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By Root 750 0

webs) or that of the forest tent caterpillars. Still, it was eye-catching enough. On some maple branches as many as a third of the leaves were folded over, much as one might fold a piece of paper and then glue it together to make an envelope. It was the typical work of one of the thousands of species of microlepidopterans, and I was tempted to let it go at that—I knew a little caterpillar would be inside and feeding. So what? But for some obscure reason I idly opened several of these folded-over leaves, and to my great surprise saw neither caterpillars nor masses of feces. I was puzzled, because caterpillar feeding damage was evident—these caterpillars had eaten the underside of the leaves. Many birds learn to open leaf “envelopes,” and these envelopes could have been easily opened. But if I had been a bird, I would have been disappointed—at least at first. I looked into many of them and wondered: where are the caterpillars?

No caterpillars were visible, but each leaf envelope contained a thin dark object an inch or more in length. At one end it was as thin as the lead in a mechanical pencil, and at the other end it was about one-tenth of an inch wide and had an opening: it was a long, tapering tube. I eagerly broke several of these rather dry and friable tubes open, and again at first found nothing. However, I eventually did find what I was looking for: the tiny, almost transparent caterpillars. They were near the bottom of the tube rather than at the top. It turned out that they retreat to the bottom of their tube as soon as a predator opens the leaf envelopes they are in. But where did the tubes come from?

Fig. 18. Small moth caterpillar that feeds on the undersurface of leaves and makes a tube house in the space created by folding the leaf into a sandwich.

The tubes were made by the caterpillars, and from their own fecal pellets. Thus, these caterpillars were not soiling and spoiling their food, but instead using their own waste to make a retreat to hide in. They silked their fecal pellets onto the mouth of the “door” of their house, to gradually build a wider, longer tube as they grew. The beauty of this caterpillar’s behavior is in the mind’s eye, but in still another caterpillar it also enhances the fall foliage show.

It is mid-October. The aspen leaves have turned a rich deep golden yellow. On a slight breeze after a light frost, the leaves come tumbling down. Unlike those of the red maples, for example, where random blotches of yellow, red, purple, and pink may intermingle, the poplar leaves are uniformly, unvaryingly gold. But under some trees are exceptions—many of the bright yellow leaves have a conspicuous pea-green spot near the petiole, between the mid-vein and the nearby subsidiary vein. The color, and the very specific spot on the leaf where it is always located, catches the eye—you look around and find another, and another. And each time, the green spot is in exactly the same place on the leaf. The singular green spots in otherwise rapidly aging bright yellow leaves took me by surprise. They had to be caused by an outside agent. And they were.

Examining the green spots under a microscope, I could see through the transparent leaf epidermis, and beneath, within the leaf tissue itself, was a little pale green caterpillar with a trail of black fecal pellets behind it. This “leaf miner” caterpillar also rides the leaves down to the ground and then feeds on them. But it is too small to be eaten by birds, and it is probably also too small to be able to chew through a leaf petiole. This caterpillar could surely do its eating and growing early in the leaf’s life cycle, but such a tiny caterpillar would very quickly dry out in summer heat and dry air. It would need to reach moist ground to pupate. It must leave the tree crown to do that, but for such a tiny larva to leave the leaf and enter the atmosphere in the dry summer heat could cause rapid death from desiccation. However, by shifting its caterpillar stage to late in the summer and early fall, when the weather is cooler and more moist and when the tree