The gills of fishes have a great many blood vessels running through
them. The walls of these blood vessels are very thin, and the oxygen
from the air that is in the water passes into the blood that is in the
gills, and then this blood, all full of oxygen, circulates through the
fish's body.

You see in fishes the blood vessels come into the gills and get the
oxygen.

In insects it is different. There are air tubes running like tiny pipes
all through the gills and into the body of the insect. The oxygen of the
air that is in the water passes out through the walls of these tubes
into the blood of the insect.

Yes, John, in fishes the blood comes to the air, in insects the air goes
to the blood. The air passes into the air tubes of the insects, and thus
is carried all through their bodies.

The blood takes the oxygen out of the air.

Without oxygen in the blood no animal could live.

Now let us go back to our May flies. They remain in the larval state a
year, and some species remain two years. Think of living in the mud for
two long years!

In the mud they creep about, eating, eating, eating. Then some summer
day they leave the mud and swim to the surface of the water.

Pop! they are gone.