Communication between species need not threaten the sender. Certain butterfly caterpillars signal their presence with vibration as katydids do. They rub special organs, called vibratory papillae, across a rough part of their exoskeleton. This creates an acoustic signal, sort of like rubbing thimble-clad fingers across a washboard in jug-band music. But the caterpillar aims its vibratory signal not at another member of its own species—caterpillars are immature butterflies and don’t mate—but at ants. These signals call the ants, which then gather sugary secretions the caterpillars produce from special glands on their backs. Caterpillars also produce an odor that alerts the ants to predators. In return for the food, the ants act as bodyguards, swarming around the caterpillar and keeping it safe from wasps and other predators. In some species, ants carry the caterpillar into their nest, feeding it there. The survival odds of a caterpillar sans ants falls to zero, creating pressure to keep a cadre of ants close by.
The caterpillars may use vibration to call ants because ants use vibration to call ants. Many ant species uses vibration to communicate within the colony and to call for help when necessary. In one species scientists have studied, a victim of a collapsed tunnel buzzes the ground to attract rescuers from its colony, who dig it out. Other species use vibrations to notify colony mates of food sources. The caterpillar vibrations may attract ant attention, leading to the discovery of the nutritious secretions.
The ability to signal a different species does not represent a mere quirk of evolution. Two other groups of butterfly caterpillars that associate with ants also call their companions by vibrating the substrate. Neither uses vibratory papillae, though, and scientists do not know how they produce the signals. In contrast, several groups of caterpillars that neither produce sugary secretions nor associate with ants do not send the these buzzing signals.