by Whit Gibbons

October 9, 2016

Autumn is a time for caterpillars, and those of the imperial moth are showy and impressive - big around as a thumb and as long as a finger, with formidable looking spikes on the body. Yellow spots resembling portholes are on the body segments along the sides. The one we found was a cinnamon orange color. Many are bright green.

We admired the huge caterpillar wearing its gaudy costume as it crawled onto my hand. I knew it would soon weave a cocoon, enter the pupal stage underground and later turn into a magnificent yellow-and-purple imperial moth, a member of the silkworm family.

What I didn't know about them was more extensive. Why was this one reddish instead of green? Why do they have little hornlike protuberances on the head? What is the function of the yellow spots? You don't need to understand the biology of a plant or animal to wonder and speculate.

Like frogs and toads, moths and butterflies lead a double life. Caterpillars and tadpoles are dramatically different in appearance, feeding habits and other aspects of their ecology from their adult forms. And scientists are a long way from understanding all of the ecological factors involved in that transition.

The ecology of some, perhaps most, insects is complicated. Clearly they have many secrets to reveal, and their small size makes certain observations difficult. The wide variety of caterpillar shapes, colors and ornamentation reinforces the idea that their ecology and behavior are highly complex. Research on a seemingly unimpressive western caterpillar supports this conviction. The study also demonstrates how a single scientist can discover an intriguing biological phenomenon.

Biologist Erick Greene conducted a study on the larvae, or caterpillars, of a small southern Arizona moth that lays eggs on oak trees. The moth has two life cycles during a year, laying eggs that hatch into caterpillars in spring that become moths in summer. These then lay eggs that become caterpillars later in the summer. This behavior is not particularly unusual. What the caterpillars do in the two different seasons is.

Those that hatch in the spring feed on the male flowers, catkins, of the oak trees. The caterpillars not only eat the catkins but also look almost exactly like them. Thus when a caterpillar is having a meal, it looks like the food it is eating - great camouflage for a caterpillar that might otherwise be a tasty morsel for a bird.

But what about the larvae that hatch in the summer, a time when the catkins are gone? To look like a catkin on an oak tree that has only leaves and twigs would be like inviting a bird to dinner. Well, sure enough the moth has a different strategy for that time of year.

The caterpillars eat leaves instead of catkins, and they quickly assume the shape, texture and brown color of a tiny oak twig. The caterpillars from the two different seasons do not vary genetically, so an obvious ecological question for the investigator to pursue was, how does the caterpillar know whether to look like a catkin or a twig?

After conducting feeding experiments, Greene found that the season a caterpillar was born was not important. Instead its future appearance was determined by what it ate. Caterpillars he fed catkins began to look like catkins, even in summer, when catkins would not normally be present. Those fed leaves, which have a different chemical composition in that they have tannins (that are absent in catkins), began to mimic twigs.

As we continue to uncover such intricate ecological strategies in something as simple as a moth, we realize how much we do not know. How many wonderful discoveries are out there waiting to be revealed? With greater understanding of ecosystems and their interconnecting parts, we learn to appreciate our natural environments. And the really pressing questions remain: Why are some imperial moth caterpillars cinnamon orange instead of green? And what on earth is the purpose of those yellow portholes, other than to look pretty?

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