Besides, there are grave objections in the case of our Wasp. Smell is a passive rather than an active sense; it does not, like touch, anticipate the impression: it receives it; it does not inquire after the scented effluvium: it accepts it when it comes. Now the Ammophila’s antennæ are always moving: they investigate, they anticipate the impression. The impression of what? If it were really an impression of smell, repose would serve them better than a perpetual quivering.
But there is more to be said: the olfactory sense goes for nothing when there is no smell. Now I have tested the Grey Worm for myself; [[345]]I have given it to young nostrils to sniff, nostrils much more sensitive than mine: not one of us has perceived the faintest trace of smell in the caterpillar. When the Dog, famed for his scent, becomes aware of the truffle underground, he is guided by the tuber’s savour, which is highly appreciable by ourselves, even through the thickness of the soil. I admit that the Dog has a more subtle sense of smell than we have: it is exercised at greater distances, it receives more vivid and lasting impressions; nevertheless, it is impressed by odorous effluvia which becomes perceptible to our own nostrils under the proper conditions of proximity.
I will allow the Ammophila, if you like, a scent as delicate as that of the Dog, more delicate even; but still a smell is needed; and I ask myself how that which is inodorous at the very entrance to our nostrils can be odoriferous to an insect through the intervening obstacle of the ground. The senses, if they have the same functions, have the same excitants, from man to the Infusoria. No animal, so far as I know, can see clearly in what to us is absolute darkness. True, it may be said that, in the zoological progression, perception, always fundamentally the same, has varying degrees of power: this species is capable of more and that species of less; what is perceptible to one is imperceptible [[346]]to another. This is perfectly right; and yet the insect, generally considered, does not appear to possess exceptional keenness of scent: the effluvia that attract it are perceived without a sense of smell of unusual delicacy. When Dermestes, Silphæ and Histers pour into the chalice of a carrion-scented arum lily, never to come out again; when swarms of Flies buzz around a dead Dog’s blue and swollen belly, the whole neighbourhood reeks with the stench. It hardly requires a scent of exquisite accuracy on the insect’s part to discover putrid meat and rotten cheese. Wherever we see its hordes gather, with scent for their undoubted guide, we ourselves are cognizant of a smell.
There remains hearing. This is another sense about which entomologists are not adequately informed. Where is its seat? In the antennæ, we are told. Those fine, quivering stalks would seem fairly well suited to be put in motion under the impulse of sound. In that case the Ammophila, exploring the region with her antennæ, would be warned of the presence of the Grey Worm by a slight noise coming up from the ground, the noise of the mandibles nibbling a root, the noise of the caterpillar wriggling its hind-quarters. What a faint sound and how difficult to transmit through the spongy cushion of the earth! [[347]]
It is less than faint, it is non-existent. The Grey Worm is nocturnal in its habits. By day it skulks in its lair and does not stir. It does not nibble either; at least, the Grey Worms which I unearthed upon the Wasp’s indications were nibbling nothing, for the very simple reason that they had nothing to nibble. They were completely motionless and therefore silent in a layer of earth devoid of roots. The sense of hearing must be rejected with that of smell.
The question recurs, more abstruse than ever. How does the Ammophila go to work to recognize the spot beneath which the Grey Worm lies? The antennæ are, beyond a doubt, the organs that guide her. They do not, in this case, act as olfactory instruments, unless we admit that their dry and tough surface, which has none of the delicate structure required for the ordinary sense of smell, is nevertheless capable of perceiving scents that are non-existent to us. This would be equivalent to admitting that coarse tools tend to perfection of work. Nor do they act as instruments of hearing, for there is no sound to be discerned. What then is their function? I do not know and I despair of ever knowing.
Inclined as we are—and it could not well be otherwise—to judge all things by our standard, the only one in any way known to us, we [[348]]attribute to animals our own means of perception and do not dream that they might easily possess others of which it is impossible for us to have an exact idea because there is nothing like them in ourselves. Are we quite certain that they are not equipped, in very varying degrees, for the purpose of sensations as foreign to ourselves as the sensation of colours would be if we were blind? Has matter no secrets left for us? Are we so very sure that it is revealed to the living being only by light, sound, taste, smell and touch? Physics and chemistry, young though they be, already declare to us that the dark unknown contains an enormous harvest, in comparison with which our scientific sheaf is the merest penury. A new sense, perhaps that which dwells in the grotesquely exaggerated nose of the Rhinolophus,[5] perhaps that which dwells in the antennæ of the Ammophila, would open to our search a world which our physical structure no doubt condemns us to leave for ever unexplored. Cannot certain properties of matter, which have no perceptible action upon us, find a receptive echo in animals, which are differently equipped?
When Spallanzani,[6] after blinding some Bats, [[349]]released them in a room converted into a maze by means of cords stretched in every direction and of heaped-up brambles, how were those animals able to find their way about, to fly quickly, to move to and fro, from end to end of the room, without hitting the interposed articles? What sense analogous to any of ours guided them? Would some one tell me and, above all, make me understand? I should also like to understand how the Ammophila infallibly finds her caterpillar’s burrow with the aid of her antennæ. It is not a case of the sense of smell: we should have to presume it to possess an unparalleled delicacy, while recognizing that it is exercised by an organ in which no provision seems made for the perception of smells.
What a number of other incomprehensible things do we not ascribe to the insect’s sense of smell! We are satisfied with a word: the explanation is ready-found, without laborious search. But, if we care to consider the matter thoroughly, if we compare the requisite array of facts, then the cliff of the unknown rises abruptly, not to be climbed by the path which we insist on following. Let us then change our path and admit that animals may have other means of information than our own. Our senses do not represent the sum total of the methods [[350]]whereby an animal communicates with that which is not itself: there are others not capable of comparison, however remote, with those which we possess.
If the act of the Ammophila were an isolated fact, I should not have lingered over it as I have done; but I propose to speak of others stranger still, which will carry conviction to the most exacting mind. After relating them, therefore, I shall return to the subject of special senses, irreducible senses, unknown to us.