Before they become butterflies, some caterpillars transform themselves into ants. Rather than living out in the open and braving predators while they grow up, these caterpillars sneak into ant nests for free food and lodging—or they just eat the ants. Either way, scientists have found, part of the caterpillars’ disguise involves mimicking the sounds of an ant queen. When worker ants hear this tune, they drop everything to tend to the mooching invaders.
At the beginning of its life, a caterpillar of the genus Maculinea hangs out on its favorite plant and munches away, like any other future butterfly. Its food plants are within the foraging grounds of Myrmica ants, the caterpillar’s targets. Once it finishes growing, the caterpillar waits until an evening between 6 and 8 PM—prime foraging time for the ants—then lets go of its perch and falls to the ground. There, it waits.
When a Myrmica ant that’s out searching for food stumbles across the caterpillar, it stops and feels it with its antennae. This assessment may take only a few seconds, or may be an hours-long “adoption ritual,” explains University of Turin biologist Luca P. Casacci. Then the ant picks up the giant baby (“Whoops, how did one of our larvae get out here?”) and carries it back to the ant nest.
How the caterpillar takes advantage of its new home depends on its species. Casacci studied two species of Maculinea that share an ant host, Myrmica scabrinodis. One caterpillar, M. teleius, has a “predatory” strategy. It hides out in far corners of the nest until it gets hungry, then binges on ant larvae. The other caterpillar, M. alcon, is described by scientists as a “cuckoo.” Real cuckoo birds leave their eggs into other birds’ nests; the cuckoo caterpillar plays nice and convinces its hosts to care for it. Worker ants feed and tend to the caterpillar as if it were one of their own larvae.
Ants rely mostly on chemical cues to recognize each other, communicate, and coordinate their activities. Parasitic caterpillars take advantage of this system, Casacci says. The caterpillars use chemicals on the outsides of their bodies to imitate the smell of the ants; this hastens their adoption and lets them safely hang around the nest.
But scientists recently discovered that Myrmica ants also communicate with sound. They make noise by dragging their hind legs across spikes on their abdomens. Ants that have different ranks within the colony make different sounds. One study found that Maculinea caterpillars could mimic these sounds, in addition to mimicking the ants’ smell. So Casacci brought caterpillars and their host ants into the lab for some recording sessions.
Casacci and his coauthors made recordings of queens and worker ants, as well as cuckoo and predatory caterpillars—both before and after they were adopted by the ant colony. Analyzing the components of the insects’ sounds, they found that caterpillars sound more like ant queens than like worker ants.
The cuckoo caterpillars, M. alcon, made louder or more intense sounds once they were inside the nest. This made their rasping call even more similar to a queen’s. But predatory M. teleius caterpillars got quieter after being adopted. Since these caterpillars “spend much of their lives hidden in some remote chambers of the nest,” Casacci says, they may be trying “not to attract the attention of workers.”
To see how much attention these queen-like calls got from workers, the researchers set up boxes of dirt with speakers buried in the corners. They they played their ant and caterpillar recordings from these speakers and watched to see how ants walking around in the boxes reacted.
Two sounds got the biggest reaction out of the ants: the recording of a queen, and the recording of a cuckoo caterpillar that had already been adopted. In both cases, ants walked toward the speaker and felt around it with their antennae. Sometimes the ants even started digging, as if trying to free another ant trapped under the soil. (In other experiments, scientists disturbed ant nests and saw that the workers rescued cuckoo caterpillars before rescuing their own larvae.)
“When producing sounds similar to queens, Maculinea larvae achieve the highest status within ant societies,” Casacci says. They don’t just imitate worker ants—they imitate the ants’ boss. This means the ants don’t only tolerate the caterpillars, but feed and care for them. Predatory caterpillars, meanwhile, imitate queens for just long enough to get inside the nest. Then they lie low.
When they’re done with the ant colony, all these caterpillars will stop humming the queen’s song and transform themselves once again. This time they’ll announce the change with a flutter of wings, a sound the ants will ignore.
Image: Luca P. Casacci
Sala M, Casacci LP, Balletto E, Bonelli S, & Barbero F (2014). Variation in butterfly larval acoustics as a strategy to infiltrate and exploit host ant colony resources. PloS one, 9 (4) PMID: 24718496