The grown-up May flies mate, and then the female drops her eggs on the surface of the water. When she does this a fish will very often jump up and seize her, for fish are very fond of May flies, and lucky are the May flies to escape these ravenous enemies.
The eggs are heavy and sink to the bottom, where they hatch into these queer-looking larvæ that eat and grow and shed their skin just like the dragon fly larvæ.
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.