Caddis-worms are able to remain on the level of the water indefinitely with no other support than their house; they can rest in unsinkable flotillas and can even shift their place by working the rudder.
How do they do it? Do their sticks make a sort of raft? Can the shells contain a few bubbles of air and serve as floats? Let us see.
I remove a number of Caddis-worms from their sheaths and put the sheaths in the water. Not one of them floats, neither those made of shells nor those of woody materials. The Worm also, when removed from its tube, is unable to float.
This is how the Worm manages. When at rest, at the bottom of the pond, it fills the whole of the tube of its sheath. When it wishes to reach the top of the pond, it climbs up the reeds, dragging its house of sticks with it; then it sticks the front of its body out of the sheath, leaving a vacant space in the rear, like the vacuum in a pump when one draws out the piston. This promptly fills with air, enabling the Worm to float, sheath and all, just as the air in a life-preserver holds a person up in the water. The Caddis-worm does not need to cling to the grasses any longer. It can move about on the surface of the pond, in the glad sunlight.
To be sure, it is not very talented as a boatman. But it can turn round, tack about and shift its place slightly by using the front part of its body, which is out of the tube, as a rudder and paddle; and that is all it wishes to do. When it has had enough of the sun, and thinks it time to return to the quiet of the mud-bed at the bottom, it draws itself back into its sheath, expelling the air, and at once begins to sink.
We have our submarines—the Caddis-worms have theirs. They can come out of the water, they can dip down and even stop at mid-depth by releasing gradually the surplus air. And this apparatus, so perfectly balanced, so skillful, requires no knowledge on the part of its maker. It comes into being of itself, in accordance with the plans of the universal harmony of things.
CHAPTER III
THE MASON-BEES
At a school where I once taught, one subject in particular appealed to both master and pupils. This was open-air geometry, practical surveying. When May came, once every week we left the gloomy schoolroom for the fields. It was a regular holiday. We did our surveying on an untilled plain, covered with flowering thyme and rounded pebbles. There was room there for making every sort of triangle or polygon.