y 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."

[Illustration:
_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."

"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