Caterpillars and related bugs can fight off insect predators by vomiting a surfactant solution over unwitting attackers, scientists have found.

Grazing insects have long been known to defend themselves against attackers by regurgitating fluid - but the repellent effect had always been attributed to toxins in the liquid that the insects had taken up by eating poison-producing plants.

However, not all plants produce such toxins, says Michael Rostás of the University of Würzburg, Germany. Rostás decided to test his theory that insects which graze on a wide variety of plant species must have developed their own defence mechanisms, independent of their plant host, to tackle attackers.

Along with Katrin Blassmann at the University of Basel, Switzerland, Rostás examined the oral secretions of several caterpillar species, to test whether the plant toxins were needed. Rostás found that toxin-free secretions were enough to fend off predatory ants, because the fluid contains a complex mixture of surfactants. 

'If you only used water, it would just roll off the ant, because the ant cuticle has a very hydrophobic surface,' says Rostás. 'But if you add surfactants, the liquid spreads over the surface of the ant, and at that point the ant lets go of the caterpillar and starts grooming itself.'

Rostás is currently working to identify the surfactants the caterpillars are producing, which includes a range of peptides, he says. 

'These surfactants are probably the first line of defence, and all insects probably have it,' adds Rostás. 'If the insect also feeds on a plant that is quite toxic, that may also help as a second line of defence.'

James Mitchell Crow

Enjoy this story? Spread the word using the 'tools' menu on the left.