Azteca aurita

Longino, J. T., 2007, A taxonomic review of the genus Azteca (Hymenoptera: Formicidae) in Costa Rica and a global revision of the aurita group., Zootaxa 1491, pp. 1-63 : 54-55

Azteca aurita group

Diagnosis. Queen and worker: Palpal formula 4,3; middle and hind tibia lacking apical spur; anteromedial border of clypeus strongly convex and extending well beyond anterolateral clypeal lobes, HLB/HLA> 1.04. Queen: general body size small, similar in size to major workers; integument extremely smooth and shining, glass-like, with appressed pubescence extremely dilute; pilosity, when present, a stubble of short, stiff, fully erect setae; petiole bluntly subpyramidal to bilobed, never flat and scale-like.

Worker: Head always cordate, with variable tendency for posterolateral portions of occipital border to be drawn out into angular projections; scape, tibiae, lateral and posterior margins of head, and mesosomal dorsum devoid of setae; mandibles either of two forms, both unique to the species group: (1) dorsal surface strongly flattened, densely and finely striate, mat, or (2) dorsal surface convex and shiny, masticatory margin strongly sinuous, with large, projecting apical tooth; petiole as in queen.

Biology. Members of the A. aurita species group are widespread but rare. They construct carton nests on the branches of trees, nests which are always bare of epiphytes (they do not form ant gardens).

The diminutive and highly derived queens of the group suggest a social parasitism syndrome (Forel 1928, Hölldobler and Wilson 1990). Species in the A. aurita group have queens that are about the same size as workers, and the gaster is very small in proportion to the rest of the body. This contrasts with more typical Azteca species, which have queens much larger than workers and with large gasters, presumably full of resources for founding new colonies on their own. It is difficult to imagine the small aurita-group queens doing so, and a more likely scenario is for aurita-group queens to insinuate themselves into established colonies of other species, killing the host queen and having the host workers rear the parasites' offspring. It is not even clear how they function once established; aurita-group colonies are enormous, and it seems paradoxical that such small queens could generate sufficient eggs to populate them. The morphology of A. nanogyna carries this paradox to an extreme, and a possibility in this case is that A. nanogyna is a workerless social parasite.

The A. aurita group is perhaps the most circumscribable set of species in the genus, with a distinctive suite of characters. The reduced palpal segmentation and the lack of tibial spurs are both unique to the group and, being losses, are likely apomorphic traits that support the monophyly of the group. The group is most similar to A. trigona and A. chartifex . Shared worker traits include reduced pilosity, cordate head shape, and the construction of large, pendant, epiphyte-free carton nests. Although many worker series of A. trigona have shiny mandibles, a few have faint aciculate sculpture. Also, the mandibles are somewhat flattened, approaching the condition in the A. aurita group. Azteca trigona workers retain at least a few erect setae on the mesosoma, and often a few setae occur on the tibiae. The queens of the two groups could not be more different. Queens of A. trigona are much larger than workers, have broadly cordate heads much wider than long, a mat to sublucid integument, a strongly flattened and scale-like petiole, and no stubbly pilosity.

Knowledge of the taxonomy and natural history of this group is in its incipient stages, but what we do know suggests that additional study would be highly rewarding.

Key to A. aurita group queens

1. Color uniform orange; CI> 80 .................................................................................................................... 2

- Color uniform brown; CI <80 ..................................................................................................................... 4

2. Pilosity absent on posterior and posterolateral borders of head, dorsum of mesosoma, petiole, and gaster (tibiae, scape, and all of face and mandibles below level of lower margin of eyes with dense, short, white erect pilosity); HW <1.15mm .............................................................................................................. aurita

- Short erect pilosity present on all margins of head and on dorsum of mesosoma, petiole, and gaster; HW> 1.15mm ........................................................................................................................................................ 3

3. Dense, short, erect pilosity on scape and tibiae; head relatively narrower (CI <92) ....................... pilosula

- Scape and tibiae lacking erect pilosity; head relatively broader (CI> 92) ................................... lallemandi

4. Gastral dorsum lacking erect setae; HLA> 1.35mm ..................................................................... schimperi

- Gastral dorsum with erect setae; HLA <1.35mm ....................................................................................... 5

5. HLA about 1.3mm ........................................................................................................................ lanuginosa

- HLA about 0.86mm ........................................................................................................................ nanogyna

Key to A. aurita group workers (the worker of A. nanogyna is unknown)

1. Dorsal surface of mandible flat, opaque, densely striate; scape relatively long (SI> 80) .......................... 2

- Dorsal surface of mandible convex, shiny; masticatory margin concave, with enlarged apical tooth; scape relatively short (SI <75) .............................................................................................................................. 3

2. Head relatively broad (CI> 105); posterolateral margins of vertex rounded and cordate, not bluntly angulate................................................................................................................................................. lallemandi

- Head relatively narrow (CI <106); posterolateral margins of vertex bluntly angulate ......... aurita , pilosula

3. Pubescence dilute and tightly appressed; color usually brown with orange head .......................... schimperi

- Pubescence more abundant, giving somewhat wooly appearance; color all brown ..................... lanuginosa