the head behind the eyes being more convex. The types were from "Canas Gudas", Costa Rica (Pittier). Finally, there is an araneoides var. canalis Enzmann 1947, with type locality Panama, Chiriqui. Without seeing types, I am reluctant to further characterize these taxa, but presumably one or more of them will match the form from southwestern Costa Rica.
Systematics
Worker neck shape and male eye development vary concordantly; these vary discordantly with worker gaster sculpture. One process that would generate this pattern is current selection maintaining the parapatric forms. This could occur in the presence of gene flow. Transitions from convex to tapered neck (workers), shiny gaster to opaque gaster (workers), and bulging to non-bulging eyes (male) could be independent step clines. With this scenario, there would be no genotype clusters in sympatry, a requirement for Mallet's species definition (Mallet 1995). Alternatively, a historical process of dispersal and vicariance could have occurred, generating a set of discrete historical entities. A possible historical scenario begins with a form in northwestern Costa Rica, derived from northern Aphaenogaster, that has workers with convex necks and shiny gasters, and males with non-bulging eyes. A southern expansion into the Talamancas and onto the Osa Peninsula is accompanied by the development of an opaque worker gaster. The lineage then wraps around the southern Talamancas, invading the wet forests of the Atlantic slope, accompanied by the development of a tapered worker neck and bulging male eyes. The climatic warming of the current interglacial allows the lineage to move upslope on both sides of the continental divide, such that the current proximity of the Penas Blancas and Monteverde populations is a recent event. Some gene flow may be occurring between the previously separated populations, such that the Penas Blancas and Rio San Lorenzo populations may have resulted from hybridization or introgression, with the shiny gaster trait "leaking" eastward.
Literature Cited
Emery, C. 1890. Studii sulle formiche della fauna neotropica. I-V. Boll. Soc. Entomol. It. 22:38-80.
Enzmann, J. 1947. New forms of Aphaenogaster and Novomessor. Jour. New York Ent. Soc. 55:43-46.
Forel, A. 1899. Formicidae. Biol. Cent.-Am. Hym. 3:1-160.
Forel, A. 1912. Formicides nŽotropiques. Part IV. 3me sous-famille Myrmicinae Lep. (suite). Mem. Soc. Entomol. Belg. 20:1-32.
Mallet, J. 1995. A species definition for the modern synthesis. TREE 10:294-299.
Page author:
John T. Longino, The Evergreen State College, Olympia WA 98505 USA. longinoj@evergreen.edu
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