When March comes, the Caterpillars leave their nest and their pine-tree and go on their final trip. On the twentieth of March I spent a whole morning watching a file about three yards long, containing about a hundred of the Caterpillars, now much faded as to their coats. The procession toils grimly along, up and down over the uneven ground. Then it breaks into groups, which halt and form independent processions.
They have important business on hand. After two hours or so of marching, the little procession reaches the foot of a wall, where the soil is powdery, very dry, and easy to burrow in. The Caterpillar at the head of the row explores, and digs a little, as if to find out the nature of the ground. The others, trusting their leader, follow him blindly. Whatever he decides will be adopted by all. Finally the leading Caterpillar finds a spot he likes; he stops, and the others break up into a swarming heap. All their backs are joggling pell-mell; all their feet are raking; all their jaws are digging the soil. Little by little, they make a hole in which to bury themselves. For some time to come the tunneled soil cracks and rises and covers itself with little mole-hills; then all is still. The Caterpillars have descended to a depth of three inches, and are weaving, or about to weave, their cocoons.
Two weeks later I dug down and found them there, wrapped in scanty white silk, soiled with dirt. Sometimes, if the soil permits, they bury themselves as deep as nine inches.
How, then, does the Moth, that delicate creature, with her flimsy wings and sweeping antennæ-plumes, make her way above ground? She does not appear till the end of July or in August. By that time the soil is hard, having been beaten down by the rain and baked by the sun. Never could a Moth break her way through unless she had tools for the purpose and were dressed with great simplicity.
From some cocoons that I kept in test-tubes in my laboratory I found that the Pine Moth, on coming out of the cocoon, has her finery bundled up. She looks like a cylinder with rounded ends. The wings are pressed against her breast like narrow scarfs; the antennæ have not yet unfolded their plumes and are turned back along the Moth’s sides. Her hair fleece is laid flat, pointing backwards. Her legs alone are free, to help her through the soil.
She needs even more preparation, though, to bore her hole. If you pass the tip of your finger over her head you will feel a few very rough wrinkles. The magnifying-glass shows us that these are hard scales, of which the longest and strongest is the top one, in the middle of her forehead. There you have the center-bit of her boring-tool. I see the Moths in the sand in my test-tubes butting with their heads, jerking now in one direction, now in another. They are boring into the sand. By the following day they will have bored a shaft ten inches long and reached the surface.
When at last the Moth reaches the surface, she slowly spreads her bunched wings, extends her antennæ, and puffs out her fleece. She is all dressed now, as nicely as she can be. To be sure, she is not the most brilliant of our Moths, but she looks very well. Her upper wings are gray, striped with a few crinkly brown streaks; her under-wings white; throat covered with thick gray fur; abdomen clad in bright-russet velvet. The tip end of her body shines like pale gold. At first sight it looks bare, but it is not: it is covered with tiny scales, so close together that they look like one piece.
There is something interesting about these scales. However gently we touch them with the point of a needle, they fly off in great numbers. This is the golden fleece of which the mother robs herself to make the nest or muff for her eggs at the base of the pine-needles which we spoke of at the beginning of the story.