Nicholas Wade in The New York Times:
It may be hard to imagine a ménage à trois, satisfactory to all parties, in which one member tries to dislodge another with a toxic gas and a third eats the offspring of the other two. But such an arrangement exists, and one of its members may even be sitting quietly in your kitchen’s spice rack. The story begins with the Large Blue, a butterfly that lays its eggs on the wild oregano plant. The caterpillar munches on the plant’s flower buds for two weeks and then one night drops to the ground. Most ants forage at noon, but by timing its descent at dusk, the infant caterpillar gets adopted by a red ant known as Myrmica that forages only at day’s end. The caterpillar deceives an ant into thinking it is a stray grub from the ant’s own nest. It does so by adopting the grub’s posture and by exuding a scent that mimics that of the ant’s own species. Taken underground to the Myrmica nest, the adopted caterpillar doesn’t remain a helpless foundling for long. It starts to acquire influence in the ant society by imitating the little clucking sounds made by the ants’ queen. And having gained high status in the nest, it can fulfill the purpose of its visit: to feast on the ants’ larvae. The ants themselves use their larvae as a food source when times are tough, so for their queenly guest to behave like a cannibal may not strike them as all that abhorrent.
The caterpillar gorges on the ant grubs for 10 months, increasing its weight nearly 50 times until it is time to turn into a pupa and then a butterfly. The Large Blue’s association with ants has been known for more than a century. Only recently have researchers started to explore how the butterfly pulls off the feat of detecting the underground nests of a single species of ant to which its caterpillars are adapted. (The butterfly, widespread in Europe, seeks out a single species of the Myrmica family of ants, but the particular species varies from one region to another in the Large Blue’s territory.) Considerable puzzlement ensued when experiments to test the butterfly’s ability to sense the presence of Myrmica came up blank. “It was a huge mystery that none of the experiments seemed to work,” said Naomi E. Pierce, an expert on butterfly-ant interactions at Harvard University. A surprising solution has been proposed by researchers led by Dario Patricelli and Emilio Balletto at the University of Turin in Italy and Jeremy A. Thomas of the University of Oxford. They have developed evidence that the oregano plant is the crucial mediator between the ants and the Large Blue butterfly.