Part 12

"When they hatch out, what do they look like?" asked Jack, who grew daily more interested in the creatures about him, and who, in the years to come, was destined to be a great scientist.

"It looks a little like the mother," said Ben Gile, taking out his pipe, "but not much. It goes through a great many changes before it is really grown up. All told, the growth takes from a few months to a whole year. The young one, called a nymph, is an ugly little fellow, dingy black with six sprawling legs, two staring eyes, and a big lower lip which covers up its cruel face like a mask. It is a true ogre, lurking under stones and in rubbish at the bottom of the pond seeking whom it may devour. It eats the smaller and weaker nymphs."

"Oh," said Betty, studying the picture the guide had drawn, "what an ugly, ugly fellow!"

"It changes its skin a good many times, and sometimes it looks a little better while the skin is still clean and light gray. But it soon turns dingy again. See these three little leaf-shaped gills I've drawn?"

"They are like the screw on a steamer," commented Jimmie.

"They are, a little. Well, this chap uses these gills for the same purpose as the steamer uses its screw--to scull through the water."

"What happens when it changes?" asked Jack.

"After the nymph has its full growth, some sunny morning soon after daylight, it makes its way up out of the water on to a stem and waits quietly for the old dark skin to split. Then out crawls a soft-skinned creature with gauzy wings. But the body is so moist and weak it has to wait awhile for the warm sunshine to harden the skin and strengthen the muscle. When this is done the new dragon-fly, with its glistening body, flies out from the pond in the bright, warm light."

"Then does it live forever?" asked Betty.

"No; it dies after twenty-five to forty-five days of its flight. Here, Jack, catch that fellow!"

There was a wild scramble, but every time Jack just missed the dragon-fly. Finally Betty lent him her broad hat, and at last Jack caught the insect.

"Gee! aren't its eyes big?"

"And beautiful, too," said the guide. "They are made up of thousands of facets (a facet is just a small, plain surface) as many as thirty thousand facets in one eye. Some look up, some look down, some look out, some look in; so that there is nothing that escapes the sight of this hawk of the air. Look at the wings on this fellow, and look at the picture I drew for you of the nymph. Well, this fellow's wings begin in the nymph as tiny sacs, or pads, made by the pushing out of the wall of the body. Running all through between the two layers of the wing are thickened lines of chitin, which divide and subdivide, forming this fine network. In the new wing, protected by these thickenings, are air-tubes, which divide and branch into all parts of the wing. But as the wing reaches its full growth most of the air-tubes die." The guide paused. "We are talking too much and fishing too little. Time to go on. Put out the fire, boys. Be sure that it's out. Run water all around it. Now we're off!" And up, up, up the brook they went.

VII

THE SWIMMING-POOL

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."