Chapter xix
AN UNKNOWN SENSE
I have described the Ammophila’s hunting tactics in detail. The facts which I ascertained seem to me so rich in results that, even if the harmas laboratory supplied me with nothing more, I should think myself indemnified by this one observation. The surgical methods adopted by the Wasp with the object of paralysing the Grey Worm are the highest manifestation in the realm of instinct that I have hitherto met. This inborn science is eminently calculated to give us food for thought. What a subtle logician, what an unerring operator is that unconscious physiologist, the Ammophila!
He who would witness these marvels for himself can hardly count on what a country walk may happen to show him; besides, if the lucky opportunity did present itself, he would not have time to profit by it. An observation, which I kept up for five hours on end, without even then managing to complete the experiment and obtain the proofs which I anticipated, [[342]]is one that, to be properly conducted, should be made at leisure in one’s own garden. I owe my success, therefore, to my rustic laboratory. I make a present of the secret to whosoever would continue those magnificent studies: the harvest is inexhaustible; there will be sheaves for all.
When we follow the Ammophila’s hunting in the due sequence of her actions, the first question that suggests itself is this: how does the Wasp go to work to recognize the spot beneath which the Grey Worm lies?
There is nothing outside, nothing, at least, perceptible to the eye, to indicate the caterpillar’s hiding-place. The soil that conceals the quarry may be grassy or bare, flinty or earthy, smooth or seamed with little cracks. These varieties of appearance are matters of indifference to the huntress, who prospects every spot without showing preference for one more than another. At no place where the Wasp stops and digs with some persistency do I see anything particular, in spite of all my attention; and yet there must be a Grey Worm there, as I have but now convinced myself, five times in succession, by lending a helping hand to the insect, which was at first discouraged by a task out of proportion to its strength. Sight, therefore, is certainly out of the question here.
What sense, then? That of touch? Let us [[343]]inquire. Everything tells us that the organs of search are the antennæ. With their tips, bent like a bow and quivering with a continual vibration, the insect tests the ground, giving a number of little taps. When some crack shows, the restless threads enter and sound it; when some grass-tuft spreads its tangled root-stock along the ground, the quivering of the antennæ redoubles as they grope among its knots and angles. Their tips are applied for an instant to the spot explored, moulding themselves, so to speak, upon it. They suggest two tactile filaments, two long fingers of incomparable mobility, which gather information by feeling. But the sense of touch can play no part in revealing what is underground: the thing to be felt is the Grey Worm; and the worm is lying snug in its burrow, at a depth of some inches below the surface.
We thereupon turn our thoughts to the faculty of scent. Insects, there is no denying, possess the sense of smell, often very highly developed. The Necrophori,[1] the Silphæ,[2] the Histers,[3] the Dermestes[4] hasten from every side to the spot where lies a little corpse of which [[344]]the ground is to be purged. Guided by scent, these grave-diggers hurry towards the dead Mole.
But, while the presence of the olfactory sense in insects is indisputable, we still ask ourselves where it is seated. Many declare that the seat is in the antennæ. Let us admit this, though it is difficult to understand how a rod consisting of horny segments, jointed end to end, can fulfil the office of a nostril which is so very differently constructed. The organization of one apparatus having naught in common with the other, can the impressions received by both be of the same nature? When tools are dissimilar, do their functions remain alike?