Acromyrmex

A genus of fungus growing ants. Primarily a Neotropical group, Acromyrmex occurs from the southern USA south to southern South America. They cultivate fungus within their nests and, like Atta, they are true leaf-cutters. Most Acromyrmex inhabit tropical forests, grasslands, and disturbed areas. Acromyrmex versicolor is atypical in that it is found in dry desert areas of the southwestern USA and northern Mexico.

Identification
The genus Acromyrmex occupies a morphological "middle ground" between the highly polymorphic genus Atta and the monomorphic genus Trachymyrmex. Acromyrmex is separated from Atta by the presence of 3 pairs of spines or teeth on the promesonotum (only 2 pairs in Atta), and from Trachymyrmex by the presence of spines or teeth on the promesonotum (rather than tubercles), by the absence of shallow antennal scrobes (often present in Trachymyrmex), and its polymorphic worker caste (monomorphic in Trachymyrmex).

Biology
Longino (Ants of Costa Rica) - Acromyrmex species are true leaf cutters. Compared to Atta, they have a greater predilection for flowers and fruits. Their colonies are smaller (thousands of workers rather than millions), and foraging columns are usually single file and sparse. Their nests are usually in pre-formed cavities, and they do little excavating of their own.

Morphologically they are sandwiched between Atta and Trachymyrmex. Atta have very large colonies, strong worker size polymorphism, and a smooth gastral dorsum. Trachymyrmex have smaller colonies, usually smaller monomorphic workers, and a tuberculate gastral dorsum. Acromyrmex have intermediate colony sizes, strong worker polymorphism (but the major workers are not as highly differentiated as Atta majors), and the gaster is tuberculate. Single workers of Acromyrmex can be difficult to differentiate from Trachymyrmex in a key.

In Costa Rica there are few species of Acromyrmex. In lowland and seasonally dry habitats there are usually one or two forms of the Acromyrmex octospinosus species complex. In moist to wet montane areas Acromyrmex coronatus dominates.

Schultz et al. (1998) have discovered cryptic species in the A. octospinosus complex in Panama, and one should expect similar complexity to occur in Costa Rica. I recognize three species in the complex in Costa Rica (octospinosus, Acromyrmex echinatior, Acromyrmex volcanus), but the morphological basis for the distinctions is slight and some concentrated research on the group could easily redraw the species boundaries.

There is a large literature on Acromyrmex that is not reviewed or cited here.

Worker Morphology
These characters are collated in the table "WorkerMorphology". View table.

Nomenclature

 *  ACROMYRMEX [Myrmicinae: Attini]
 * Acromyrmex Mayr, 1865: 83 [as subgenus of Atta]. Type-species: Formica hystrix, by monotypy.
 * Acromyrmex raised to genus: Emery, 1913b: 251.
 * Subgenera of Acromyrmex: nominal plus  Moellerius.