Scientists have discovered that certain caterpillars manufacture and secrete their own insect repellent, a new study shows. According to a report published today in the Proceedings of the National Academy of Sciences, this defensive mechanism could help explain how the creatures managed to become so widespread.

Scott R. Smedley of Trinity College and his colleagues studied larvae of the European cabbage butterfly, Pieris rapae. Native to North Africa and Eurasia, the species was accidentally introduced into Canada around 1860 and has since spread over most of North America. To try and help explain the creature's extraordinary adaptive fitness, the scientists investigated the properties of the clear oily fluid that the animal secretes, which collects at the tips of tiny hairs that run the length of its body (see image). They tested the chemical composition of the liquid and found that it consisted of compounds that they dubbed mayolenes.

The team further determined that the chemicals are similar to those used by plants to defend against insects and disease. Ants that came in contact with a P. rapae caterpillar spent significantly more time cleaning themselves than did ants exposed to a mealworm lacking this protective mechanism. What is more, when given a choice between two meals--an egg that was doused in a synthetic version of the secretion and a control egg--the ants preferred the control egg, suggesting the caterpillar's fluid serves as a deterrent. In fact, the authors conclude that "the secretion could clearly be effective against arthropods other than ants, including such enemies as wasps, bugs, beetles, spiders, harvestmen, and parasitoids."