A larva of swallowtail butterfly Papilio xuthus which resembles bird droppings (left) in the early stages of development and one that resembles its host leaves (right) just before it transforms into a butterfly is seen in this undated handout photo. Japanese scientists have discovered that a hormone known as juvenile hormone is the secret behind these dramatic changes. REUTERS/Handout/Ryo Futahashi

CHICAGO (Reuters) - A hormone is the secret behind the unusual ability of young swallowtail caterpillars to disguise themselves as bird droppings and then copies of the leaves they live on before becoming butterflies, Japanese researchers found.

Writing on Thursday in the journal Science, the researchers said a special hormone -- juvenile hormone -- keeps larvae of the butterfly Papilio xuthus, which is commonly found in Japan, in their black and white bird-excrement camouflage.

As they reach the last stage of caterpillar development, levels of this hormone drop, triggering a transformation into the green leaf phase.

“We found that juvenile hormone works as a switch for the camouflage pattern. That is a novel aspect of this hormone,” Haruhiko Fujiwara of the National Institute of Agrobiological Sciences in Japan, who worked on the study, said in an e-mail.

Juvenile hormones are known to regulate many aspects of insect development including molt -- when an insect sheds its outer shell -- and metamorphosis -- as when a caterpillar becomes a butterfly, he said.

What Fujiwara and colleagues discovered, however, was that juvenile hormone also appears to govern this camouflage process. He said the hormone may regulate genes involved in color, pattern and surface formation.

As for the bird-poop disguise, Fujiwara said it likely keeps the larvae safe from hungry birds until they are more mobile, but they did not study this.