nerve-fibrils pass inward toward the centre, and enter ganglia, which in turn are in immediate connection with the great nerves of the balancers. There is but one nerve in the insect's body that is larger than the balancer nerve, and that is the optic nerve; hence, it is natural to infer that the balancer nerve leads to some special sense centre. This centre in my opinion is, unquestionably, the seat of the auditory function.

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.

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.

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]

Seeing that Will's experiment as described by him was incomplete, I took a pair of beetles belonging to the same family (genus Prionus), and determined the true location of their ears by a system of rigid exclusion. These beetles, when irritated, make a squeaking chirrup by rubbing together the prothorax and mesothorax.

When I irritated the female she began to chirrup, and the male immediately turned toward the small paper box in which she was confined. I then removed the antennæ of the male, and again made the female stridulate; the male heard her, and at once crawled toward her, although his antennæ were entirely removed.

This showed conclusively that the organs of audition were not located in the antennæ, as Will supposed and as Lubbock advocates. I then removed the maxillary palpi of the male, after which the insect remained deaf to all sounds emanating from the female.

Again, I took an unmutilated male, which at once turned and crawled toward the chirruping female. I then removed its labial palpi, leaving maxillary palpi and antennæ intact; it heard the female and made toward her. The maxillary palpi were then removed (the antennæ being left in situ), and at once the creature became deaf.

If the maxillary palpi of long-horned beetles be examined, certain vesicular organs, each containing a microscopic hair, will be observed in the basal segments; these, I take it, are auditory vesicles. In some of the Coleoptera I have found auditory rods in the apical segments, though this is by no means a common occurrence. In Cicindelidæ and Carabidæ these auditory vesicles are exceedingly small, and require a very high-power objective in order to be clearly seen.

In justice to other observers I must say, however, that I am inclined to believe that in all beetles the antennæ in some way aid or assist audition, but they are adjuncts, as it were, and not absolutely necessary. It is a matter of easy demonstration to show that some of these insects hear less acutely where they are deprived of their antennæ. I presume they are about as necessary in audition as are the external appendages of the human ear; this, however, is mere supposition, and has no scientific warrant for its verity.

I have purposely said but very little about the senses of touch, taste, and smell in this discussion of the senses in the lower animals. These three senses have been so exhaustively treated by Lubbock in his Senses, Instincts, and Intelligences of Animals, that I could not hope to introduce any new data in regard to them. Graber, Frey, Leuckart, Farre, Hertwig, and a host of others have likewise investigated these senses most thoroughly.

As to the senses of sight and hearing, the matter presented a different aspect. I was confident that I could add somewhat to the knowledge already formulated, consequently I have treated these senses at some length. Technicalities and the details of microscopic investigation, especially microscopic anatomy, have been omitted; they have no place in a work like this.