The big-headed ant (Pheidole megacephala) is considered one of the world's worst invasive ant species. As the name implies, its colonies include soldier ants with disproportionately large heads. Their giant, muscle-bound noggins power their biting parts, the mandibles, which they use to attack other ants and cut up prey. In a new study, researchers report that big-headed ant colonies produce larger soldiers when they encounter other ants that know how to fight back. The new findings appear in the Biological Journal of the Linnean Society.  Big-headed ants are world travelers, hitching rides with humans to get around. Scientists have found them in more than 1,600 sites across the globe (see map). Their arrival at a sufficiently warm destination (they cannot tolerate cold weather) spells almost certain doom for native ants, spiders, beetles and other invertebrates that are unaccustomed to their brand of warfare.

"If you think about the worst invasive species, ants frequently show up on those lists, and big-headed ants are among the most problematic," said University of Illinois entomology professor and animal biology department head Andrew Suarez, who led the new study with postdoctoral researcher Bill Wills. "They are very aggressive. And unlike a lot of native ants, they produce large numbers of queens, so they have incredibly high potential for reproduction."

Big-headed ants spread out, assembling multiple nests that cooperate on defense, reproduction, territorial expansion and food procurement, the researchers said.

Soldiers and non-soldiers in a Pheidole megacephala colony are sisters, so genetic changes do not account for their differing morphology. Changes in nutrition during development are primarily responsible for their different shapes and sizes, Suarez said.

"We know that you can feed larvae differentially and that will change hormone levels that will cause a completely different developmental pathway to turn on, so you get this giant head, you become a big-headed ant rather than a small worker," he said.