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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 . 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.
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 , 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.
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 . 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.
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 .
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