"Why, they have a tube at the end of their bodies, near the tail, through which they breathe; and that part must, you know, be at the top to get air. Besides this, its tail and its breathing tube both end in a sort of funnel, made up of hairs placed somewhat in the form of a star, and covered with oil, so as to keep off water, and these buoy or float it up. When it wishes to sink, it just folds up its funnels, and shuts up in them a little bubble of air, which it breathes under the water; and when it wishes to rise, it opens its funnels, and they float it to the top again. Here is a drawing which will show it to you.
Larva of the common Gnat floating in water, greatly magnified. aa, the body and head of the larva; b, the respiratory apparatus, situated in the tail; c, the larva, not magnified.
"They are hatched in a few days, and then the boat of empty eggs floats about until it is destroyed by the weather. And to show you how good a boat it is, I will tell you what a gentleman did to prove it. Mr. Kirby, who is very fond of natural history, and has written a great deal about insects, says that he put half a dozen of these gnat-boats in a tumbler half full of water, and then poured upon them a stream from the mouth of a quart bottle, held up a foot above them, and he could not sink them. More than that, the water would not stay in them. If you push one to the bottom with your finger, it will come up to the top directly, and you cannot see any water in it."
"Why, this is a noble boat, Uncle Philip, that will never sink."
"True, boys; but listen, and you will find that before it can use its wings the gnat has to sail in another boat still, much more dangerous than this is. After it is hatched, it has to pass through several shapes before it gets to be such an insect as you see. Here is a picture which will show you its different shapes.