THE DRIVER-ANT. 201 



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 hy 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 corite 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, convert- 

 ing 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 sun- 



