Researchers have translated odours emitted by insects to defend themselves against predators into sounds. This allowed them to compare the effect of such odorous cocktails on predators with the effect of sounds on humans. 'We sent the volatiles via an algorithm to a synthesizer and then tested the sounds on volunteers,' says entomologist Jean-Luc Boevé (RBINS). 'People reacted to the sounds just as strongly or weakly as the predators to the smells.'

Chemical signals play a crucial role in the insect world, including as a defence weapon. Take the larvae of sawflies. They are often attacked by ants. The larvae try to keep them at bay by emitting a cocktail of chemical substances that the ants cannot tolerate. Many insect species have similar defence tactics. But how do you measure the effect of that smell on the predator?

There are tests in which ants can perceive substances separately or in a mixture, and their response is measured. But that can be difficult: you need to find sometimes rare insect species in the field and/or rear them in the lab. Entomologist Jean-Luc Boevé (Royal Belgian Institute of Natural Sciences) and informatics engineer Rudi Giot (Institut Supérieur Industriel de Bruxelles) chose an alternative and more original approach: sonification.

Audible chemicals

If you know from a prey insect the chemical substances and their concentrations, you can convert them into sounds. ‘Take a small molecule, such as acetic acid, that evaporates very quickly,' says Boevé. ‘We gave it a high tone, larger molecules a lower one. Other characteristics influenced the duration or the timbre. Then, the higher or lower concentration of a substance is reflected in its volume: louder or quieter.'

Boevé and Giot sent the chemical parameters through a synthesizer and allowed volunteers to hear the single and the mixed sounds. They measured how far the volunteers were walking away from the speakers. Some subjects described certain sounds as unpleasant and frightening. Some sounds would indeed fit in the soundtrack of a horror film. Boevé: 'To our surprise, the tests showed that the humans reacted against the sounds as the ants did against the odours.’

The researchers hope that this method can be complementary to existing techniques for testing defensive odours in insects. 'Sonification is already used to recognize earthquakes in seismological data, or hacked networks in internet data,' says Giot.

The study is published in the journal Patterns.

Soundclips (below)

You first hear the sound clips from three separate substances, cis,trans-dolichodial (lasts 22 seconds), benzaldehyde (9 seconds) and heptacosane (19 seconds), and then the chemical secretion emitted by three sawfly species: Craesus septentrionalis (24 seconds), Nematus lucidus (22 seconds), and Nematus pavidus (22 seconds)