Tracking down insects' molting hormones.

During each of the many stages an insect larva passes through before becoming an adult, it must shed its outer shell, or cuticle. Scientists know that a brain peptide called eclosion hormone initiates the events that culminate in this molting, or ecdysis.

Now, a study of the tobacco hornworm (Manduca sexta) suggests that the hormone works by setting off another peptide, which actually triggers ecdysis. Dusan Zitnan and his colleagues injected a synthetic version of the peptide into larvae, pupae, and adult insects. The early stages of ecdysis began within 2 to 10 minutes, they report in the Jan. 5 Science.

Zitnan, now at the Slovak Academy of Sciences in Bratislava, Slovakia, and his coworkers conducted the research at the University of California, Riverside.

"The new report . . . indicates that the long-held view that [eclosion hormone] is the sole mediator of ecdysial behavior and physiology is too simplistic," asserts James W. Truman in an accompanying comment. Researchers now need to define how eclosion hormone and the newly discovered peptide work together, adds Truman, a zoologist at the University of Washington in Seattle.

During their research on molting, Zitnan and his colleagues found that certain glands expand just before ecdysis and shrink once ecdysis has begun. They discovered in those glands cells that produce a peptide and release it into the insects' bloodlike fluid, called hemolymph, immediately before ecdysis starts. They named the peptide M. sexta ecdysis-triggering hormone.