Two or three days after the fishing expedition the boys had gathered together at the swimming-pool, Ben Gile with them. They had been racing, and climbing trees, and were very warm. "Come, boys," said the guide, "let's sit down a minute before you go into the water. It won't do to bathe when you're too warm. Look round on the stones under the water and see what you can find."

"Look at this," called Peter; "it's just like a sponge."

"It is a fresh-water sponge."

"I didn't suppose sponges grew in these parts at all," said Jimmie.

"Oh yes, there are many of them in the ponds."

"See this, sir," shouted Jack; "what funny little legs it has!"

"That's a May-fly or shad-fly nymph. He was hiding carefully under that stone and keeping out of the way of the dragon-fly nymph, who would so gladly gobble him up."

"It's prettier," said Jimmie, "than that dragon-fly nymph you drew a picture of."

"So it is. See, here on each side of its body are these fine little gill-plates, moving, moving, moving, so that they may get as much fresh air as possible out of the water. Each gill-plate is a tiny sac, and within these are the fine branches of the air-tubes. It's wonderful the way these creatures breathe."

"Don't they breathe just the way we do?" asked Jack.

"No; throughout the body of an insect is a system of tiny white tubes. Some day we'll look at these tubes under the microscope, and you will see that they are made up of rings. From end to end of the tube is a fine thread of chitin twisted in a close spiral like a spring. It is these little coils which look like rings. The coiled thread holds the little tube open so that the air may pass readily. But your little fellow, Jack, cannot have pores on the sides of the body like the last nymph. It lives under water, and the water would get into its tubes; instead, it has tracheal gills."

"That's a pretty big word," said Peter, looking up at the guide. He was growing impatient, and wished to begin the swim. If he had known what that swim was to mean to him, probably he would not have been so anxious.

"They aren't so hard to understand; they are just little oval sacs, inside of which is a limb of the air tube divided into tiny branches. The fresh air in the water passes through the thin wall of the gill and is taken by the air-tubes to all parts of the body, while the impure air passes out in the water. This is all that breathing means in any creature—a changing of impure for pure air."

"Then that is what my nymph is doing," asked Jack, "when it wiggles its gills so?"

"Just that. Your May-fly nymph, Jack, hatched from a tiny egg first. But it grows rapidly, and splits and sheds its skin sometimes as often as twenty times. During the last few months wings appear, which grow a little larger with each shedding of the skin. Finally, after three years—sometimes three years spent in growing and hiding away from its enemies—the little nymph floats up to the surface of the water. In a few minutes the old skin splits along the back, and from it flies forth a frail little May-fly. Its body is very soft and delicate. Its four wings are of a gauzy texture. At the tip of the body are two long, fine hairs. Its jaws are small and weak, but the life of this little creature is so short that it never eats. Up it flies into the air with thousands of its brothers and sisters, whirls in a mad dance for a few hours, then falls exhausted to the ground to die.

"Well, now I think we'd better go into the water," ended the guide. "You boys can go in just as you are." For three little boys had been sitting undressed in the bright sunshine. "Good for their pores," Ben Gile had told them, which is all very true.

Soon there was the greatest splashing and paddling and shouts of, "My goodness, isn't the water cold!" "Can you swim this way?" "How far can you go, anyway?"

Jimmie and the guide were swimming around near the shore when suddenly, two hundred feet ahead of them, they saw Peter disappear in what they supposed was shallow water. Jack was half-way the distance between the guide and Peter. It did not take him an instant to realize what had happened. But before he could get to the place where Peter had gone down, the lad had come up, struggled, and gone down again. As he came up once more Jack caught him by his curly hair, turned over on his back, holding Peter's head high out of the water, and swam calmly for the shallow place. Once there, the old man took Peter in his arms and hurried to shore, where they rolled him until they had the water out of him. Not a word was said, and modest, quiet Jack did not seem to think that he had been brave.

When Peter opened his eyes he said, "Guess my pores weren't in the right place."