Erick Greene has spent half his life up a tree looking for caterpillars and has found an astonishing one.
It undergoes a transformation of its whole body shape, its color and its behavior - depending on what it eats.The geometrid caterpillar, which lives on oak trees in the Southwest, becomes a mimic of what it consumes.
If it feeds on the drooping chains of tufted oak tree flowers called catkins, it is transformed into an imitation catkin: It turns yellow and grows tufts and a row of false pollen sacs.
If the young caterpillar feeds on oak leaves instead of catkins, it is transformed into a twig mimic: It turns a green-gray color and develops a straight body with the few small bumps of an oak twig.
The imitation is so good that Greene, an entomologist at the University of California at Davis, will often stare straight at one and not see it among the catkins: He has to work by feel.
The imitation goes beyond shape and color. The catkinlike caterpillar has a small mouth and head, with weaker muscles suitable for eating the soft pollen sacks of catkins. The twiglike caterpillar has large and powerful jaws suitable for chewing up the leathery leaves of oaks.
And somehow, the caterpillar knows which version it is, so that a twig-mimic set down in catkins will flee, and when it finds a suitable spot on a branch, will stand up and pose, motionless, as a twig. A catkin-mimic set on feed.
This elaborate masquerade is intended to fool birds. It succeeded in fooling biologists until Greene sorted it out last year.
The caterpillar is called a geometer, or by its common name, inchworm, because it moves by repeatedly arching and extending its whole body. After spinning a cocoon and undergoing metamorphosis, it becomes the emerald moth, Nemoria arizonaria, a bright green insect with a thin purple line at the leading edge of its wings.
The trouble the geometer faces is birds. Greene says that birds can clear a tree of thousands of caterpillars in a short time. More than half the entire population is commonly wiped out, making the odds of survival for caterpillars rather poor.
To adapt, the geometers have over eons taken on the looks of the places they frequent. But those looks change.
The mechanism evolution has wrought to do this job is a tannin trigger, Greene discovered. Tannin is a bitter-tasting substance that many plants produce in their leaves as a defense against insect attack.
Greene raises caterpillars to adulthood, and recently he was surprised to find two that looked very different but produced the same emerald moth. Biologists had known of the existence of the caterpillar that was the twig mimic, but not the catkinlike version.
"The signficance of all this, I think, is to show us how very little we know, even about our own back yards," said Greene.