7 FEBRUARY 2008
Before it becomes a gorgeous, showy butterfly, the swallowtail larva is a master of disguise. Its first four incarnations, called instars, look like, well, bird turds. This pretty well takes them off our menu, and ditto for the birds that might otherwise consider, ahem, plating the caterpillar.
But at the fifth and final instar, the critter does an extreme makeover and begins to look just like a leaf. It trades an appearance that is unappetizing to one that is undetectable (more or less).
And what accounts for the change, apparently, is a decline in a compound called juvenile hormone. When Japanese researchers painted an artificial type of juvenile hormone on the larva early in the fourth instar, 67 percent of the fifth instars looked like bird droppings.
They failed to make the expected, extreme changeover into a leaf-look-alike.
Juvenile hormone turns out to regulate several genes that control how the larva colors itself, and whether it produces hard structures that give it that dunglike decor.
Photo courtesy Ryo Futahashi
Caterpillar camo: It's the hormones speaking!
Turdo-mimicry is a common defensive strategy among insects, says Haruhiko Fujiwara, in the Department of Integrated Biosciences at the University of Tokyo, although it's rarer to see such a radical makeover from turd-like appearance to leaf-camo. He speculates that the jumbo fifth instar, which is six to seven centimeters long, is forced to change its camo strategy, dropping that repulsive appearance in favor of one that mimics its surroundings. The fifth instar would be "rather distinctive to predators, because we do not know such a big bird dung, so the larva should change to another type of camouflage."
To explore the chemical control of the transition, Fujiwara and colleague Ryo Futahashi looked at the balance between juvenile hormone and ecdysone, a hormone that causes molting and metamorphosis (change in shape). "Juvenile hormone has various functions, but the best-known function is to control the molt and metamorphosis in many insects," Fujiwara told us by email. He adds that juvenile hormone is known as a "status quo" hormone, because it usually counteracts the molting hormone.
Photo courtesy Ryo Futahashi.
While new methods for blocking insect growth and development are always interesting, especially those that affect a relative of the pestiferous locust, Fujiwara said his goal was not to control insects, but to achieve a more basic understanding of the regulation of metamorphosis. "Our aim is basically to clarify the developmental mechanisms controlled by this hormone. In this research, we found that the expression of many genes is changed by juvenile hormone simultaneously. This type of gene switch should be used in other animals, and so this research is suggestive for understanding that mechanism in general."
However, juvenile hormone is used to control mosquitoes and other insects by preventing maturation.
Evolution has given butterflies a wealth of pigmentation patterns, and closely related species can look extremely different. But while the patterns on butterfly wings "are very well studied," Fujiwara says, "the pattern on the caterpillar has not been understood well so far." Yet when it comes to avoiding predators, caterpillars need all the help they can get, he adds. "The camouflage pattern is a very important phenotype because they cannot escape from predators by flying away."
- David Tenenbaum
• Juvenile Hormone Regulates Butterfly Larval Pattern Switches, Ryo Futahashi and Haruhiko Fujiwara, Science, 22 Feb. 2008.