Sometimes the hair is not hollow; in this case, the root of the hair is intimately associated with nerve-filaments which take up vibrations.

It is highly probable that the majority of the lower animals, especially those which are sound-producers, can hear just as we hear. It is also highly probable that the so-called deaf animals can hear, just as we hear when we have either been born deaf, or through disease have lost the power of hearing—by feeling the sound waves.

Owing to our own lack of acuteness, all of the problems involved in this question of audition in the lower animals will, probably, never be definitely settled; yet, reasoning by analogy, we can, approximately, solve some of them.

By far the larger number of entomologists locate the auditory organs of insects in their antennæ. I have only to mention the names of such men as Kirby, Spence, Burmeister, Hicks, Wolff, Newport, Oken, Strauss, Durkheim, and Carus, who advance this opinion, to show what a formidable array of talent maintains it. Yet my observations lead me to believe otherwise, though these authorities are in part correct. As far as Lepidoptera are concerned, and certain of Hemiptera, they are right—the antennæ in these creatures are the seat of the organs of audition. But in Orthoptera, in most of Coleoptera, Hymenoptera, and Diptera, and in certain bugs (Hemiptera), they are located elsewhere. The habit that almost all insects have of retracting their antennæ when alarmed by noise, or otherwise, has done much to advance and strengthen the opinion that these appendages are the seat of insect ears; yet I am confident that in nine cases out of ten the antennæ are retracted through fear of injury to them, and not through any impression made on them by sound. The antennæ are the most exposed and least protected of any of the appendages or members of the insect body; hence their retraction by insects when alarmed is an instinctively protective action. They shelter them as much as possible in order to keep them from being injured. Again, although the antennæ of most insects are provided with numerous sensitive hairs, or setæ, we have no right to assume that these hairs are auditory; no "auditory rods," otoliths, etc., are to be found generally in antennæ, yet there are exceptional instances. Leydig found auditory rods in the antennæ of Dyticus marginalis (Furneaux[17]), the giant water-beetle, and I myself have observed them in Corydalis cornuta and other neuropterous insects. I am inclined to believe that the entire order of Neuroptera has antennal ears, and should therefore in this respect be classed with Lepidoptera.

In grasshoppers and crickets the ears are situated in the anterior pairs of legs. If the tibia of a grasshopper's anterior leg be examined, two (one before and one behind) shining, oval, membranous disks, surrounded by a marginal ridge, will be at once observed. These are the tympana or ear-drums of the ear of that leg. Where the trachea, or air-tube, enters the tibia it becomes enlarged and divides into two channels; these two channels unite again lower down in the shaft of the tibia. The tracheæ of non-stridulating grylli are much smaller than those of sound-producing grasshoppers. The same may be said of the tibial air-tubes of the so-called dumb crickets. In grasshoppers and crickets the ear-drums lie bathed in air on both sides—the open air on the external side and the air of the air-tube, or trachea, on the inside. Lubbock calls attention to the fact that "the trachea acts like the Eustachian tube in our own ear; it maintains an equilibrium of pressure on each side of the tympanum, and enables it freely to transmit atmospheric vibrations."

In grasshoppers the auditory nerve, after entering the tibia, divides into two branches, one forming the supratympanal ganglion, the other descending to the tympanum and forming a ganglion known as Siebold's organ. This last-mentioned ganglion is strikingly like the organ of Corti in our own ear, and undoubtedly serves a like purpose in the phenomenon of audition. The organ of Corti is composed of some four thousand delicate vesicles, graduated in size, each one of which vibrates in unison with some particular number of sound vibrations. The organ of Siebold in the grasshopper's ear begins with vesicles, of which a few of the first are nearly equal in size; these vesicles then regularly diminish in size to the end of the series. Each of these vesicles contains an auditory rod, and is in communication with the auditory nerve through a delicate nerve-fibril. I have observed that each of these nerve-fibrils swells into a minute ganglion immediately after leaving its particular vesicle; the function of these ganglia is, I take it, to strengthen and reënforce nerve-energy. No other observer mentions these ganglia, as far as I have been able to determine; they may have been absent, however, in the specimens studied by others, yet in the specimens studied by myself—the "red-legged locust" (Melanoplus femur-rubrum, Comstock)[18] and the "meadow grasshopper" (Xiphidium), they were always present.

That grasshoppers, locusts, and crickets can hear, no one who has observed these creatures during the mating season will for one instant deny; they hear readily and well, for in most of them the sense of hearing is remarkably acute.

Immediately behind the wings of flies two curious knobbed organs are to be observed; these are considered to be rudimentary hinder wings by entomologists, and are called the halteres. Bolles Lee and others of the French scientists call them balanciers. This latter name I consider the correct one, for these organs unquestionably preside over alate equilibrium: they are true balancers. I do not propose to enter into any discussion as to whether these organs are rudimentary wings or not; suffice it to say that they appear to me to be organs fully developed and amply sufficient to serve the purposes for which they were created. Whether or not in the process of evolution there has occurred a change of function, is a point which will not be discussed in this paper. As they now exist, I deem them to be auditory organs of Diptera (flies, gnats, etc.).

The semicircular canals are, to a great extent if not entirely, the seat of equilibration in man. Any derangement or disease of these canals interferes with equilibration; this is well shown in Ménière's disease, in which there is always marked disturbance of the equilibrating function.

If the balancers of a horsefly be removed, the insect at once loses its equilibrium; it cannot direct its flight, but plunges headlong to the ground. The same can be said of Chrysops niger—in fact, of the entire family of Tabanidæ, of the gall gnat (Diplosis resinicola, Comstock), and of the March flies (Bibionidæ). These widely differing flies constitute the material from which I have derived my data; I will venture to assert, however, without fear of contradiction, that what has been said about the flies mentioned above is equally true of all flies.

When the knobbed end of the balancers of the horsefly (Tabanus atratus, Comstock)[19] are examined with the microscope, the cuticle will be found to be set with minute hairs or setæ; some of these hairs penetrate both cuticle and hypoderm, are hollow, and receive into their hollows delicate nerve-fibrils. These