Caterpillar disguise: you are what you et.

Caterpillar disguise: You are what you eat

A biologist has found what he calls the "best worked-out" example of diet influencing an animal's development: a caterpillar that grows to look like either a flower or a twig, depending on what it eats after hatching. The caterpillar provides a good model for studying how environmental cues turn genes controlling development on and off, a subject about which little is understood, says discoverer Erick Greene of the University of California, Davis.

Although several insects develop into different forms depending on external variables, this is the first known case in which diet dramatically influences an individual insect's appearance, Greene says. Nemoria arizonaria caterpillars that hatch in the spring feed on an oak's male flowers, called catkins, and soon begin to take on catkins' golden color and fuzzy appearance. They also develop rows of dots resembling catkin pollen sacks on their backs. In stark contrast, summer-born N. arizonaria--hatched after catkins have disappeared -- are greenish-gray and look like oak twigs, Greene says.

To test how three environmental variables might influence the caterpillar's appearance, Greene subjected eight groups of recently hatched N. arizonaria to different temperature, day-length and diet regimens for 15 days. He found that only the diet, composed of either catkins o r leaves, mattered. Then, using artificial diets, he discovered that plant chemicals known as tannins alone will prompt a caterpillar to develop a twig-like appearance. Greene still does not know which tannins are important or how tannin levels affect develoment.

Although catkin types survive better than twig types, the twig-like caterpillars probably maintain their evolutionary foothold because they allow the species to produce a second yearly brood, Greene suggests in the Feb. 3 SCIENCE. Because female N. arizonaria don't wait a full year to become moths and lay eggs, catkin mimics always produce twig-like young, whereas twig mimics always produce offspring destined to become catkin-like.

Greene discovered the charlatan catkin while studying insect-eating birds. The moment came when "one of the catkins started to walk away from me," he recalls. "I j ust about fell out of the tree when I found it."