How butterflies can eat cyanide.

You're a plant. How about deploying built-in cyanide bombs to defend against those that would eat you? It's a nifty tactic that some plants actually use, but some caterpillars are so tough that they eat those cyanide bombs for breakfast.

For lunch and dinner, too. The caterpillars of the tropical butterfly Heliconius sara eat nothing but passion vines, which means they should be dead. That's because the leaves of these vines have evolved powerful pesticides called cyanogens, chemicals that can release hydrogen cyanide gas.

"To our knowledge, this is the first example of an insect that is able to metabolize cyanogens," say Helene S. Engler of the University of Texas at Austin and her colleagues. In the July 13 NATURE, they outline how such a daredevil diet can be possible.

The passion vines, in the genus Passiflora, contain cyanogenic glycosides--essentially sugar compounds hitched to a cyanide group. In the plant, the compounds lie harmlessly in one cellular compartment while an enzyme whose action releases the poison lies in another. When the average leaf-chomping marauder bites down, the compartments burst and their contents mix. Too bad for the marauder.

Analyses of the cyanide-related compounds in both vines and the H. sara caterpillars, however, revealed that caterpillar enzymes prevent the release of poison gas. The caterpillars switch a harmless sulfur-based chemical construct, or thiol, into the place of the cyanide group, thereby inactivating the cyanogen.

Plants still can fight back, according to study coauthor Lawrence E. Gilbert, also of Texas. Some passion vines grow hooked hairs that can impale a caterpillar.