Popular Science Monthly/Volume 39/October 1891/Hearing in the Lower Animals



SLOW as we have been in recognizing that man owes the superiority of his form and his attributes to the experience of his entire animal ancestry, we have been prompt to attribute to other animals psychical and sensorial qualities more or less nearly identical with ours. There really seems to be some insolence in assuming that hearing, as we enjoy it, is refused to the immense majority of living beings, and especially to all the invertebrates. This arises partly from the fact that we are hardly acquainted with our own senses, and that many of our faculties are still left for us to discover while we use them in a way as constant as unconscious, and partly from our natural tendency to base on distant organic analogies functional assimilations which are far from being always admissible.

Here is a pigmentary spot to which we assign the dignity of the eye; there, a hair, an otocyst, which we call the auditory organ; and at the same time we assume that the spot is for seeing and the hair for hearing. As sight and hearing are known to us only as we have them, there results a deplorable misinterpretation of the sensorial function in animals.

Can we, however, have any conception of the power of smell of a ray or a rat? Are there not some insects which can supply the place of nearly all the other senses by the richness of their smell? And do we, aerial animals, know anything about the kind of sense of smell that water animals have?

The comparative physiology of the sight would show curious differences between us and any animal. Ants might have theories respecting luminous undulations that would seem very strange to us. Do they know what we understand by color? Do they comprehend anything but quantity and direction where we perceive light and tonality, form and perspective? Is the luminous world for them and for many other insects nothing else than a special palpitation of disturbed space which they analyze, orienting themselves in it by the perception of the force and direction of the direct or reflected disturbances? Do the handsome colors of butterflies prove that they are sensitive to the colors of flowers; and can we make sight play a certain part in the curious mimetic adaptations of different animals? Selection may serve us better than we should do ourselves without our aiding it; and it is not always by tactics that an animal escapes its enemies.

Mimicry of form and color addresses itself to the sight of the enemy, and there doubtless also exists a mimicry of odor. This, however, only proves that the enemy hears and sees without there being any calculation on the side of the interested animal. There is nothing astonishing in the thought, for we frequently meet persons to whom particular colors are unknown, and who have, therefore, only an incomplete idea of light; and if we look for sensorial memory and aesthetics behind the senses, the divergencies will be found still stronger.

Hearing, according to an opinion that appears solidly established on facts, is well developed in animals quite distant from us. Instances in point are the spider of Grétry and other spiders which seemed to have real musical tastes; eatable crustaceans, which can not be fished for successfully except in the most rigorous silence; the crabs of Minasi, which stopped in the midst of most lively frolics when a bell called them to order; the prawns of Hensen, which leaped when the slightest sound reached them; the shrimp, which exhibited in the hairs of their tails what were taken to be organs of hearing.

Dahl has ascertained similar facts concerning spiders. Romanes remarks that these insects approach instruments having a pleasant sound, and cites an observation of Reclain, who, during a concert at Leipsic, saw a spider come down a chandelier while a solo was performed on the violin, and go back very quickly as soon as the orchestra set in. He, however, expresses doubts as to the meaning of these facts; and many authors, including Lubbock and Forel, have not been able to ascertain that insects hear.

Hearing means perception of noises and sounds; and it is this perception that we refuse absolutely to every being deprived of a sacculo-cochlear apparatus. If an animal hears because certain hairs of its body are set into vibration by certain disturbances of the air, then a barley beard, a piece of velvet, and a brush vibrating harmonically, hear likewise. If we should invest the most hopelessly deaf man with a stiff-jointed armor, like the armor of the men-at-arms of the middle ages, and should put his head into a vessel capable of vibrating and trembling, he would come to a stand at the first shock like the crabs of Minasi, or would leap like the prawns of Hensen. It is not the sound that will affect him, for he is deaf, but the trepidation, which, disagreeable to a mollusk, would be intolerable to a crustacean enveloped in rigid pieces, adjusted and in contact. Nothing is more like the noise of a fly's buzzing than the sound of a tuning-fork or of soft instruments. The same flight of a fly and its energetic efforts to free itself when captured, also produce a disturbance of the air like that of the tuning-fork; what is a humming sound to us is to the spider a beating of wings.

Romanes has cited, as apposite to this subject, an interesting observation by Boys, which is very instructive, and admits of an easier interpretation than the author seems to have believed. Boys remarked that, on lightly touching with a tuning-fork a point of a spider's web, the insect turned at once to the side of the instrument, and tried the rays of its web with its fore legs to discover what was vibrating; then, coming closer and closer, reached the instrument and tried to seize it as it would have done with a fly. We know that spiders do not enjoy a very delicate sight, and that their sense of smell is not remarkable; they are accorded, on the other hand, a very fine hearing, which serves, according to the authors, the satisfaction of their musical tastes and the gratification of their carnivorous appetites. I believe, however, that they are absolutely deaf and nearly blind, but are remarkably well endowed with what we might call a sense of trembling a sense which suffices for the needs of the immense majority of animals, and which is complicated somewhat late among the higher animals with sonorous perceptions. Sound is, in fact, like color, of recent acquisition in the animal series; it is a thing of sensorial operations that require a remarkable degree of perfection.

We have an organ, the cochlea, which permits us to appreciate fast rhythms, under the form of sensations to which we can attribute a place in a whole of continued sensorial affections of the same character, and group them in series. The spider perceives a trembling, feels the thread that vibrates most, runs along it, stops an instant at the branchings, and arrives at the point where the force, the form of tremor characteristic of this or that prey, suggest to her the instinctive and sometimes intelligent manœuvres, as we saw in the case of the tuning-fork, which will bring that prey into her possession. The same automatism studied by Fabre is observed in the spider; and the point of departure of that series of adapted acts is always the perception of a trembling.

The center of the web, the meeting point of all the radiating threads, is a veritable center of information, and the point to which the spider returns to get the right direction. Furthermore, the extreme delicacy of the threads, the tension of which is augmented by the weight of the spider, informs her of disturbances of distant origin, which, are communicated only by the air and arrested by this light screen. Boys's spider glided along the thread toward the tuning-fork, as the spider did for Reclain's violin solo, and came back when Boys touched a point of the web with, the tuning-fork. This experiment sufficiently explains the musical interest that led the Leipsic spider to the violin whose vibrations reached it. But at the first tutti or concerted crash the whole room trembled, including the web, and our spider ascended precipitately.

To a web-making spider, the regular trepidation or oscillatory vibration, which is sound to us, means simply a struggling prey to be taken; a spider placed upon a table approaches the source of a sound quite automatically. Sound, as we define it in the sense of a continued sensation, is a dead letter to it. Forel has remarked that insects continue insensible to the sound of the voice if we interpose a screen before the mouth and cut them off from the disturbance of the air. When animal nature acquires the sense of sound as the extreme limit of its perceptions it is to suffer itself to be fascinated, hypnotized, and charmed like the serpent. That continued fixed sensation is to the serpent what the fixity of its look will be to the bird which it will smite in its turn. It is an extreme perception, exceedingly troubling, and away at the extremity of physiological equilibrium, a source of sensorial inhibition, extra-cerebral.

Fishes, which have the power of orientation through the remarkable development of the canals of the labyrinth and their lateral organs directed to all points of their liquid horizon, with such susceptibility to trepidation and disturbances that they perceive the slightest, are only deaf-mutes. I have studied, in my thesis on the Auricular Sense in Space, the transformation of the sense of disturbance into a hearing sense, and the mechanism of auricular orientation in the whole animal series. I only say that all animals possessing the otolithic apparatus in any of its forms perceive, of disturbances of the medium, the intensity, the direction, and the number only, but can not convert them into continuous sensations like those we call sounds. All the experiments and observations invoked as making manifest that invertebrates have hearing only prove the perception of disturbances and tremors, more or less rhythmical modifications of the ambient medium. Of what use would be the perception of sounds in mediums where hardly any sounds are produced? The mollusk fastened to its rock recognizes the approach of a prey or an enemy; it feels the waves breaking upon it; it usually lives on a well-conducting ground, and by its foot-organ knows all that is passing there. This otolithic apparatus is admirably well adapted to give notice to the mollusk or to the insensitive armor of the crustacean of the least disturbances that pass through the mass into which it is plunged. Every one is acquainted with the experiment of the balls in contact series. A disturbance reaching the end of the chain is betrayed only by the last ball, which is free, and expends in oscillations the shock that is transmitted to it. Equally well known is the process of placing in contact with the vibrating body, plate, or membrane a light substance—sand or cork—the oscillations of which reveal a vibration which our eyes can not discover on the trembling body. In the same way every disturbance traverses the indifferent mass of the animal, and the otolith free in the otocyst, collects it, and announces the slightest shocks to the nervous tissue on which it reposes. The preotolithic formations serve in a similar way. Animals furnished with otoliths can thus analyze rhythms and disturbances which are synthetized in our cochlea into sounds of different tones. The otolithic bell can only reveal a trepidation, and continues unfit to provoke a continuous sensation other than that which results from the persistance of nervous, terminal, or central impressions, a limit beyond which it can not estimate the pitch without such an arrangement as that of the cochlear formations.

As does the spider in the center of its web, stretching with its weight all the vibrating cords that converge toward its fore legs, furnished with otoliths, so can any insect standing on slight legs, stiff and flexible at once, which draw from the ground the slightest tremblings as its antennae do from the air, distinguish between a thousand significant disturbances, without, after all, perceiving any sensation like what we call sound.

There are, in fact, two fundamental senses which are two forms of touch. The first is immediate touch, under the form of contact when the surface of the object is accessible, or of smell or taste when it is in a state of division, in which it is revealed by its molecular atmosphere. The second touch, at a distance, which is extremely varied, comes by means of the modification of an interposed medium originating in the object that is perceived. Perceptions of electricity and heat are common to both forms.

We ought, a priori, to refuse to attribute to insects, whose sensorial organs are so different from ours, senses like those of man. Should not their psychology with more reason be referred to them as a class than to ours? And while it may be legitimate for man to expect to find some of his feelings among the vertebrates which have the most evident relationship with him, that inverse anthropomorphism has a curious appearance which lends our thoughts, wishes, needs, senses, and affections to beings so different from us in all respects. It would be at least prudent not to ascribe similar functions to different organs; and our language, the product of our cerebration, would be impotent to represent the emotions of an arthropod or a mollusk; for beings that differ in structure differ also in thought and instinct. The cochlea being adapted to the hearing of simple or composite sounds, tones, or noises, audition proper does not exist where there is no cochlea. We are, in return, much less well endowed than the spider and crawfish to perceive rapid tremors and vibrations, of which we can make continuous sounds only when they exceed forty in a second.—Translated for the Popular Science Monthly from the Revue Scientifique.