Baby-Eating Freeloac1er Ants Are Welcome.

• Position: INSECT ELONES

It might seem surprising that a colony of ants would tolerate the type of guests that gobble up both their grub and their babies, but research shows there likely is a useful tradeoff to accepting these parasite ants into the fold: they have weaponry that is effective against their host ants and a more-menacing intruder ant.

Rachelle Adams, assistant professor of evolution, ecology, and organismal biology at Ohio State University, Columbus, wanted to better understand the dynamics of the symbiotic relationship between parasitic Megalo-myrmex symmetochus ants and Sericomyrmex amabilis hosts. The host ants spend their lives farming fungus. The party crashers come in, do not contribute to the work, and eat both fungus and ant larvae. This lopsided arrangement can go on for years.

When they began the study, Adams and her collaborators knew that there were some obvious explanations for such behavior in nature. It could be that the parasites smell like the hosts and therefore go unnoticed, a situation called "mimicry." It could be that they smell like nothing and essentially are invisible intruders, a strategy called "insignificance."

However, when the researchers analyzed the odiferous hydrocarbons on the ants' bodies--the ant world's way of cueing the insects into whether they are encountering a nest mate or an outsider--they found that neither of these explanations told the entire...