| Polyergus | |
|---|---|
| Polyergus breviceps | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Insecta |
| Order: | Hymenoptera |
| Family: | Formicidae |
| Subfamily: | Formicinae |
| Genus: | Polyergus Latreille, 1804 |
| Species | |
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Polyergus, also called Amazon ants, is a small genus of 6 described species (and several possible undescribed species) of "slave-raiding" ants. Its workers are incapable of caring for brood, in part due to their dagger-like, piercing mandibles, but more importantly, because in the evolution of their parasitism, they have lost the "behavioral wiring" to carry out even rudimentary brood care, or even to feed themselves. Polyergus species subsist solely as a specialized brood-acquiring caste, maintaining a worker force by robbing brood of particular species in the closely related genus Formica in massive colony-to-colony raids. The captured ants are generally referred to as "slaves" in scientific and popular literature, though recent attempts have been made to apply other human cultural models, such as describing the Polyergus individuals of a colony as "raiders" or "pirates" and the Formica workers as "helper-ants", or "domesticated animals". Biologists describe the system simply as social parsitism by Polyergus on the host Formica species.
Polyergus obtains its Formica work force by stealing pupae from nearby Formica colonies and carrying them back to its own nest. Back in the Polyergus nest, Formica workers are eventually helped to emerge from the cocoons by Formica workers already living there. The new workers quickly assimilate the characteristic odor of the mixed-species population of the Polyergus colony—completely without violence or coercion. The Formica workers that emerge in the mixed-species colony go on to nurse, forage, and perform other colony upkeep duties.
As far as is known, all established Polyergus colonies have only one queen. To found a new colony, a lone Polyergus queen invades a nest of the host species, or encounters and moves in with a colony-founding queen of the host species and her first few workers. In the latter case, the host queen is allowed to survive until she has reared a number of host workers, something the Polyergus queen cannot do herself. A young Polyergus queen kills the existing Formica queen (immediately if sufficient workers are present, later if these are not yet reared) and becomes accepted by the Formica workers. These proceed to rear the first and all subsequent Polyergus brood. Clearly, this complicated and lengthy process often fails, as Polyergus colonies are relatively rare, even though each mature colony produces dozens or hundreds of new potential queens each year. To counteract the natural mortality of the Formica worker population, Polyergus workers must conduct regular raids over a 6-8 week period, every summer over the 10-15 year life span of their colony.
The raids of Amazon ants have been sensationalized as "blood baths" in popular writing. However, careful observation has demonstrated repeatedly and for all species that their brood-stealing raids are highly efficient and virtually non-violent. It is rare for even a single member of the raided Formica colony to be injured or killed. Instead, the host colony is overwhelmed, apparently by its own formic acid and alarm pheromones produced in the panic, when the marauders enter their nest en masse. Commonly, the queen and workers of the raided Formica colony evacuate and climb up nearby vegetation where they simply "wait for the problem to go away" while the Polyergus workers find and run off with their brood.