But the insect makes no mistake: the excavations under my pencil-marks bear witness to that; it always directs its apparatus towards the hollow of a cell. How is it apprised whether the part below is empty or full? Its organs of information are undoubtedly the antennae, which feel the ground. They are two fingers of unparalleled delicacy, which pry into the basement by tapping on the part above it. Then what do those puzzling organs perceive? A smell? Not at all; I always had my doubts of that and now I am certain of the contrary, after what I shall describe in a moment. Do they perceive a sound? Are we to treat them as a superior kind of microphone, capable of collecting the infinitesimal echoes of what is full and the reverberations of what is empty? It is an attractive idea, but unfortunately the antennae play their part equally well on a host of occasions when there are no vaults to reverberate. We know nothing and are perhaps destined never to know anything of the real value of the antennal sense, to which we have nothing analogous; but, though it is impossible for us to say what it does perceive, we are at least able to recognize to some extent what it does not perceive and, in particular, to deny it the faculty of smell. As a matter of fact, I notice, with extreme surprise, that the great majority of the cells visited by the Leucopsis' probe do not contain the one thing which the insect is seeking, namely, the young larva of the Mason-bee enclosed in its cocoon. Their contents consist of the refuse so often met with in old Chalicodoma-nests: liquid honey left unemployed, because the egg has perished; spoilt provisions, sometimes mildewed, or sometimes a tarry mass; a dead larva, stiffened into a brown cylinder; the shrivelled corpse of a perfect insect, which lacked the strength to effect its deliverance; dust and rubbish which has come from the exit-window afterwards closed up by the outer coating of plaster. The odoriferous effluvia that can emanate from these relics certainly possess very diverse characters. A sense of smell with any subtlety at all would not be deceived by this stuff, sour, 'high,' musty or tarry as the case may be; each compartment, according to its contents, has a special aroma, which we might or might not be able to perceive; and this aroma most certainly bears no resemblance to that which we may assume the much-desired fresh larva to possess. If nevertheless the Leucopsis does not distinguish between these various cells and drives the probe into all of them indifferently, is this not an evident proof that smell is no guide whatever to her in her search? Other considerations, when I was treating of the Hairy Ammophila, enabled me to assert that the antennae have no olfactory powers. To-day, the frequent mistakes of the Leucopsis, whose antennae are nevertheless constantly exploring the surface, make this conclusion absolutely certain.