Dolichoderinae, Formicidae, Hymenoptera, Insecta, Arthropoda, Animalia
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Additional images:
Worker, mandible (small, large), petiole, lateral view (small, large), tibia (small, large).
Range
Costa Rica to southern Brazil. Costa Rica: southern Pacific lowlands.
Identification
Worker
Measurements (n=5 workers from Costa Rica to Brazil): HLA 0.88 (0.84-0.96), HW 0.97 (0.87-1.05), SL 0.78 (0.64-0.85), CI 108 (100-110), SI 87 (74-89). Palpal formula 5,3; middle and hind tibia with prominent pectinate apical spur; dorsal surface of mandible smooth and shining, with moderately abundant small piligerous puncta; medial and lateral clypeal lobes at about same level; head with strongly convex sides, strongly cordate posterior margin; in lateral profile promesonotum forming single strongly protruding convexity, posterior mesonotum dropping abruptly to much lower metanotal groove and dorsal face of propodeum; scape and tibia lacking erect setae; sides and posterior margin of head lacking erect setae; posterior pronotum with conspicuous cluster of 6 or more long erect setae, mesonotum with 0-4 shorter erect setae, dorsal face of propodeum with 4-6 erect setae at variable angles to surface; color red brown.
Natural History
Azteca chartifex occurs in wet forest habitats. Colonies are polydomous, occurring in clusters of large, pendant carton nests. The carton is dry and paper-like. The nests are never penetrated by epiphytes or other plant roots, and in this regard are very different from the ant gardens of A. gnava and A. nigra. They can occur in very exposed and highly insolated environments, and seem more abundant in seasonal moist to dry habitats than in weakly seasonal wet forest.
I have observed two colonies in Costa Rica, both in the lowland forest of the Osa Peninsula. Both colonies were in regenerating second growth forest. One colony was on a large Inga (Fabaceae) tree and several adjacent Psidium (Myrtaceae) trees. There were about eight large nests within a 10m radius. Individual nests were up to 2m long and tapering. I cut into several nests and dissected one nest thoroughly, finding only workers and larger brood. This particular colony was relatively long-lived: I first observed it in 1990, and when I walked by the same site six years later the colony was still there and looked relatively unchanged. The second colony I observed was a single large nest on a palm trunk, about 10m high. It was in an area of dense vegetation and it is likely there were other nests in the vicinity. Forel's subspecies laticeps was collected from a carton nest on Psidium (the Champion series from Chiriqu’, Panam‡).
Comments
The production of large pendant carton nests by Azteca is a common phenomenon in moist to wet forests from Panama southward through tropical South America. Some of them are made by the A. aurita group, but the majority are made by the A. trigona group. The workers of the A. trigona group exhibit a strongly hump-shaped promesonotum which drops steeply and abruptly to the much lower dorsal face of the propodeum. Other characters exhibited by but not unique to the group are 5,3 palpal formula, prominent meso and metatibial spurs, broad heads (CI > 99), and few to no metatibial setae which, if present, are very short and inconspicuous. The queens have very broad, strongly cordate heads (CI 109-135). The only other Azteca queens with heads that proportionately broad are A. gnava, which have strongly setose tibia and a 6,4 palpal formula. The species group as a whole has a sharp geographic boundary: it is common in central Panama, but Costa Rica is the far northern limit of the group, with a single rare species in the southern Pacific lowlands.
The species-group taxa associated with this group are A. trigona and its synonyms and infraspecific forms festai, gaigei, mathildae, mediops, subdentata; A. chartifex and its infraspecific forms cearensis, decipiens, lanians, laticeps, multinida, spiriti, stalactitica; A. barbifex; and A. severeni. Queens are known for only two of these: A. trigona and A. barbifex. I have examined the types of most of the taxa and made measurements of HLA, HW, and SL. When the data for workers are all plotted together, they form one continuous cloud of points along one line of allometry. However, when I examine just series from Panama and Costa Rica, two groups emerge. One group has the largest workers (selecting one of the larger workers of each series) with HW 1.21-1.36mm, CI > 111, and the posterior margin of the head with a very deep, V-shaped medial impression. Another group has the largest workers with HW 0.93-1.06mm, CI < 111, and the posterior margin of the head with a shallower, less strongly V-shaped medial impression. The former I identify as A. trigona; the latter as A. chartifex. The various subspecies of A. trigona and A. chartifex fall within these respective size ranges, with the exceptions of A. trigona gaigei, with HW 1.06mm, and A. chartifex lanians, with HW 1.22mm. Azteca severeni, with HW 1.11mm, is intermediate.
Queens are remarkably rare in this group. I have been able to examine and measure eight queens, including the holotype queen of A. trigona and the syntype queen of A. barbifex. Six of the queens, which I identify as A. trigona, form a cluster with HW 1.64-1.82mm. A queen from Bolivia has a very broad head (click here for image), with HW 2.07mm, and the queen of A. barbifex is distinctly smaller, with HW 1.20mm. Azteca barbifex workers are similar in size and shape to A. chartifex workers. Thus it is possible that there are two main lineages, A. trigona having large queens and workers, and A. chartifex having small queens and workers. Azteca barbifex workers fall well within the cloud of points formed by A. chartifex and its subspecies.
The very broad heads and short, small mandibles suggest very powerful cutting ability, like bolt cutters. Perhaps Azteca trigona group queens found their nests in hard dead wood, and this head structure is an adaptation for quickly excavating a chamber in hard wood.
Page author:
John T. Longino, The Evergreen State College, Olympia WA 98505 USA. longinoj@evergreen.edu
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