The Origin of the Invisible Moths

Buckle up kids, this is a wild ride through evolution.

“macro photography of white moth” by Tim Goedhart on Unsplash

As a kid I was fascinated with bats. They could “see” in the dark using echolocation, they were the only flying mammal, they ate everything from blood to fruit to moths.

This was part of the reason I went on to study Biology at undergraduate and go on to get a MRes in biological sciences: I was obsessed with how natural selection drove moth-eating bats to be the ultimate predators. Flying silently at night, falling upon unsuspecting moths, using echolocation to ferret out even the most weaselly of their prey.

Lighting and its effect on bats

One of my most vivid memories is seeing bats flock to streetlights at night like an all-you-can-eat moth buffet, scooping up record numbers of moths every night.

But during my degrees, I learned that moths weren’t just passive prey. They didn’t just wait around to get eaten. Instead, they evolved sophisticated, cunning defences against their predators, some of which are being researched by the military.

This took a lot of different forms as moths evolved different strategies to not be eaten. For example, bats are best at detecting prey using echolocation. Basically, they holler into the nighttime, and the nighttime world hollers back. The bat can tell, based on how long it takes for the echoes of their calls to come back to their enormous ears, where things are, what they look like, and even what their texture is.

“brown bat flying” by Igam Ogam on Unsplash

If you’re a moth, how can you avoid that superpower? By developing special ears which let you hear when bats are coming.

Their ears are specially tuned to the frequency of bat echolocation. Not only that, but certain types of moths have specially developed ears which are tuned to the exact frequency of the species of bat that hunts them exclusively.

But some moths weren’t content with just hearing and avoiding bats. They actually developed the ability to sonar jam, sabotaging bats and hiding in plain sight.

What is sonar jam? To answer that question, put yourself in a bat’s shoes for a minute.

Imagine you fly by night, using echolocation to illustrate the air around you. As the echoes of your cries return to you, you get a painted picture of your surroundings: trees, leaves, and, crucially, moths. It’s dinnertime.

“brown and black bat opening mouth” by Todd Cravens on Unsplash

Now imagine, as you approach the moth, it somehow knows you’re coming. It starts to dodge and weave. No worries, you’re super agile and you quickly close in on your prey.

But now imagine that this moth starts making a noise — a click that’s incredibly quick, clocking in at 4,500 times per second. What effect does that have on your echolocation?

“time lapse photography of water drop” by Jordan McDonald on Unsplash

From a crystal-clear view of the night air around you, suddenly you have a blind spot. This click that the moth is making blankets their surroundings. You can’t “see” anything. You know there’s a moth there, it’s nearly in your grasp, but you just can’t quite make it out. You’re calling and calling as quick as you can to try to refresh the acoustic image, but it’s no use.

Your prey is effectively camouflaged by sound.

When approached by the bats, the moths produced their own ultrasonic clicking sounds at a rate of 4,500 times per second, blanketing the surrounding environment and cloaking themselves from sonar detection. — Joseph Stomberg, Smithsonian.

That, to me, is amazing. The ability to blind your predator by using their own weapons against them is incredible, and this instance is actually unique in the natural world. Only one species of moth can do this (so far discovered by scientists).

But what if you’re a moth, and you don’t have ears? Ears are costly to evolve, and to produce sonar jamming behavior uses up calories and resources which can be scarce. Not all moths can afford to evolve ears and sonar jamming equipment.

Nevertheless, you probably still don’t want to be eaten. So what do you do?

You grow a furry coat to help you hide in plain sight.

“brown fur coat” by Clem Onojeghuo on Unsplash

If you can’t get away from a bat if you’ve been detected, the only escape is to avoid being detected in the first place.

Deaf moths can’t detect bats, so they need to rely on passive defenses to survive. One method is to grow a muffling coat.

Called “acoustic stealth,” this adaptation lets moths evade detection by a whopping 38%. Acting as a lightweight, porous sound absorber, this grants moths the “acoustic stealth” they need to provide protection at all ultrasonic frequencies which are likely to be fatal to them. Bats might call but unless they get lucky with angles and echoes, they’ll have no idea the moth is there, sitting cozily in its fur coat.

A frequent theme in ecological biology is trying to learn to perceive the world in non-human specific ways. Invisibility to us is all about sight. But to bats and moths, whose depends primarily on sound, their invisibility cloak will be based on acoustics rather than visual cues.

It had never even occurred to biologists until recently that moths could use sonar jamming — that was only discovered in 2009. The moth’s furry coat was published in October, 2018.

We’re learning all the time about the incredible and jaw-dropping ways the natural world evolves in ways we can’t anticipate from our biased human perspective.

These two types of invisible moth just show us what an amazing journey it can be.

Biology MSc. Psychology nerd. She/her. Get my FREE 5-day Medium Starter Kit to make money writing about what you love:

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