Formicidae, Hymenoptera, Insecta, Arthropoda, Animalia
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Identification
xanthochroa is an obligate Cecropia ant (Longino 1989, 1991a,b).
Worker: scapes and tibiae with abundant, conspicuous setae; strongly polymorphic, largest worker HW approaching 1.40mm. In Costa Rica, workers of xanthochroa are very difficult to distinguish from workers of constructor. In xanthochroa, the largest workers approach the queen in color, with the sides of head orange, and the sides of the head are relatively rounded. In constructor, the largest workers are uniform brown, and the head is more quadrate (figure). At some upland wet forest sites in Costa Rica, where A. xanthochroa colonies appear somewhat stunted, worker series of A. xanthochroa and A. constructor are nearly indistinguishable.
Queen: scapes and tibiae with abundant, conspicuous setae; color mottled orange; HL greater than 1.90mm.
Colonies of xanthochroa and constructor are relatively easy to distinguish in the field because of differences in nest structure. xanthochroa usually maintains a vertical fissure near the base of the tree, and does not have entrances near the central nest. constructor does not maintain a fissure near the base of the tree, but rather has multiple fissures near the nest. See additional details under Natural History.
Range
Western Panama to southern Mexico.
Natural History
See also general treatment of the Cecropia-Azteca association in Costa Rica.
In Costa Rica, A. xanthochroa is one of the most common Cecropia ants in wet forested areas. In the Atlantic lowlands, colonies are most frequently found in C. obtusifolia, a common tree of disturbed areas. Cecropia insignis, a tree more often in primary forest, is dominated by A. ovaticeps. As one moves up-slope, both C. obtusifolia and A. ovaticeps drop out, and A. xanthochroa is then frequently found in C. insignis throughout the rest of its elevational range (up to 1100m in the Penas Blancas Valley east of Monteverde). Above this limit, C. insignis is replaced by the non-myrmecophytic C. angustifolia. Founding queens may be found in C. angustifolia saplings, well above the elevational limit of C. insignis, but never in mature trees. These queens are presumably doomed dispersants from lower elevations. On the Pacific slopes of northwestern Costa Rica, a narrow band of C. obtusifolia separates cloud forest C. angustifolia from the common C. peltata which is throughout the seasonally dry lowlands. Azteca xanthochroa is common in this narrow band, but does not occur at lower elevations where C. peltata dominates (Longino, 1989).
In C. obtusifolia, colonies maintain a longitudinal fissure near the base of the tree, from which very large workers emerge when the tree is disturbed. These large workers bite only infrequently, perhaps because they have difficulty maneuvering their large heads or obtaining a grip with their large mandibles (Perlman, pers. com.). Many smaller workers emerge from branch tips, however, and these are very aggressive and readily bite. All reproduction is concentrated in a single carton nest in the bole, and there are no entrance holes near this central nest. Internal communication is maintained with all branch tips, which contain only workers, coccoid homoptera, and cached muellerian bodies.
Nest structure appears quite different in C. insignis, perhaps due to the much thicker wood of the bole, or to the fact that it is inhabited mainly at higher elevations where it is much cooler and wetter. For example, in the Penas Blancas Valley (800-900m), colonies are smaller and less aggressive. They occur high in the tree, sometimes in only a portion of the crown, and they do not maintain a basal fissure.
Literature Cited
Longino, J. T. 1989. Geographic variation and community structure in an ant-plant mutualism: Azteca and Cecropia in Costa Rica. Biotropica 21:126-132.
Longino, J. T. 1991a. Azteca ants in Cecropia trees: taxonomy, colony structure, and behavior. Pages 271-288 in C. Huxley and D. Cutler, editors. Ant-plant interactions. Oxford University Press, Oxford, U.K.
Longino, J. T. 1991b. Taxonomy of the Cecropia-inhabiting Azteca ants. Journal of Natural History 25:1571-1602.
Page author:
John T. Longino, The Evergreen State College, Olympia WA 98505 USA. longinoj@evergreen.edu
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