Those brushes along their sides are the gills they breathe with. See the gills moving swiftly back and forth; they look as though the larva wished to swim with them, but this is not why it moves them so constantly. The continual motion of the gills stirs up the water and keeps our larva supplied with fresh air. Nellie is asking what gills are. Well, gills in fishes and in such insects as have gills, and in crabs and lobsters and other creatures that live in the water, are parts that often look like fringes or flat plates. 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. They were so quick about it we could not see what happened. The larval skin burst open and forth leaped the May fly, like a winged fairy from a prison cell. They do not come out slowly and wait for their wings to dry like the dragon fly. [Illustration] They spring out all of a sudden and fly away, leaving their cast-off skin in the pond. Unless their motions were quick they might be snapped up by the fish that are so fond of them. But though they seem to emerge thus quickly into perfect winged May flies, they are not quite done with infancy. They are still wrapped about by a very delicate skin that they have to get rid of. So they fly to a bush near the water and stay a little while until this skin splits and comes off, and they are free. In spite of their quick motions when they spring from the water, many of the May flies fall back into it and are caught by the fish. [Illustration here, as the text is broken]