No tickling: common caterpillars deploy defensive hair. (This Week).

Caterpillars of the European cabbage butterfly, which has invaded most of North America, turn out to bristle with a kind of defense system that scientists have not documented before.

The caterpillars sprout hairs that carry droplets of a novel predator repellent derived from a fatty acid, says Thomas Eisner of Cornell University. Ants tend to avoid the caterpillars or spend an unusual amount of time cleaning themselves after contact, Eisner and his colleagues report in an upcoming issue of the Proceedings of the National Academy of Sciences.

"Here's something new about a species that's dirt-common," marvels entomologist May Berenbaum of the University of Illinois at Urbana-Champaign. Eisner's team named the oddball repellents mayolenes after her. A footnote in the paper clarifies that this is an honor.

The repellent-dotted species, Pieris rapae, hitchhiked to Canada in 1860 from its native Eurasia and North Africa. The adult butterfly's chalky wings, with a dark spot or two, have become a familiar sight coast to coast in the United States. The repellent's defensive powers could easily have sped the species' proliferation, Eisner says.

Soft and slow, caterpillars often turn to chemical warfare, he explains. For example, some carry poisons that their fathers transferred to their mothers along with sperm. Other caterpillars, when attacked, regurgitate the remains of noxious plants.

Researchers had previously documented defensive glandular hairs in beetle pupae, and entomologists have speculated that the little droplets atop the hairs of the cabbage butterfly caterpillars might also offer some kind of protection. Eisner says he knows of no previous attempts to test the idea.

In one behavioral experiment, his team confined a predator, an ant species from the Northeast, with a European cabbage butterfly caterpillar or a mealworm of similar size. The ants probed the caterpillars much less often than they poked at mealworms. In another experiment, the scientists washed the caterpillars with solvent and then offered them to the ant. The deterrent effect had disappeared.

To look into the chemistry behind the deterrence, the researchers collected droplets from hundreds of caterpillars. The deterrent chemicals fell apart easily when exposed to heat and acids, so the team developed novel techniques. The researchers learned that mayolenes are derived from a common fatty substance, linolenic acid. However, the final compounds more closely resembles substances produced in plants' injury responses than in other caterpillar defenses, say the researchers.

Eisner says that he himself can't sniff anything repellant in the mayolenes, much to Berenbaum's relief. "If they're named after me, I sincerely hope they don't smell bad," she says.