Jaws of Mason-wasp.—Greatly magnified.
"I do not know, boys, that the masons we have been talking of, show us any tools like those with which men who are masons work; but they show us, at any rate, how to make mortar by kneading or working it together; and they certainly show us that we were not the first who built walls. But there is another kind of mason who works in stone. He picks out the stones which are of proper size, and he fastens them together with mortar really as men do."
"Pray tell us of him, Uncle Philip."
"I will. The insect I mean is the caddis-worm, which is to be found sometimes in ponds, and very often in springs of fresh water. There are several sorts of them, but the one I am thinking of now, builds a stone house to live in. These worms are in the habit of making a little tube, sometimes of sand, or shell, or wood, or leaves, or stones, to live in; and their skill consists in joining these perfectly, and making them stick together. But we are talking now of the caddis-worm that uses stone. What the worm has to do is to make a tube out of small stones, that shall have a hollow about as large as a wheat straw, and be perfectly smooth inside. This is a pretty hard task—at least it would be very hard to us. When the stone-mason wishes a stone of a particular size or shape, and cannot find it, he takes his hammer and breaks one until it will suit; but the caddis-worm has no hammer, and must take the stones just as it finds them. The little insect then has to pick out a great many stones before he gets the right one, because they have so many little rough points about them that it is very difficult to get those which will make the tube perfectly smooth inside. Remember, too, that the bottom or lower side of this stone case has to be pretty nearly smooth, so that the worm can drag it along on the bottom of the spring or pond (for it never comes out of it), and you will see that the picking out of the stones alone is no trifle. But besides this, it has to fasten them together with mortar."
"And can the worm really do this, Uncle Philip? Will not the water wash the mortar all away?"
"It certainly would if it were like common mortar. It was a long time that men lived before they found out a mortar that would remain, and grow hard under water. When they want to build a wall that is to be under the water, they use a cement which is called pozzolana; it is made of lava out of a volcano, and is water-proof. Our caddis-worm has a cement too, which is better than pozzolana, and though it has been tried, it cannot be melted or dissolved in water. Here is a drawing of the stone nest of a caddis-worm."
