Moths and butterflies are delicious. At least, a flycatcher called the Eastern phoebe thinks so. Almost ten metres from her nest, Jayne Yack, from Carleton University in Canada, ties a cotton thread around the abdomen of a plump-bodied moth and tethers it to a tree in plain view of the flycatcher.
The phoebe soon swoops in and snatches the moth. The sound of the bird’s aerial attack is recorded with strategically placed microphones so that Yack can understand whether such insects can hear a potential predator approaching.
Insects like moths and butterflies can tap into wavelengths to which we are oblivious, but it’s a secret sensory world that we are slowly beginning to understand.
Human and insect ears perceive sound in roughly the same way. Sound travels through the air in waves. The sound wave then strikes a thin membrane and makes it vibrate – like hitting a drum with a drumstick.
In humans, the vibration of this membrane is transmitted to the cochlea in the inner ear – in insects to a structure called the chordotonal organ. In both the end result is the same – auditory nerves send electrical signals to the brain in order to notice a sound.
Imagine the symphonies of sounds that we are missing
For three decades Yack has been studying hearing in a range of insects including caterpillars, bark beetles, moths, and butterflies. The latter have ears on all sorts of body parts - thorax, abdomen, legs, mouth, and even their wings.
The vibrating membrane is a different size and thickness in every species, which means it resonates in response to different frequencies. Our tympanic membranes, for instance, vibrate only to frequencies between 20 - 20,000 Hz.
Anything higher or lower, and the membrane doesn’t budge, so you would not hear a thing. This limits us to a relatively small slice of the auditory spectrum. Imagine the symphonies of sounds that we are missing out on, simply because our ears are tuned to a different wavelength.
"I’m always interested in putting myself in the shoes of the insect and trying to imagine what the world is like for them," says Yack. She wants to know not just what they hear, but also why they hear it.
You just press the button and the moths go into loops and spirals
Moths' ears for instance, are tuned to hearing the high frequency echolocation calls of bats. Since moths are nocturnal and bats eat them, being able to hear a foraging bat is in a moth’s best interest.
Yack has tested this using a cheap ‘dog trainer’ - a device that emits a loud, high frequency whistle that both dogs and moths can hear. Moths normally fly in relatively straight lines but, explains Yack, if you ‘zap’ it with a high frequency sound wave, they immediately dart left and right, up and down, and do loop-the-loops - performing evasive flight manoeuvres to outwit a bat.
"It’s fun," she says. "You just press the button and the moths go into loops and spirals." Just by mimicking a bat’s call, the moth’s behaviour changes completely.
Butterflies, however, always fly in jerky, erratic patterns making them tricky to catch. They do so to stay out of the way of their main predators – birds. Yack believes that a constant aerial threat makes their unpredictable flight necessary all the time. So if butterflies are already on the defensive in flight, Yack thinks they might be using their ears to listen for incoming predators when they’re perched.
That's because when butterflies perch, they fold their wings and expose their ears – which are present on the underside of their wings. It's a bit like how we sometimes turn our head towards a noise to hear it better.
The moth was the bait needed to record the subtle sounds of the bird’s wings
And as butterfly ears face backwards, they can hear a bird approaching best from behind.
The ones with the most sophisticated ears belong to the family Nymphalidae (sometimes called brush-footed butterflies) - a large family of about 6,000 species. Many of these, like the tropical Morpho peleides that Yack studies, live in the forest and sit in the undergrowth or on branches.
To understand how they hear and what they hear, she first had to record the sound of a hunting bird. A bird on the prowl does not sing or call – instead it glides in as silently as possible to avoid alerting its prey. This is where the sacrificial moth comes into play. The moth was the bait needed to record the subtle sounds of the bird’s wings as it was foraging.
Back in the lab, Yack attached an electrode to the auditory nerve of the butterfly and listened in on the nerve’s activity.
When she played back the sound of the bird’s wings, the butterfly’s auditory nerve went into overdrive - meaning its ears are tuned into the same low frequency sound of their avian predators.
Animals are detecting the world in such different ways
This highlights the intricate forces that drive the evolution of predator prey relationships in these creatures. As evolution drives the hunters to make less noise, their prey develop more sophisticated hearing.
"All around us we’ve got these animals that are detecting the world in such different ways," says Yack. Their senses allow them to tune into the wavelengths they need to detect to stay alive. "They tap into that one band of energy that is most important to them – if that’s a bat calling, or another insect talking to them, or a predator."