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Encyclopedia of Social Insects

Living Edition
| Editors: Christopher K. Starr

Azteca

  • John T. LonginoEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-319-90306-4_13-1
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The genus Azteca is a dominant component of arboreal ant communities in Neotropical forests. Colonies are large and can be spread over large portions of forest canopy or second-growth vegetation, with workers foraging on dead insects, small live prey, extrafloral nectar, and honeydew. They are highly specialized for life in the vegetation, showing a broad range of arboreal nesting habits. Some species are involved in coevolved mutualisms between ants and plants, being obligate inhabitants of myrmecophytes. The most evident and well-known of these associations is with Cecropia, a genus of common second-growth trees found throughout the Neotropics.

Systematics and Morphology

Azteca is in the subfamily Dolichoderinae [1]. The subfamily is mostly tropical, with some radiations in hot deserts and the temperate zones of Australia and southern South America. All lack a stinger and instead rely on defensive chemicals that they daub or smear on their enemies. Most species of the subfamily, including all Azteca, have a characteristic odor when crushed which, once learned, is a useful field character. Within the subfamily the genus is a well-defined monophyletic group. Its sister taxon is Gracilidris, a poorly known South American genus with just two known species. The estimated age of the most recent common ancestor of the two genera is 45 million years, and the age of the extant Azteca species (crown group age) is at least 10–15 million years.

Morphologically the genus is relatively uniform, which makes species-level identifications difficult. The workers have a smooth integument, without any coarse sculpture, and lack spines of any kind (Fig. 1). Species vary in size, but workers are continuously polymorphic within colonies, and the largest workers have disproportionally large heads. As colonies grow, the number and size of the largest workers gradually increase. Queens are usually easier to identify, varying in size, head shape, and color. However, obtaining queens is difficult, because most species are monogynous, with a single colony queen that is highly protected in the deepest recesses of the nest. Males are small, somewhat nondescript, wasp-like insects, also not readily identified to species.
Fig. 1

Typical Azteca worker (A. xanthochroa, Costa Rica)

At present 112 species and subspecies are recognized, but this number means very little. These names have accumulated since the mid-1800s, many based on workers only, and there is no real understanding of species diversity in the group. In a regional work on the fauna of Costa Rica [2], 24 species were encountered. In this case sampling was relatively thorough, and the identifications were all queen-based. But the entire continent of South America is largely unexplored with respect to Azteca species diversity.

In most species of Azteca, the queens are much larger than the largest workers and have clearly evolved for independent colony founding. The A. aurita group is distinctive in having queens that are little different in size from the largest workers. These queens are distinctive in other ways, having highly smooth and polished integument, the head with posterior borders drawn out into sharp angles or points, and falcate mandibles. This queen morphology is typical of species in other ant genera that are temporary social parasites, usurping the queen’s place in host colonies as a means of establishing their own colonies. Although there is no direct evidence, it may be that the A. aurita group similarly relies on dependent colony founding, using established colonies of other Azteca species.

Habitat, Foraging, and Nesting

The geographic range of Azteca extends from southern Mexico to Paraguay and southernmost Brazil on the mainland, and in the Lesser Antilles in the Caribbean. Azteca365体育网站 prefer lowland habitats, from wet to seasonal dry forest, mature or second-growth vegetation, and mangroves. They drop out at the lower edge of cloud forests, and no species are known to be montane specialists. Within habitats they prefer the most insolated parts of the environment: high canopy, low scrubby vegetation, road edges, and mangroves. They are less abundant in highly shaded understory. They typically forage in the vegetation, but in seasonal habitats they will forage on the ground and in leaf litter.

All known species nest and forage arboreally, either in or on vegetation. Most are polydomous, with the colony dispersed among multiple nests. Many Azteca species are generalist omnivores with large, territorial colonies. They feed on small insect prey, scavenge dead insects, and make heavy use of extrafloral nectar and honeydew from aggregations of Hemiptera. Aggressive workers blanket large swaths of vegetation, often covering multiple tree crowns and contributing to a mosaic of dominant ants that form exclusive territories. They often strongly defend stable resources such as extrafloral nectar and Hemiptera. Species with generalist foragers may be abundant in agroecosystems and contribute to biological control of pests.

A common feature of many species is the use of carton, masticated and moistened plant fibers that are shaped into structural elements such as walls or partitions within cavities or whole external nests suspended from branches. Sometimes carton is associated with the nest itself, where brood is sheltered, or it may be used to make small, dispersed pavilions that shelter honeydew-producing Hemiptera. Azteca brevis365体育网站 uses carton to make surface galleries that are perforated with many small holes. Workers are ambush predators, emerging from the holes to attack and immobilize prey.

Within the arboreal realm, Azteca species exhibit a wide variety of nesting habits. The diversity of nesting behaviors can be illustrated with a set of examples. Azteca tonduzi, which has one of the smallest queens in the genus, nests in small-gauge dead stems. Azteca instabilis, which has one of the largest queens in the genus, builds large carton nests in the dead centers of live tree trunks. These nests are highly protected and can be very long-lived. Azteca trigona and relatives build large, pendant carton nests (Fig. 2), forming very conspicuous polydomous colonies that are a major component of many South American rainforests. Azteca ulei and relatives build similar exposed carton nests but incorporate epiphyte seeds in the carton. The seeds sprout and grow, resulting in ant gardens, nests that are festooned with epiphytes [3]. These epiphytes are from multiple plant families and most are specialized associates, found only on ant gardens.
Fig. 2

Large pendant carton nest of Azteca cf. trigona (Panama)

Many Azteca are live stem specialists, but species show a gradation in degrees of host specificity. Live-stem species have reduced external foraging, relying on food generated inside the stems. For example, A. forelii nests in live stems of a variety of large trees, from multiple plant families. Carton galleries – remarkably similar to termite galleries in appearance – traverse trunks and stems, connecting chambers in live branches. The inner surfaces of the chambers are densely encrusted with Coccoidea, scale insects that tap into plant tissues from inside the stems. The workers are rarely exposed on the surface, traveling beneath the galleries. It is unknown whether the ants are parasites or mutualists with their host trees, but there is no specialization by the plants to house ants. This type of generalized use of live stems grades into more specialized associations, in which sets of Azteca species are associated with particular groups of plants, and the plants exhibit specialized and derived traits that facilitate mutualistic interactions with the ants [4]. The ants exhibit specialized colony founding behavior, with queens searching out specific host plants. Workers restrict their foraging and patrolling to the host plant, defending it against herbivores and encroaching vegetation. In return, the plants provide nest sites for the ants in the form of pre-formed hollow chambers (domatia) and, in the most derived systems, also feed the ants with specialized food bodies.

Myrmecophytes

When early European naturalists began exploring the tropics, they were drawn to highly specialized ant-plant associations, and often the first knowledge of particular ant lineages was in the context of these associations. In the Neotropics, Azteca was first observed as an associate of Cecropia (Fig. 3), a Neotropical genus of about 70 species. Cecropia are small- to medium-sized pioneer trees that are common along roadsides, in abandoned pastures, in forest gaps, and along river margins. The young trees seem to spring up from the competing vegetation in early successional habitats, and the stems are inhabited by aggressive ants that boil out when the tree is disturbed. The trees have multiple traits that favor ants: hollow cylindrical sections of stem (internodes) that serve as ant nesting space, thin spots on internode walls that are an entry point for founding queens (prostomata), and glycogen-rich food bodies (Muellerian bodies) that sprout from the base of the leaf petiole. Several species of Azteca are specialized associates of Cecropia, and the habit has evolved at least twice in different Azteca lineages.
Fig. 3

The Azteca-Cecropia association. (a) Crown of small Cecropia tree. (b) Dense pad of trichomes (trichilium) at the base of leaf petiole, with food bodies (Muellerian bodies). (c) Carton nest of Azteca merida in Cecropia trunk, with workers swarming over surface and forceps. (d) Cecropia sapling with preformed thin spot (prostoma) that founding queens use to enter internode. (e) Founding queens of two species, A. constructor and A. xanthochroa, in Cecropia internode

Azteca-Cecropia associations are not tightly species-specific but instead are interacting communities of multiple species. Local populations of one to several Cecropia species are inhabited by communities of several Azteca species. The internodes of Cecropia saplings contain multiple Azteca queens, commonly of multiple species. Sometimes there is one queen per internode, and sometimes there are multiple queens in internodes. As incipient colonies develop, there is competition, and ultimately one colony with one queen comes to dominate the growing tree. Mechanisms of coexistence among these Azteca species have been the subject of ecological studies [5].

Although the Cecropia-Azteca community is the most conspicuous Azteca-plant association in the Neotropics, it is not the only one. Members of the A. pittieri complex are specialized associates of the tree species Cordia alliodora (Fig. 4). Cordia alliodora is a common second-growth tree in Central America and northern South America. The tree provides multiple nodule-like domatia at stem tips, and a single Azteca colony typically dominates a tree. A similar association is between A. longiceps and the tree Triplaris melaenodendron, which has hollow stems. Neither of these associations involves food bodies, but the walls of the domatia are always covered with sap-feeding Coccoidea.
Fig. 4

Azteca-Cordia association. (a) Domatium of Cordia alliodora. (b) Opened domatium, showing workers of Azteca pittieri, a worker larva, and scale insects on the inner surface of the node

References

  1. 1.
    Ward, P. S., Brady, S. G., Fisher, B. L., & Schultz, T. R. (2010). Phylogeny and biogeography of Dolichoderine ants: Effects of data partitioning and relict taxa on historical inference. Systematic Biology, 59, 342–362.
  2. 2.
    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, 1–63.
  3. 3.
    Davidson, D. W. (1988). Ecological studies of neotropical ant gardens. Ecology, 69, 1138–1152.
  4. 4.
    Davidson, D. W., & McKey, D. (1993). The evolutionary ecology of symbiotic ant–plant relationships. Journal of Hymenoptera Research, 2, 13–83.
  5. 5.
    Yu, D. W., & Davidson, D. W. (1997). Experimental studies of species-specificity in Cecropia-ant symbioses. Ecological Monographs, 67, 273–294.

Copyright information

© Springer Nature Switzerland AG 2020

Authors and Affiliations

  1. 1.Department of BiologyUniversity of UtahSalt Lake CityUSA