It has been demonstrated beyond doubt that analogous hollow hairs, or setæ, are prominent factors of audition in many animals, notably crustaceans, such as the lobster, the crab, and the crayfish, and many of the insect family; hence, it is logically correct to conclude that the hollow hairs on the balancers of flies are likewise auditory hairs. Moreover, there are grouped about the bases of these knobbed organs certain rows of vesicles, which contain auditory rods almost identical in appearance with the auditory rods of the grasshopper. Indeed, I have found those in the upper row of vesicles to be precisely similar in appearance to the rods found in Melanoplus. I have determined that in the horsefly (_Tabanus atratus_) there are six rows of these vesicles, and that they are graduated in size. There are in the knobs of the balancers minute spiracles (I do not think that these have been pointed out before by any other observer) through which air passes into the large, vesicular cells which make up the greater portion of the knobs; spiracles are also to be found in the shafts of the balancers, thus providing an abundance of air to the internal structures of these organs and allowing for the free transmission of sound vibrations. I am well aware of the fact that in considering these organs to be the ears of flies, I antagonize Lee and others who consider them olfactory in character.[20] The position I take in regard to these organs is, however, a tenable one, and one that cannot easily be overthrown. [20] Bolles Lee, _Les Balanciers des Dipteres_; quoted also by Lubbock, _Senses, Instincts_, etc., pp. 110, 111. The ears of Lepidoptera (butterflies) are situated in their antennæ. This fact has been clearly demonstrated by Lubbock, Graber, Leydig, and Wolff. Newport has made an especially exhaustive study of the antennæ of insects; and he, too, places the organs of audition in these appendages.[21] But in Coleoptera my experiments and microscopical researches compel me to assert that I differ somewhat from the conclusions of the above-mentioned authorities. These gentlemen locate the ears of beetles also in their antennæ. Lubbock bases his conclusions on an experiment of Will--an experiment which, if it had been carried a little further, would have demonstrated the fact that the ears of beetles are not in their antennæ, but are, on the contrary, in their maxillary palpi. [21] Newport, _The Antennæ of Insects_, Entomol. Society, Vol. II. Will put a female Cerambyx beetle into a box, which he placed on a table; he then put a male Cerambyx on the table, some four inches from the box. When he touched the female she began to chirrup, whereupon the male turned his antennæ toward the box, "as if to determine from which direction the sound came, and then marched straight toward the female." Will concluded from this that the ears of the beetle were located in its antennæ.[22]