In Nature we can find many examples of Tunnels used for similar purposes. The silken tunnel of the Wax-moth larva has already been mentioned, and we now come to Tunnels where earth in some form, and not silk, is the material of which they are constructed.
The lowermost figure on the left-hand side of the illustration represents that well-known and most destructive burrower, the Ship-worm (Teredo), which, by the way, in spite of its popular name, is not a worm, but a mollusc. This creature has a peculiar interest for engineering, inasmuch as its mode of working gave Brunel the first idea of subaquatic tunnelling in loose, sandy soil, just as the Victoria Regia leaf gave to Paxton the idea which afterwards developed into the Crystal Palace.
The plan adopted by the Ship-worm is at the same time simple and effective. It feeds upon wood, and gradually eats its way through almost any timber that may be submerged. It does not, however, merely bore its way through the timber, but lines its burrow with a coating of hard, shelly material. Taking this hint, Brunel proceeded in the same fashion to drive his tunnel through the very ungrateful soils which form the bed of the Thames.
He built a “shield,” as he called it, of iron, exactly fitting the tunnel, and divided into a number of compartments, each of which could be pushed forwards independently of the others. In each compartment was a single workman, and, as he excavated the earth in front of him, he pushed forward his portion of the shield, while the interior was cased with brickwork, just as a Teredo tunnel is cased with shell.
Above the Teredo is represented another marine tunnel-maker, as it appears in its burrow.
This is the mollusc popularly known as the Piddock, and scientifically as Pholas dactylus. It may be found abundantly in all our chalk cliffs, boring its tunnels deeply into the stone, and aiding the sea in its slow, but never-ending task of breaking down the cliffs on one side, while it gradually rears them up on another. As the material into which the Piddock burrows is so hard, there is no need for lining the tunnel, as is done by the Teredo. In this point, too, our engineers follow its example. When their tunnels pass through comparatively soft ground, they line it with masonry, proportioning the thickness of the lining to the looseness of the soil. But, when they come to solid rock, they are content with its strength, and do not trouble themselves about the lining.
The mode of action adopted by the Pholas has long been a disputed point, and even now appears to be not quite settled. I think, however, that William Robertson has proved by his experiments that the shell and the siphon are both brought into requisition. The shell perpetually rotates in one direction, and then back again, just like the action of a bradawl, and, by the file-like projections on its surface, rasps away the chalk, converting it into a fine powder. This powder, being of course mixed with water, passes into the interior of the animal, and is ejected through the siphon.
There are many species of Pholas which burrow into various substances, even in floating cakes of wax and resin. The same species, too, will burrow into different substances, and it is worthy of notice that those specimens which burrow into soft ground attain a much larger size, and their shells are in better preservation, than those which force their way through hard rock.
The uppermost figure represents a very remarkable tunnel, having the peculiarity of being built instead of sunk. It is the work of an African Ant belonging to the genus Anomma, and popularly known as the Driver-ant, because it drives away every living creature which comes across its course of march.
There are many Ants which seem to rejoice in the full blaze of the tropical sun, running about with ease on rocks which would scorch and raise blisters on the hand if laid on it, and finding no difficulty in obtaining the moisture needful for the mud walls of their habitations. But the Driver-ants cannot endure the sun, and, unless compelled by necessity, will not march except at night, or at all events during cloudy days. Should, however, they be absolutely forced to march in the sunshine, they construct as they go on a slight gallery, which looks very much like the lining of a tunnel stripped of the surrounding earth. If their path should lead them to thick herbage, sticks, &c., which form a protection from the sun, the Driver-ants do not trouble themselves to make a tunnel, but take advantage of the shade, and only resume the tunnel when they reach the open ground.