The Moth: Ultrasonic Hearing


We probably don’t give much thought to the humble moth, but the amazing thing that most of us don’t realise, is that some moths have the most advanced hearing mechanism on the planet! Their hearing has evolved so they can avoid being eaten by bats, as they can detect them approaching and get out of the way.

Moths are related to butterflies and belong to the genetic family, Lepidoptera. There are around 160,000 different species of moth, and scientists believe there are many more that have yet to be recognised.

Moth

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A lot of them are nocturnal, but there are other crepuscular species (most active at twilight), while some are diurnal species, who come out during the day.

The word “moth” is believed to come from the old English word “mohðe”, dating from the 16th century, or the German word “motte”. The word “maða” was used during the same era to describe the moths’ larvae and their habit of devouring clothes when they weren’t being worn. Larvae produce caterpillars, grow in cocoons and emerge as moths, which can fly immediately.

 

Moths v butterflies

Although moths are related to butterflies, there are differences between the two species. The butterflies are known as a “monophyletic” group. This means they all share a common recent ancestor. The current group comprises all descendants of that ancestor.

Moths are not classed as a monophyletic group. Scientists have attempted to group the Lepidoptera into “super-families” of natural groups. However, they have failed, because it has been found some of the groups are simply not monophyletic.

One major physical difference between moths and butterflies relates to their antennae. Butterflies normally have thin antennae, which have a small ball at the end. Moths’ antennae have no ball at the end and are normally feathery.

 

Ultrasonic hearing

Moths have evolved super-sensitive hearing, enabling them to pick up bats’ ultrasonic probes. This means the moths can avoid becoming the bats’ prey. However, the greater wax moth is unique in that it can hear the highest-known sound frequency on the planet.

Its hearing goes up to around 300 kHz – more than 100 kHz higher than some bats’ hearing. The greater wax moth is armed to deal with anything that the bat can throw at it. Its ears are actually a pair of eardrums on its flanks. Each eardrum vibrates four receptor cells that can hear frequencies of 300 kHz.

This is way beyond the capabilities of any other species, and perhaps more importantly, higher than any bat can squeal. A moth can hear literally any bat approaching, even those whose cries are out of some other moths’ hearing range.

 

Bats v moths

The hearing ability of bats and moths is very keen for both species. Moths are known to have ultrasonic hearing abilities. Their ears are connected to nerve cells and these are tuned to pick up on a range of bat frequencies. Moths’ ears can detect bats from a distance of 30 metres away on average.

Moths can hear bats before the bats sense their presence, and they have excellent defence mechanisms to avoid being eaten. Some moths will change their direction mid-flight if they hear a bat. Others simply close their wings and drop quickly to the ground until the danger has passed. The moths’ ears are so effective that they can reduce the bats’ success rate as a predator by around 50%.

Bats often have massive ears that look out of proportion to the rest of their body. They have the best hearing of all land mammals. They rely on their hearing for night-flying when they make high-pitched squeals known as “ultrasounds”.

People can’t hear most of these sounds, but when the ultrasounds hit something, such as a tree or a wall, they bounce back like an echo so the bats can hear them.

The sounds warn the bats where the objects are and prevent them from crashing into things in the dark. The process is called “echolocation” and most species of bats have this skill.

 

Scientific tests

It was acoustical engineer James Windmill, of the University of Strathclyde in Scotland, who discovered the amazing hearing ability of the greater wax moth, whose Latin name is Galleria Mellonella. He published the results of his studies in Biology Letters.

Working in collaboration with bio-acoustician Hannah Moir, of Leeds University, they played sounds of different frequencies to wax moths. Using high-tech equipment, such as lasers, the researchers measured the vibrations of the moths’ eardrums and recorded the activities of their auditory nerves using electrodes.

It was found that moths reacted most to a frequency of 80 kHz – the usual frequency of courtship calls. When a frequency of 300 kHz was played, the moths’ eardrums vibrated and their neurons reacted, proving that they could still hear this exceptionally high frequency.

Prior to the latest research, it was believed the North American gypsy moth had the best hearing range of any insect. Tests showed it could hear frequencies of 150 kHz. The greater wax moth easily broke this record.

Animal communication expert William Conner, of Wake Forest University in North Carolina, said the astounding findings should prompt researchers to rethink their study methods. Most scientists would never test at such high frequencies and few would even have the necessary equipment. However, Conner says the surprising results should make scientists think about other insects’ hearing capabilities, as the greater wax moth may not be on its own in having super-sensitive hearing.

Humans have a keen sense of hearing, and we are highly sensitive to certain sounds – noise nuisance can have a direct impact on our health.

As experienced acoustic consultants, Sound Planning offers a wide variety of noise control services and products.

For more information, please contact us.

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