Most people are familiar with the army ants of South America and the driver ants of Africa featured in many nature documentaries, but few are aware army ants also occur in North America. There are 30 species of army ants from the Neivamyrmex genus and 1 species from the Novamyrmex genus living on this continent. North American army ants differ from those of South America and Africa. North American army ants cross the landscape in more narrow spear-headed swarms than those of their tropical cousins. Nevertheless, they are just as predatory.
A species of North American army ant. Notice how thick their antenna are. Years ago, I witnessed army ants tearing apart an earthworm in Columbia County, Georgia. This photo is by Alex Wild from the below link.
Army ants don’t live in permanent nests. Instead, they alternate between foraging and stationary phases. During foraging phases they roam across the land searching for food to feed their larva. They mostly eat other ants and are built to subdue other species. They have muscular bodies and thick antennas that other ants can’t bite through. When the larva go into the pupa stage, army ants enter the stationary phase and live within a swarm of their own bodies. Colonies produce new queens every 3 years, and the colony will split into 2 after the new queen is born. Most army ant colonies perish when the queen dies, but some manage to track down a closely related queen and will merge with that colony.
There are over 200 species of army ants worldwide including 5 genera in the Americas and 2 genera in Africa and Asia. A study of army ant genetics determined some genera of American army ants are closely related to African army ants. They diverged 100 million years ago before Africa and South America drifted apart. Other genera of army ants are not closely related to other army ants and are examples of convergent evolution.
Range map of army ants in North America. Map is also from the below reference.
Cold climate apparently is a limiting factor for army ant distribution, but it might not be the temperatures. I noticed in the map of their distribution that the northern limits of their range approximately corresponds to the southern limit of Ice Age glaciers. Like many species of trees, they simply have been unable to colonize deglaciated territory even though they can survive the cold temperatures of Iowa and Nebraska.
Reference:
Most of the information for this blog entry comes from Alex Wild, a Texas entomologist. This links to his website.
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