Caterpillar stunned by its own peptide.

Scientists have studied the tobacco hornworm (Manduca sexta) for decades, learning about its behavior, physiology and biochemical makeup. But the big, fat caterpillar still holds a few surprises.

In his quest for biologically based weapons against insect pests, toxicologist Gary B. Quistad tried injecting tobacco hornworms with various components extracted from their blood. One had a dramatic effect: It temporarily paralyzed the caterpillar. The insect "gets extremely rigid, as if frozen rockhard," says Quistad. "It will bounce if you drop it. Then, after 20 minutes to a half-hour, it will fully recover."

He and his colleagues at Sandoz Crop Protection in Palo Alto, Calif., went on to pinpoint an unusual 23-amino-acid peptide as the hornworm's innate paralytic compound. Quistad speculates that the peptide may play a role in clotting, normally remaining harmless in the blood. Exposure to air -- or the stress of a needle prick--may activate its paralytic powers by somehow changing its peptide structure, he suggests.

Quistad has since found this or a related peptide in three of 11 other species of insect pests. But the peptide did not cause paralysis when injected bact into their bodies, he reports.