by Whit Gibbons

August 7, 2005

I never cease to marvel at how complicated the ecology of some, perhaps most, animals really is. Insects in particular still have many secrets to reveal, in part because there are so many species yet to be discovered, and also because their small size makes certain observations difficult. As more and more caterpillars appear over the course of the summer and into fall, the wide variety of shapes, colors, and ornamentation reinforces the idea that caterpillars may be highly complex in ecology and behavior. The following discovery supports this conviction. It also highlights some of the amazing discoveries modern scientists are capable of making.

The larvae, or caterpillars, of a particular species of moth that lives in southern Arizona had never been described before. Then, a scientist discovered that they live on oak trees and have two life cycles during a year. Adults first become active in late winter and produce eggs that hatch into larvae in spring. Adults are not seen again until summer, when they lay eggs and larvae begin hatching soon after. So far, this may not sound particularly unusual, certainly not that different from a lot of other insects. But what the caterpillars do at the two different times of year is the intriguing part.

Larvae that hatch in the spring feed on the male flowers of the oak trees. These yellow oak flowers, or catkins, are familiar to anyone who has taken a careful look at an oak tree in the spring. Not only do the moth caterpillars eat the catkins, they look almost exactly like them. Thus, when a caterpillar is having a meal, it looks like the food it is eating. Obviously, this is great camouflage for a caterpillar that, if it were easily seen, might otherwise become a nice morsel for a bird.

So, this species of moth has developed a fantastic system of providing its young with food while at the same time protecting them from predators. But wait. What about the larvae that hatch out in the summer, a time when all 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 larvae 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 to look like a catkin or like a twig? The investigator did experiments by raising caterpillars to find out. He captured female moths that were ready to lay eggs in April and in July. He also collected and froze catkins and leaves of oak trees. Then, when eggs hatched at the two different times of year, he was able to feed half the larvae one diet and half the other.

As it turned out, the season a caterpillar was born was not the important factor. Instead its future appearance was determined by what it ate. Thus, caterpillars that were fed catkins began to look like catkins, even if it were summer, when catkins would not normally be present. Those fed leaves, even at a time when leaves would not be present, began to look like twigs.

Further experiments and chemical testing suggested that caterpillars were responding to particular compounds, called tannins, that are present in the oak leaves. Therefore, a larva eating leaves containing tannins assumes the shape of a twig. If the tannins are absent from the diet, as in the catkins, the larvae develop into the catkin type of caterpillar.

As we continue to find out about such intricate schemes of ecology and evolution, we begin to realize how much we do not know. How many wonderful discoveries are out there waiting to be revealed? With greater understanding of ecosystems and all the interconnecting parts, perhaps we will learn to better appreciate our natural environments.

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