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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 A. octospinosus species complex. In moist to wet montane areas A. 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, echinatior, 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.
Schultz, T. R., D. Bekkevold, and J. J. Boomsma. 1998. Acromyrmex insinuator new species: an incipient social parasite of fungus-growing ants. Insectes Sociaux 45:457-471.
John T. Longino, The Evergreen State College, Olympia WA 98505 USA.firstname.lastname@example.org
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