Azteca

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 : 4-5

[[ Genus Azteca ]]

The dolichoderine genus Azteca is a strictly neotropical group of arboreal ants (Emery 1893, Forel 1928). They are abundant in lowland habitats from Mexico to Argentina, occurring as both generalized foragers and as specialized inhabitants of myrmecophytic plants. Azteca species exhibit a variety of nesting habits, including the construction of carton nests, the occupation of live and dead plant stems (Forel 1899, Ule 1901, Emery 1913, Davidson 1988, Ayala et al. 1996), and the formation of ant gardens. Ant gardens are arboreal ant nests which sprout epiphytes from carton nest material (Ule 1901, Wheeler 1921, Longino 1986, Davidson 1988, Corbara et al. 1999, Kaufmann & Maschwitz 2006). Striking cases of symbiosis occur between Azteca and highly specialized myrmecophytic plants, the most notable case being the relationship between Azteca and Cecropia ( Müller 1876, 1880-1881, Bequaert 1922, Wheeler 1942, Benson 1985, Longino 1991a, b). Also, Azteca ants have developed complex trophic relationships with many species of coccoid Hemiptera (Wheeler 1942, Johnson et al. 2001, Davidson et al. 2003). Azteca workers are often found tending mealy bugs (Pseudococcidae) and soft scales (Coccidae). For Azteca species that nest in live stems, the interior walls of the nest are often encrusted with mealy bugs and scales. Species building carton nests and ant gardens maintain dense populations of mealybugs and scales under the carton of the main nest or under small carton "pavilions " scattered over the vegetation. Very little attention has been paid to the taxonomic diversity of Coccoidea associated with Azteca , and usually only cursory observations of their presence are made during field collections. Because of the richness of the ecological interactions among Azteca , plants, and hemipteran symbionts, Azteca species have been and will continue to be subjects in the study of adaptation and coevolution, and therefore taxonomic work on the genus is particularly important.

The taxonomic bounds of the genus have not changed since its inception (Forel 1878, Shattuck 1992). Members of the genus can be recognized by the combination of (1) a thin, somewhat flexible cuticle, (2) anterolateral margins of clypeus extending anterior to mediolateral regions (with the exception of the aurita group, as reported here), (3) mandible with 7-9 teeth, (4) at least larger workers with cordate head shape, with margin of vertex concave, (5) surface sculpture (other than on mandibles) smooth, micropunctate, microalveolate, or combinations of these, (6) the total absence of coarse surface elements such as spines, tubercles, carinae, rugae, striations, or large puncta, (7) a distinctive petiole which is strongly sloping anteriorly and has a rounded posteroventral lobe, and (8) worker caste polymorphism.

The Asian genus Philidris (former Iridomyrmex cordatus group) is highly convergent with Azteca . In contrast to Azteca , the anterolateral margins of the clypeus are posterior to the mediolateral portions, and the mandible has 10-12 teeth. Male characters (Shattuck 1992) and recent molecular evidence (P. S. Ward, pers. com.) ally Philidris with other Asian dolichoderines and confirm that the similarity is due to convergence.

The relative clarity of the generic status of Azteca is not mirrored in species-level taxonomy. Several factors contribute to taxonomic confusion in Azteca , some historical, some biological. The only revision of the genus Azteca is that of Emery (1893). Over 140 species-group names were subsequently published by Forel, Wheeler, and others, with no attempts at revision. Many species were described from workers only, with no biological data. Since it is often particularly difficult to separate Azteca species with workers only (Longino 1991a, b, 1996), many named Azteca species are difficult to circumscribe.

Wheeler and Bequaert (1929) belatedly stated "Apparently the females [i.e., queens] furnish more reliable characters for identification than the workers in the genus Azteca ." An analogy can be drawn between the taxonomy of Azteca and the taxonomy of many plants. Botanists typically shun sterile material because it is often more plastic within species and less differentiated between species than reproductive material. Such is the case in Azteca . Workers are polymorphic within colonies, and colonies exhibit prolonged ontogenetic changes in worker morphology (pers. obs.). In contrast, queens are much less variable morphologically and exhibit strong interspecific differences. Within a single locality, species with strongly differentiated queens may have workers that are barely distinguishable.

Correlated with sharp differences in queen morphology are distinctive nesting habits. Nesting habits show great interspecific variation and little intraspecific variation. For example, queens of Cecropia-inhabiting species colonize very young Cecropia saplings. These queens are often very abundant in the environment, colonizing saplings and apparently competing for domination of saplings (Longino 1989b). I have made extensive collections of neotropical arboreal ants by breaking live and dead branches, searching for carton nests and ant gardens, and dissecting other myrmecophytes such as Cordia , Acacia , Triplaris , Tococa , and Ocotea . The Azteca species which dominate Cecropia trees are found only in Cecropia trees. In spite of high queen density and competition for saplings, I have never encountered one of these Azteca species, either colonies or founding queens, in any plant cavity other than that of a Cecropia . Thus, when only workers are available, biological data on nest site can be of critical diagnostic importance.

Because of the unreliability of worker morphology, many names in Azteca may remain in nomenclatural limbo indefinitely. Identities of species based solely on a type series of workers, with no data on queen morphology or nesting behavior, will only be resolved by a thorough understanding of the subtle differences between workers of all the species at the type locality. In the mean time, it is important to have species descriptions and a nomenclature for this important genus of neotropical ants.