Summer World_ A Season of Bounty - Bernd Heinrich [39]
I remember my own first finding of a caterpillar as an exquisite experience. I was in elementary school and was picking berries in the woods. I was startled to discover among the raspberries something very beautiful—a plump green body decorated with red tubercules sprouting short black bristles—and I have been enamored of caterpillars ever since. As a graduate student I chose to study how the tobacco hornworm caterpillar is programmed to handle leaves without moving from its attachment point at the base of the leaf, and without leaving any scraps. Avoiding being eaten is usually an even greater problem for a caterpillar than finding enough food. To discover how caterpillars might escape detection by birds, I first used students as stand-ins for the birds.
I got reacquainted with caterpillars during a summer in the late 1970s at the University of Minnesota Field Station at Lake Itasca, where I helped teach a field ecology course. Each of the three instructors designed “field projects” for our select group of about a dozen graduate students in biology. These field projects had to involve the local flora and fauna, and I was spending a couple of days getting oriented in the local woods, looking for potential projects. It was then that I found the clipped-off partially eaten leaf of a basswood tree.
In my previous fieldwork with bumblebees (in Maine) I had studied individual bees and found that they became specialists, developing individual skills for finding and working on specific kinds of flowers. In any one field with several kinds of flowers, one bumblebee might, for example, search out the clover flowers while ignoring most of the goldenrod. Meanwhile, another in the same field might visit the goldenrod and ignore the clover. The bees maintained their specialties independent of the abundance of other flowers. They developed “search images” of what flowers to look for. Birds (and humans) presumably also use search images to help them find specific caterpillars. It helps to know what you are looking for, but this knowledge usually comes at the cost of not noticing the remainder.
To demonstrate the significance of the search image to my students, I “planted” four stick-mimicking caterpillars (Geometridae) on a poplar sapling, and then gathered the students around it and asked them to search. I told them that there were four caterpillars directly in front of them, but gave no clue of what these looked like. (Two of the geometrids mimicked live green twigs, and two mimicked dead brown twigs.) No “successful foragers” were allowed to give clues to anyone else about what they had found or where they had found it.
I had expected that it might take seconds, or at the most a minute or two, to find any one of the caterpillars, which were only a few inches in front of our eyes. I was indeed surprised to discover that despite their earnest and continuous searching, few of these naive but eager hunters discovered a caterpillar within the first half hour. But those who did eventually find one then located the other, similar one in a minute or less. That is, as predicted, after the students knew what to look for, everyone’s performance increased enormously. This generalization has large implications. If birds search for caterpillars similarly to bumblebees and students, then there is a great advantage for each caterpillar species in having a different disguise from the next: that is, one not included in the existing repertoire of the predator’s search images. This implies that it helps to be rare and different.
There is an enormous variety of caterpillars among the 250,000 species of moths and butterflies. Some species look like leaves or parts of leaves; others resemble twigs, bird droppings,