February 21, 2007—This larval swallowtail butterfly's clever disguise (left) is literally a load of crap.

In its larval stages, the Asian swallowtail—also known as the Chinese or Japanese yellow swallowtail—mimics the appearance of bird droppings to prevent predators from gobbling it up.

In its last phase of development, the insect turns green (right) to blend into the leaves on which it lives.

A new study shows that a single juvenile hormone is responsible for switching on and off the "spectacular diversity" of the caterpillar's colors, Japanese researchers say.

"[Juvenile hormone] has been known to be involved in molt, metamorphosis, and some other events," study co-author Haruhiko Fujiwara of the University of Tokyo told National Geographic News in an email.

"But in this report, we found that [the hormone] regulates pattern change, which is an original finding."

Hormone levels drop when the caterpillar leaves the bird-droppings stage and begins its green color transformation.

The substance also has the power to shape the caterpillar's texture and pigment distribution, enhancing its disguise.

The study, conducted by Fujiwara and Ryo Futahashi, also of the University of Toyko, appears tomorrow in the journal Science.

—Christine Dell'Amore