Azteca trigona group
- Azteca barbifex
- Azteca chartifex and its infraspecific forms
- Azteca severini
- Azteca trigona and infraspecific forms
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 Azteca aurita group, but the majority are made by the Azteca trigona group. The workers of the A. trigona group exhibit a strongly hump-shaped promesonotum (e.g., Azteca chartifex) 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.
Queens are known for only two of the species in this group: 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, 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.