Ants are well known for their social lifestyle and communication is key for their survival. However, some creatures take advantage of the ants' communal existence to hitch a free ride. Kartsen Schönrogge from the Centre for Ecology and Hydrology, UK, explains that the caterpillars of some Maculinea butterflies parasitise red ant nests. According to Schönrogge, some Maculinea species treat their hosts as a larder, raiding and consuming the ants' brood, while other species mimic ant larvae to trick the workers into feeding them, just like cuckoos. But the relationship between the butterflies and their hosts is extremely close: Maculinea rebeli caterpillars depend entirely on Myrmica schencki ants while Maculinea arion are only nurtured by Myrmica sabuleti: caterpillars that arrive in the wrong species' nests are slaughtered. Efficient communication between the ants and their lodgers is essential.
Curious to find out more about the systems that the caterpillars and their hosts use to communicate, Schönrogge and his colleagues Jeremy Thomas, Emilio Balletto, Francesca Barbero and Simona Bonelli began listening to the ants (p. 4084). But why turn to the sense of hearing, when insects mainly communicate through smell? Schönrogge explains that Maculinea butterflies and the Myrmica ants produce sounds, which were mainly thought to provide information about the sender's location, and when M. rebeli caterpillars make sounds like an M. schencki queen, the ants behave like her courtiers, even though she isn't there. According to Schönrogge, M. rebeli caterpillars are such convincing mimics that they are able to trick ants into believing that they are larvae so that worker ants rescue the caterpillars before they rescue their own young when under attack. Curious to know whether the cuckoo caterpillars are better acoustic mimics of their hosts than predator caterpillars that simply consume brood, Schönrogge and his colleagues decided to analyse the sounds produced by all four species.
Digging up ants' nests in Italy and the UK and collecting butterfly eggs to grow into caterpillars, Barbero travelled to Schönrogge's UK lab to use his custom built ant-recording studio to listen to the insects. Focusing on workers and queens of both ant species, and larvae and pupae from both butterflies, Barbero recorded the insects' frequency (pitch), pulse length and pulse repetition frequency.
Analysing the recordings, the team could see that the main difference between the worker and queen ants' buzzes was their pitch. Workers have a high-pitched buzz, around 1400 Hz, while the queens buzz at 800 Hz (regardless of species) and the butterfly pupae all buzz at 800 Hz. The pupae were mimicking the queens to raise their status in the worker ants' eyes and ensure that they get fed first. The team analysed the pupae's buzzes and found that the predatory M. arion pupae were as good at mimicking the ants as the cuckoo-like M. rebeli pupae, although both caterpillars pulsed more slowly than their ant hosts.
Schönrogge admits that he is surprised that there are not more differences between the caterpillars' buzzes given that M. rebeli have to inspire more cooperation from their ant host than the predatory M. arion pupae. He adds that it is even more surprising that the ants and insects are able to produce such similar buzzes given their differences in size and the instruments that they use. ‘That is the part that most people find amazing,’ says Schönrogge.