"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."
A. May-fly.
B. Nymph of May-fly.
"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."