If natural selection operates at the level of each individual, who every day struggles to survive and reproduce, how is it possible that the same colony of ants "produces" workers of exaggeratedly different sizes - on the one hand, platoons of "minor" specimens with bodies and content chief, the super-soldiers on the other, with extra-large heads and jaws? What is the point, moreover, if all these workers are sterile?

These questions questioned Charles Darwin, who, because of them, almost questioned some of the principles of his evolution theories. A study just published in Nature now seems to provide a possible answer: it is the colony itself that maintains a balance between the various sub-classes of workers, regulating the growth of super-soldiers with the activation of an organ that develops only in the final phase of the larval stage.

Rule 30-70: the secret of ants' success

Not a simple accessory. The discovery of a team of biologists from McGill University (Canada) was a surprise. Among some species of warrior ants that live mainly in the Mexican deserts, there are specialized specimens that develop head and mandibles of anomalous dimensions, to be used to block access to the anthill. In these workers (called super-soldiers) a sort of disk at the level of the wings is formed for a short time, at the end of the larval stage, an organ that first appears and then disappears, long considered useless. It was thought that the vestigial structure (that is, apparently not functional) was a simple side effect of the overdevelopment of these ants, and not one of its causes.

"We have found that these vestigial organs are not a side effect of hormones and nutrition, but are instead responsible for generating super-soldiers. Their transient presence regulates the rapid growth of the head and body of these insects ┬╗explains Rajendhran Rajakumar, one of the authors.

For nine years, the entomologist has studied the wings of the ants of the genus Pheidole, the warrior ants in which this sub-arm of workers is present. He tried to remove portions of the disc with surgical and molecular techniques, and observed that the organ directly controlled the super-soldier's head and body growth. The size reduction of the ants took place in proportion to the portion of disk removed.

The right proportions. It is the ants society itself that maintains the balance between simple soldiers and super-soldiers (generally with 90-95 per cent of minor workers and 5-10 per cent of extra-large ants). When the super-soldiers are supernumerary, the growth of the vestigial discs is inhibited through particular pheromones. For researchers, apparently useless organs such as the one studied could have a role so far underestimated in social insect development processes.