Butterflies scrounge off ants by mimicking the music of queens

Ants are among the most successful of living things. Their nests are well-defended fortresses, coordinated through complex communication systems involving touch and chemical signals. These strongholds are stocked with food and secure from the outside world, so they make a tempting prospect for any burglars that manage to break in.

One species of butterfly - the mountain alcon blue (Maculinea rebeli) - is just one such master felon. Somehow, it manipulates the workers into carrying it inside the nest, feeding it and caring for it. The caterpillar does so little for itself that it packs on 98% of its eventual adult weight in the company of ants. How does it do it?

Partly, the caterpillar secretes chemicals that imitate those found on ant larvae, and it mimics their actions too. But that can't be the only explanation for ant workers will actually rescue alcon blue caterpillars over their colony's genuine larvae. And if food is short, they will even kill their own young to feed the parasitic impostors. In the entire colony, only one individual is treated with as much respect as the caterpillars - the queen.

Now, Francesca Barbero from the University of Torino has found out how the alcon blues manage to get the royal treatment - they "sing" in the style of queens, producing uncanny cover versions using instruments built into their bodies.

Barbero focused her attention on a species red ant called Myrmica schencki, which is frequently parasitized by the alcon blue. These ants produce sounds using instruments built into their bodies. The first segment of their abdomens has a small lip - a "plectrum" - that it scrapes along a finely-ridged surface - a "file" - on the next segment. The result is like running a gramophone needle down the ridged tracks of a vinyl record.

But the queen ant sounds very different to her minions. Her file is longer and has a wider gap between its ridges. As a result, she produces sounds with different acoustics and a lower frequency of 800Hz (compared to the 1,100 Hz whine of her workers). They also elicit different behaviour.

Barbero played the sounds of both castes to nests of workers using small speakers. White noise or silence did nothing, but the sounds of other ants brought workers running. The recruits tapped their antennae against the speakers -  a behaviour that ants use to smell their nest-mates and communicate with each other. Any ant sound did this, but the calls of the queen in particular put the workers on high alert - they stood in an "on-guard" posture that they use when they attend to royalty.

It's these queenly tunes that the caterpillars mimic. Barbero recorded the sounds made by alcon blue caterpillars as well as their pupae. The pupa plays that funky music by drawing small teeth across a comb-like structure but the larva's method is still a mystery. Regardless, Barbero's recordings picked up very similar noises from both larvae and pupae, that were closer matches for the queens' refrains than those of the workers. In fact, the butterflies appear to have evolved a sound that's as close a match as possible to that of an ant queen, while at the same time, being as distinct as possible from that of her workers. It's a very fine-tuned mimic.

The pupal calls were particularly good impressions. When they were played through speakers, worker ants behaved just as they would do when a queen was around - congregating, tapping their antennae and standing on-guard.

The larval noises are a much weaker match and not significantly more stimulating than the calls of other workers. Barbero thinks that this is because the larvae can also rely on their chemical mimicry to fool the ants. The pupae can't, for the lack the appropriate secreting organs - they have to imitate the ants through the medium of sound.

Barbero's research also shows that ants communicate with sound far more than was previously thought. Previously, scientists thought that ants only really used calls in rare situations - to signal alarm or to find buried colleagues. This study shows that the sounds also carry information about an individual's social status too.  

And if ants use noise to reinforce their internal hierarchies, it's a fair bet that other ant parasites exploit this line of communication. Other species of blue butterfly besides M.rebeli also make noises - perhaps these too are adapted to deceive ants. There are over 10,000 species of insects that parasitise ants, from flies to beetles to other ants - how many of them have evolved to sing for their supper?

Reference: 10.1126/science.1163583

More on ants and alcon blues: Evolutionary arms race turns ants into babysitters for Alcon blue butterflies

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