What sense is it that informs this great butterfly of the whereabouts of his mate, and leads him wandering through the night? What organ does this sense affect? One suspects the antennae; in the male butterfly they actually seem to be sounding, interrogating empty space with their long feathery plumes. Are these splendid plumes merely items of finery, or do they really play a part in the perception of the effluvia which guide the lover? It seemed easy, on the occasion I spoke of, to devise a conclusive experiment. On the morrow of the invasion I found in my study eight of my nocturnal visitors. They were perched, motionless, upon the cross-mouldings of the second window, which had remained closed. The others, having concluded their ballet by about ten o'clock at night, had left as they had entered, by the other window, which was left open night and day. These eight persevering lovers were just what I required for my experiment. With a sharp pair of scissors, and without otherwise touching the butterflies, I cut off their antennae near the base. The victims barely noticed the operation. None moved; there was scarcely a flutter of the wings. Their condition was excellent; the wound did not seem to be in the least serious. They were not perturbed by physical suffering, and would therefore be all the better adapted to my designs. They passed the rest of the day in placid immobility on the cross-bars of the window. A few other arrangements were still to be made. In particular it was necessary to change the scene; not to leave the female under the eyes of the mutilated butterflies at the moment of resuming their nocturnal flight; the difficulty of the search must not be lessened. I therefore removed the cage and its captive, and placed it under a porch on the other side of the house, at a distance of some fifty paces from my study. At nightfall I went for a last time to inspect my eight victims. Six had left by the open window; two still remained, but they had fallen on the floor, and no longer had the strength to recover themselves if turned over on their backs. They were exhausted, dying. Do not accuse my surgery, however. Such early decease was observed repeatedly, with no intervention on my part. Six, in better condition, had departed. Would they return to the call that attracted them the night before? Deprived of their antennae, would they be able to find the captive, now placed at a considerable distance from her original position?