A bizarre case of sound mimicry involving caterpillars and ants

Mimicry was one of the first pieces of evidence supporting Darwinism, as it wasn’t immediately obvious why a celestial creator would make one creature mimic another, while natural selection could easily explain it if the mimic gained protection or resources by resembling its “model.” The most recent issue of Science–an issue loaded with fascinating studies of evolution–has a really interesting article involving mimicry of sounds.

Ant colonies have been invaded by many species of arthropods who take advantage of the ants’ food stores or presence to gain food or protection from predators. Often natural selection has molded these arthopods to mimic the appearance of the ants. In this issue of Science, however, Francesca Barbero and his colleagues describe a caterpillar that has gone farther–it mimics not only the chemicals of the ants, but also the sounds produced by the ant queen to subdue and gain sustenance from her workers.

Caterpillars of the lycaenid butterfly Maculinea rebeli are carried by one species of ants (Myrmica schencki) into the ant nest, where they are fed by the workers, who normally feed larvae of the ants. Why do the workers feed the alien species? They are deceived by the caterpillar in two ways: the caterpillars mimic the ants both chemically and through sound!

First, the caterpillars secrete chemicals that resemble the chemicals on the surface of ant larvae. Chemical mimicy is not so unusual in insects. What is more remarkable is that both the caterpillars AND PUPAE (the next life stage of the caterpillar) of the butterfly are able to produce sounds similar to those produced by the ant queens (how the caterpillars do this is unknown.) These queens have “stridulatory organs” which they rub to produce special “queeny” sounds that induce the workers to tend and feed them. (The ant workers also make sounds, but they differ form those of the queen). Sure enough, both caterpillar and pupae are able to produce sounds more similar to those of the queen than to those of the workers. Playback experiments of recorded caterpillar and pupal calls demonstrated that these sounds induce tending behavior in the ant workers. This is simply astounding–I am not aware of any other case in which a pupa is able to produce sounds, much less sounds that mimic those of another species. It is this kind of bizarre adaptation, resulting from selection on the butterflies to get free food in the juvenile stages, that gets the juices of evolutionary biologists flowing. The bounty of natural selection is endless: you can never predict what it will come up with.

You can listen to the sounds of the ant queens and workers, and of the larvae and pupae of the butterfly here.

Queen Ants Make Distinctive Sounds That Are Mimicked by a Butterfly Social Parasite.  Francesca Barbero, Jeremy A Thomas, Simona Bonelli, Emilio Balletto, and Karsten Schönrogg, Science 6 February 2009: Vol. 323. no. 5915, pp. 782 – 785