The Dawn of Reason; or, Mental Traits in the Lower Animals. Jr. James Weir
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 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