Bert Holldobler & Edward O. Wilson
(a brief excerpt from the outstanding book Journey to the Ants by above authors)
Ants that are completely dependent on symbiotic plants are also among the most aggressive in the world. Those large enough to attack mammals, including human beings, are well-armed, quick and vicious. It is as though they have no other place to go, and with their backs to the wall they are prepared to make an extreme response to every provocation. The acacia ants swarm out almost instantly to mount and sting an offending arm and hand. When a person stands upwind and close by an acacia bush, some of the workers run to the leaves and strain to reach him, apparently aroused by his body odor alone. Larger and even more aggressive Pseudomyrmex ants inhabit Tachygalia, a small understory tree of South American forests. To brush one's bare skin against Tachygalia is like touching a nettle. The punishment in this case, however, is delivered by dozens of ants that sprint onto the body, instantly begin to sting, and hold on until picked off. As we have have walked though the rain forest undergrowth distracted, often rather carelessly in the typical naturalist's manner, we have felt the familiar burning sensation on some exposed part of our body and thought immediately: Tachygalia!
But the most effectively aggressive ant species in the world, exceeding even the tachygaliaphilic Pseudomyrmex, may be Camponotus femoratus, a large, hairy and decidedly unpleasant ant of South American rain forests. When disturbed in the slightest, the workers boil out in an angry mass over the nest surface. Just the close presence of a human being is enough to trigger the reaction. Diane Davidson, an American entomologist who has studied ant-plant symbiosis extensively, described the behavior in a letter to us as follows: "When I approached to within 1-2 m of their nests, workers of this species typically began to run back and forth and frequently jumped or fell onto me. Workers of all size classes of this polymorphic species, but usually only the major castes were capable of breaking the skin with their mandibles and causing a stinging sensation by simultaneously biting and spraying formic acid into the wound."
These ants happen to live not in the cavities of plants, but in ant gardens, which constitute the most complex and sophisticated of all symbioses between ants and flowering plants. The gardens are round masses of soil, detritus, and chewed vegetable fibers assembled in the branches of bushes and trees, ranging in size from golf balls to soccer balls, within which are grown a variety of herbaceous plants. The ants collect the material for the nests. The ants gather the seeds of the symbionts and place them in the nests. As the plants grow, nourished by the soil and other materials, their roots become part of the framework of the gardens. The ants in turn feed on the food bodies, fruit pulp, and nectar provided by the plants.
The ant gardens of Central and South America contain many plant species, representing at least 16 genera, that are found nowhere else. These specialized forms include arums such as Philodendron, bromeliads, figs, gesneriads, pipers, and even cacti.