Formicinae, Formicidae, Hymenoptera, Insecta, Arthropoda, Animalia
Additional images: minor worker scape (original, reduced); orange-yellow color variant worker (original, reduced).
Throughout mainland Neotropics. Costa Rica: throughout country below cloud forest elevations.
Minor worker: propodeum very narrow and elongate, in cross section tectiform (tent-like, like an inverted "V"), without a distinct angle separating lateral and dorsal faces; clypeus with median longitudinal keel; head relatively short and broad; scapes relatively short, almost always with abundant long erect setae; mesosoma usually densely hairy.
No other large Camponotus in Costa Rica has abundant erect setae on the scapes and the setae are distinctly longer than the width of the scape (some have suberect long pubescence that can appear like setae, but the hairs are shorter than the width of the scape).
Among atriceps material there is abundant variation in color, shininess, sharpness of lateral angles of median clypeal lobe, and pilosity. For example, some are all very dark red brown, some are bicolored red and dark red brown, and rarely they are entirely light orange.
An odd collection from Pacific coast mangroves
I was collecting in a mangrove swamp near Esterillos Este, a small town on the Pacific coast of Costa Rica. Far out into the swamp I found a Camponotus nest in a horizontal rotten log. The worker morphology is intermediate between atriceps and JTL-004. The shape of the face (both minors and majors), the length and width of the scapes, and the shape of the mesosoma is like atriceps, while the pilosity is more like JTL-004. Could this be a result of hybridization between the two species (both of which should occur in the area)? Is it a distinct species, perhaps a mangrove specialist? Is it just a genetic variant within atriceps that causes a reduction in pilosity?
Images: minor worker face view (reduced, original), minor worker lateral view (reduced, original), major worker face view (reduced, original), major worker lateral view (reduced, original).
Comparisons showing in sequence left to right or top to bottom atriceps, the Esterillos Este collection, and JTL-004: minor worker face (image), minor worker lateral view of mesosoma (image), major worker face (image).
This "species" occurs throughout the American tropics and subtropics. In Costa Rica, atriceps is found almost anywhere in the country. It is common in second growth vegetation and other disturbed areas with high insolation, and it is relatively uncommon in mature forest understory. It is extremely varied and opportunistic in its nesting habits. I most often find nests in dead wood lodged in low vegetation or on the ground. They can be quite undiscerning: I once found a vigorous nest in an old shoe along a trashy roadside. I have found them nesting in dead wood deep inside mangrove swamps. They may nest in live branches of Cecropia trees on the rare occasions when Azteca does not occupy the branches, and they occasionally occur in internodes of Cecropia saplings. They occur in canopy fogging samples from mature rainforest, and in Corcovado I collected them nesting beneath epiphytes high in the canopy. The species is often a house pest, especially in the lowlands, and can be found nesting in wall spaces, infrequently used drawers, and old cardboard boxes.
Workers are typically nocturnal foragers. Similar to most Camponotus, workers are quite timid as isolated foragers. However, when nests are broken into, workers may come boiling out and bite the attacker. Very few Neotropical carpenter ants will aggressively defend a nest site. Other species that defend nests include C. sericeiventris, and in South America a close relative of atriceps, C. femoratus.
On multiple occasions I have observed parabiosis between C. atriceps and Azteca. Near the town of Tortuguero, on the Atlantic coast, I found an "archipelago" of antgardens on understory vegetation, overhanging a trail in the forest. The nests I could reach were relatively small and with only a few epiphyte seedlings, and appeared to be satellite or incipient nests at the edge of the large antgarden colony. The Azteca species was one I have not fully identified yet, but it is near ulei and may be gnava. The Azteca were out patrolling the surface of the nests and were very aggressive, but I discovered that the nests also contained C. atriceps. When the small nests were very severely disturbed (broken open with machete or by hand), C. atriceps workers would suddenly emerge. The smaller workers would flee, but major workers attacked me and bit my hands. One of the small nests was adjacent to a cluster of rotten sticks. The sticks were hollow and most contained adult males and alate queens of Azteca. However, one contained an aggregation of C. atriceps workers.
Another case of parabiosis occurred at Santa Rosa National Park. I was night collecting along the nature trail near the Casona. A fissure near the base of a live tree was an active nest entrance where abundant workers of Azteca (instabilis complex) were very active. In amongst them were occasional workers of C. atriceps, moving in and out of the same fissure and traveling among the Azteca workers with no apparent aggression.
A third case occurred near Puerto Viejo de Limon, in beach strand vegetation beneath coconut palms. A small shrub had an external carton nest of Azteca that was very active and vigorously defended. Just beneath and connected to it was a soft rotten stick that contained a C. atriceps nest. The Azteca workers were running freely in and out of the same stick, among the Camponotus workers, with no aggression.
I observed a fourth case at La Selva Biological Station. I discovered a cluster of carton nests of an Azteca velox-like species. The nests were scattered on the leaves and stems of a cluster of small melastome shrubs. One basal, central nest was built around a dead branch, about 50cm long. Camponotus atriceps were parabiotic in the central nest and some of the satellites. The Camponotus maintained galleries apart from the Azteca, but the Camponotus and Azteca chambers were interconnected and the two species appeared to move freely among each other. The carton construction was clearly associated with the Azteca chambers in the central branch, and not with the Camponotus chambers. The Azteca appeared to be the carton makers.
A fifth observation was also at La Selva. During night collecting I observed a fissure in the base of a tree, with carton construction covering part of the fissure. Camponotus and Azteca workers were both abundant and occurred together under the carton. The Azteca were in the velox group.
An open question is whether parabiosis in C. atriceps is a facultative behavior exhibited by a single panmictic population, with non-parabiotic cavity nesting being the more common alternative nesting behavior, or are the different nesting behaviors fixed species-level traits exhibited by distinct but cryptic species? Also, is this propensity for parabiosis in C. atriceps related to the apparently obligatory parabiosis in its relative, C. femoratus? Camponotus femoratus is famous for being a highly aggressive antgarden inhabitant in South America always found living parabiotically with Crematogaster carinata (= parabiotica). Are the parabiotic atriceps somehow an intermediate stage in the development of parabiosis, from which C. femoratus was derived?
Nest foundation appears to be by single claustral queens, because I find isolated dealate queens in cavities in dead wood and under epiphyte mats.
Nests are frequently home to inquiline cockroaches and silverfish.
The species is susceptible to fungal infection; Koos Boomsma collected a dead specimen with a sprouting Cordiceps fungus. The collection was from Panama.
Camponotus atriceps was formerly known as C. abdominalis until Bolton's 1995 catalogue, in which the name was corrected.
There are dozens of infraspecific names associated with the taxon, but in a morphometric analysis Hashmi (1973) failed to find discrete taxa and synonymized them all into one polytypic species. I am not sure that this is simply intrapopulational variation. I suspect the various forms will show correlations with microhabitat and/or behavioral variables, and may reflect some degree of genetic structuring (e.g., multiple sympatric species) in material that is classified as atriceps.
Hashmi, A. A. 1973. A revision of the Neotropical ant subgenus Myrmothrix of genus Camponotus (Hymenoptera: Formicidae). Stud. Entomol. 16:1-140.
Smith, F. 1858. Catalogue of hymenopterous insects in the collection of the British Museum. Part VI. Formicidae. London: British Museum, 216 pp.
John T. Longino, The Evergreen State College, Olympia WA 98505 USA.firstname.lastname@example.org
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