Insects use stinky secretions to deter predators. Scientists converted those smells into sounds that humans hate - take a listen.

• When some insects are scared or threatened by predators, they secrete smelly compounds.
• Scientists converted the secretions into sounds, creating an eerie melody that's unpleasant to humans.

When sawfly larvae are threatened by predators like ants, they emit a cocktail of nasty smells to defend themselves. These secretions can irritate a potential foe's antennae or nose.

Scientists wanting to study these smelly compounds - to understand which aspects of them deter predators, say, and why - face numerous challenges. Orchestrating meetups between sawflies, a wasp-like insect, and ants in the lab is expensive and logistically difficult. There's also a very limited amount of the insects' secretions.

So two researchers in Brussels took a different approach: They converted the chemical defenses of 16 sawfly species into sound using a synthesizer in order to study those instead.

The results of that process, known as sonification, are anything but pleasant to the human ear, according to a study published last week in the journal Patterns. The researchers played the sounds for people, and found that they disliked them as much as ants hate the odors.

"By listening to them for the first time, I rather imagined them like some short and intriguing 'space sounds,'" Jean-Luc Boevé, an entomologist from the Royal Belgian Institute of Natural Sciences and co-author of the study, told Insider.

Take a listen. This is the sound of the chemical defense of one particular sawfly species, the Birch sawfly or Craesus septentrionalis. It's reminiscent of a haunted house soundtrack.

AylinWoodward15 · Full defensive chemical secretion of a sawfly (Craesus Septentrionalis converted into sound

Synthesizing a scent into sound

Boevé's research primarily focuses on direct analyses of the chemicals sawflies emit when threatened, but on the side, he's an amateur musician and composer. That's in part why he decided to try the sound approach - he first conceived of the idea in 2009.

"To be honest, I considered the sonification project so far-fetched myself that I set the project aside, sometimes for several months," he said in a press release.

But once Boevé and his co-author, Rudi Giot, finally got started on it, they chose 16 sawfly species' secretions to translate into sound. First, they parsed out which molecules were present in each smelly cocktail and in what amounts. Then they assigned various characteristics of those molecules - like their weight - a corresponding pitch, duration, and tone quality.

For example, smaller molecules like the acetic acid found in vinegar evaporate quickly, so Boevé and Giot assigned them high-pitched sounds. Larger molecules were given lower-pitched sounds. In total, the scientists created individual audio profiles for 20 molecules.

The sound below, for example, is of a dolichodial molecule, a compound also found in essential oils.

AylinWoodward15 · A single molecule portion of the defensive chemical secretion of a sawfly converted into sound

They then combined the sounds of each molecule present in a sawfly's chemical odor to construct the insect's soundtrack. If a molecule was highly concentrated in the insect's secretion, they assigned it a louder volume.

Boevé said he hopes the sonification process will give entomologists like him a new way to compare sawflies' chemical defenses with those from other insects. It may also offer researchers clues about which molecules repel predators most.

Unpleasant, scary sounds for humans

To test out the audio clips they created, Boevé and Giot examined people's reactions to the sounds and compared them to ants' reactions to the original scents.

They played the 16 secretion soundtracks, as well as the 20 molecule sounds, through speakers to about 50 study participants. Then the scientists measured how far people backed up to get to a "comfortable position" away from the noise.

Most of the study volunteers told Boevé and Giot that the high-pitched aspects of the soundtracks, as well as the volume, were what made them retreat. They described the sounds as unpleasant, even scary.

"Both test organisms literally moved away from a chemical versus audio source," the study authors wrote.

In other words, humans' reactions to the sounds resembled ants' responses to the odors.

"The correlation was a major surprise," Boevé said.