Earwigs are not nature's most cuddly animals. If attacked, they defend themselves with a large set of pincers (called cerci) on the ends of their abdomen. If they're disturbed enough, they'll ooze a smelly liquid, which is occasionally bright yellow, from their abdomen. Despite this, they'll often spend the winter snuggled up together.

Living in large aggregations can facilitate the spread of disease, so PhD student Tina Gasch and her colleagues at Justus Liebig University Giessen in Germany set out to see whether the earwigs' chemical secretions could be used to combat the microorganisms found in their environments. They collected three different earwig species – Apterygida media, Chelidurella guentheri and Forficula auricularia – to examine the chemical composition of their defensive ooze and to investigate whether it might kill pathogens.

First, the researchers dissected out the sacs that held the defence liquid and then used gas chromatography coupled to mass spectroscopy to identify the chemical components. They found that the secretions of all three earwig species were chemically quite simple, with no more than four unique compounds found in any species' gunge. One compound, 2-ethyl-3-methyl-1,4-benzoquinone, had never been found in insects before, but it is used as a defensive compound by harvestmen arachnids.

Next, the researchers set out to challenge the ooze with a shooting gallery's worth of potential pathogens to see whether the earwigs were mounting a chemical defence when they secreted their defensive compounds. Gram-positive and gram-negative bacteria, two species of fungi and the nematode Caenorhabditis elegans were all doused with the defensive fluid from F. auricularia, which turned out to be an effective pathogen slayer.

Finally, the researchers wanted to know whether the smell from natural aggregations of F. auricularia contained the chemicals found in the defensive emissions. They placed bamboo traps containing swabs that absorbed volatiles in areas where the earwigs liked to aggregate. Analysing the swabs, the team found that the characteristic odour of huddled F. auricularia groups contains the defensive chemicals that they found in the ooze.

While we might not like it much, the stink of the defensive ooze might be the ‘sweet, sweet smell of home’ for earwigs, which provides them with a sterile space free of pathogens.

Multifunctional weaponry: the chemical defenses of earwigs
J. Insect Physiol.