Certain tropical poison frogs excrete a lipophilic alkaloid on their skin that keeps yellow fever mosquitoes-a common vector of a wide variety of parasitic pathogens-from snacking on their blood. The repellent in question is the azabicyclo compound PTX (+)-251D (shown), which is 10 times more distasteful to mosquitoes than its nonnatural (-)-enantiomer (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0608646103). This example of chemical defense is a consequence of the frog's diet, notes lead author John W. Daly, an emeritus scientist at the National Institutes of Health. "These frogs eat a variety of insects, store the insect's alkaloids, then secrete them as poisons," he says. The repellent acts by targeting sodium channels in sensory neurons. But lest anyone be tempted to add PTX (+)-251D to a topical insect repellent, the researchers note that the (+)-enantiomer injected into mice elicits convulsions and sometimes death. Daly suggests a nontoxic analog might one day be useful to treat tents or clothing.