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Fig. 21a and b. Snake head display of the caterpillars of the Central and South American sphinx moth, Hemeroplanes triptolemus; and of the chrysalis of the butterfly Dynastes darius (drawn from photographs in Miller et al. 2006).

Although I was working on thermoregulation of the tobacco hornworm Manduca sexta sphinx moths as a graduate student, and was aware of the importance of color in butterfly thermoregulation, I did not give much thought to the occasional “black sheep” caterpillar—one that was black rather than the typical camouflage green. I saw such caterpillars occasionally but passed them off as a curiosity or an aberration to be ignored. Fortunately, others didn’t see it that way, and through studies of this black mutation fundamental discoveries were made about nature versus nurture.

In 1973, Jim Truman and colleagues determined that the black mutant is not just the result of a new gene which codes for more melanin. Instead, the melanin deposition in the caterpillar’s skin results from a lowered level of the juvenile hormone (a key hormone for development in all insects’ metamorphosis as well as in their reproduction). Applying a minute amount of juvenile hormone to a black tobacco hornworm caterpillar reverses its color back to the “normal” green. However, it is apparently not just the amount of the hormone as such that determines the degree of color change. Instead, there is a specific threshold that tips the balance; furthermore, evolution works not by varying the amounts of hormone, but by shifting the threshold where the color change occurs (Suzuki and Nijhout 2006). In a related species, the tomato hornworm, Manduca quinquemaculata, the caterpillars develop black coloration when the temperature is 68°F or colder and green when it is 82°F or hotter. Are the color shifts adaptations to temperature, in which the advantage of sunshine to speed up feeding and growth rate exceeds the disadvantage of potentially being eaten?

We humans cannot change into any radically different body color, body shape, or behavior. We have evolved to maintain a certain homeostasis, or a status quo, that has proved to be adaptive in the past. However, the genes of a butterfly are the same as those in a caterpillar. The difference is which are turned on or off, and when. It’s all “environment” as well—in this case mostly the internal environment that keeps changing through development. Whenever I see a weight lifter, a runner, a mathematician, an actor, a sumo wrestler, or a dancer, I am reminded that, like caterpillars, we do have the capacity to accomplish amazing changes, and these are sometimes in response to simple, subtle cues, which are like switches that control development. No one develops with a complete predetermination of the features and faculties that he or she will eventually come to “own.” To the contrary, although we are built from pretty much the same blueprint, many of our specific, individual “talents” can be activated only if we exceed a certain threshold of effort, but probably this threshold is also specific to each individual. I was reminded of this while training to metamorphose from a slow cold-weather-adapted animal that conserved energy and heat to one who could expend energy at high rates and dissipate heat as fast as possible. If in a caterpillar a mere visual stimulus can change gene expression affecting development, then why not exercise in us?

Once an end result is achieved, it is hard for us to imagine an alternative that has proceeded along a different developmental trajectory without crediting it to magic or “talent.” When we see in others something that we find incomprehensible for ourselves, it is easy to pass this off as “genetic.” Naturally, it is exactly that; but this description still omits the essence of development, the miracle on the miracle. The possibility of individual caterpillars to generate amazingly different forms makes me appreciate what is possible in the debate over nature versus nurture. Much of what we are and become depends on minute subtleties, and that gives me