What sense, then? That of touch? Let us 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, [46] the Silphæ, [47] the Histers, [48] the Dermestes [49] hasten from every side to the spot where lies a little corpse of which 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? 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.