At first I see nothing but the inevitable fatigue due to the incidents of exhumation, transport, and confinement in a strange place.  My exiles try to escape:  they climb the wire walls, and finally all take to earth at the edge of their enclosure.  Night comes, and all is quiet.  Two hours later I pay my prisoners a last visit.  Three are still buried under a thin layer of sand.  The other five have sunk each a vertical well at the very foot of the straws which indicate the position of the buried fungi.  Next morning the sixth straw has its burrow like the rest.

It is time to see what is happening underground.  The sand is methodically removed in vertical slices.  At the bottom of each burrow is a Bolboceras engaged in eating its truffle.

Let us repeat the experiment with the partly eaten fungi.  The result is the same.  In one short night the food is divined under its covering of sand and attained by means of a burrow which descends as straight as a plumb-line to the point where the fungus lies.  There has been no hesitation, no trial excavations which have nearly discovered the object of search.  This is proved by the surface of the soil, which is everywhere just as I left it when smoothing it down.  The insect could not make more directly for the objective if guided by the sense of sight; it digs always at the foot of the straw, my private sign.  The truffle-dog, sniffing the ground in search of truffles, hardly attains this degree of precision.

Does the Hydnocystis possess a very keen odour, such as we should expect to give an unmistakable warning to the senses of the consumer?  By no means.  To our own sense of smell it is a neutral sort of object, with no appreciable scent whatever.  A little pebble taken from the soil would affect our senses quite as strongly with its vague savour of fresh earth.  As a finder of underground fungi the Bolboceras is the rival of the dog.  It would be the superior of the dog if it could generalise; it is, however, a rigid specialist, recognising nothing but the Hydnocystis.  No other fungus, to my knowledge, either attracts it or induces it to dig.[6]

Both dog and beetle are very near the subsoil which they scrutinise; the object they seek is at no great depth.  At a greater depth neither dog nor insect could perceive such subtle effluvia, nor even the odour of the truffle.  To attract insect or animal at a great distance powerful odours are necessary, such as our grosser senses can perceive.  Then the exploiters of the odorous substance hasten from afar off and from all directions.

If for purposes of study I require specimens of such insects as dissect dead bodies I expose a dead mole to the sunlight in a distant corner of my orchard.  As soon as the creature is swollen with the gases of putrefaction, and the fur commences to fall from the greenish skin, a host of insects arrive—­Silphidae, Dermestes, Horn-beetles, and Necrophori—­of which not a single specimen could ever be obtained in my garden or even in the neighbourhood without the use of such a bait.