HEARING, SENSE OF. The power of hearing, that is to say of perceiving waves of sound which are propagated in the atmosphere, has as yet been found, apart from the four-legged vertebrates, only in certain insects. The sense of hearing in animals is used chiefly in the seeking out of the sexes by each other; it is, indeed, usually the male which produces the sounds, and by the strength of the notes attracts the female. This applies just as much to crickets and grasshoppers as to frogs and birds. In many mammals also, the voice and the sense of hearing are very clearly associated with mating: we have but to think of the belling of the stag.
When the sense of hearing serves other purposes also, such as the avoidance of enemies, or, among gregarious animals, mutual understanding, it is a derivative condition of late development, which is found only in the highest animals, the birds and mammals, perhaps in some reptiles also. The frog pays no heed to the noise made by an approaching man, it is only when it sees him that it takes to the water. Crickets and grasshoppers behave in a corresponding manner.
The structure of the auditory organs has certain features in common in all animals, therefore also in the insects which we are about to consider more closely. A tympanum is always present, that is a stretched membrane, which catches the waves of sound, and is thus made to vibrate. In the most simple form, the sensory cells which are affected by sound waves are attached directly to the tympanum (as in Lepidoptera, grasshoppers and cicadas).
Only in the green grasshoppers do we find the sensory cells separated from the tympanum, as they are in vertebrates. The transmission of sound, in these animals, takes place in the follow ing manner. The tympanic membrane is set in vibration ; these vibrations are communicated to the tracheal vesicles, which adjoin the inner side of the tympanic membrane, and it is these secondary vibrations which stimulate the sensory cells. The arrangement of the sensory cells is difficult to describe in detail, and is best shown by the accompanying figures. Usually, each sensory cell in the ear of an insect is like a stretched chord, and therefore is par ticularly sensitive to vibrations. In the green grasshoppers, the arrangement of the cells is similar to that of a pianoforte, they increase in size regularly from one end of the organ to the other. Apparently, the different-sized cells react to different sounds, but of this we do not know anything for certain.
It is remarkable that the auditory organs of insects are not situated in the head, as we are accustomed to find in vertebrates. In crickets and green grasshoppers they are situated on the front legs; in grasshoppers, cicadas and Lepidoptera on the body.
Exact experiments to prove the power of hearing in insects have been made principally by Regen, of Vienna, who used crickets. For this purpose, he made use chiefly of the well-known singing match of the male crickets, which we may so often hear in the meadows in summertime. The insects chirp alternately, and excite one another to rivalry. In Soo experiments with normal insects, this alternate chirping took place 349 times : in experi ments with insects in which the auditory organs had been pre viously destroyed, the singing match was observed only 13 times out of Soo. From this it follows, not only that the crickets are indeed able to hear, but also, that they do so with the aid of those organs, which, from their structure, we consider to be auditory organs.
Of even greater importance are the researches of this investi gator on the attraction of the female by the chirping of the male. Regen was able to show that the female cricket makes straight for the cage which contains the chirping male, although he is hidden from her sight. If the auditory organs of the female are removed, this reaction ceases immediately. It is of particular technical interest that the chirping of the male can be conveyed over the telephone in the same way as the human voice. It may then be observed that a female situated in a quite different place will spring towards the apparatus which reproduces the sounds, just as if the male were really there.
Of the other insects which possess the power of hearing only the nocturnal Lepidoptera (owlet-moths and Geometridae) have as yet been studied. These, however, show a quite different condition.
Lepidoptera are, with only very few exceptions, dumb. They thus form an exception to the rule according to which only those animals, in general, can hear which also produce sounds. They react apparently only to the sounds made by their chief enemies, bats. Eggers was indeed able to show that squeaking sounds will induce nocturnal Lepidoptera to take to flight. The insects will swerve when actually in flight as soon as they hear this noise, thus, undoubtedly, avoiding the enemy which produces it. Some species which feign death when danger threatens react also to squeaking sounds in this characteristic manner.
In addition to these examples, there are numerous observations which show that, sometimes, those insects also which have no definitely specialised auditory organs may react to sounds.
Swarms of gnats become restless at the whistle of an engine. Water-beetles hastily dive below the surface when a particular high note is sounded in the neighbourhood of their aquarium. In these cases, the sound waves apparently affect hairs of some kind or another which vibrate in harmony with a particular note. We can hardly say, in these instances, that the insects hear. Man also is able to perceive acoustic vibrations with the hand, as, for in stance, if he touches a box upon which is placed a vibratipg tuning-fork. In this instance, he perceives the vibrations by means of the sense of touch, and the previously mentioned reactions of insects must also be considered as of this nature.
In the lower animals, we frequently find sense-organs which were formerly considered, from their anatomical structure, to be auditory organs. We now know that they have a very different function, namely, to react to the stimulus of gravity. The structure and function of such a statocyst can be described in a few words. It usually consists of a ball-shaped vesicle, in the interior of which are two substances of different weight, a liquid, and a so-called otolith, which is better termed a statolith.
The latter, because of its greater specific gravity, always occupies the deepest portion of the vesicle. Such an organ serves, above all, to maintain equilibrium in flying or swimming. All flying and swimming animals take up a definite normal position during motion. Usually, they keep the back upwards (fishes, birds, most crustaceans) ; a few crustaceans swim with the ventral sur face upwards. When the animal is in the normal position, the statolith is always in contact with a definite point in the vesicle wall, which is called the "normal point." If, for any reason, the animal fails to maintain its balance, the statolith now touches a different point in the sensitive vesicle wall. The stimulated sensory cells send an impulse to the brain, and this brings about a regulatory movement of the legs, fins, or wings, which brings the animal again into the normal position. The creature is thus auto matically kept in equilibrium by the statocyst. We find such organs of equilibration in the Decapod Crustacea, in cuttlefish, and in swimming gasteropods and bivalves. Organs corresponding exactly anatomically to these are found in gasteropods and bi valves generally, but, as yet, we know nothing for certain about their function. Lastly, all vertebrates, also, have typical organs of equilibration.
The most simple method of proving experimentally that, in such cases, we are dealing with organs of equilibration is to re move the statocysts. Delage was the first to do this. If both of the balancing organs of a free-swimming gasteropod (Ptero trachea), of a crustacean (Palaemon, Mysis), or of some kind of fish are destroyed, the animal shows itself to be completely dis orientated. It revolves about its longitudinal axis in swimming, turns somersaults, and so on. In some cases, certainly, we have also to take into consideration the eyes. Crustaceans which are strong swimmers orientate themselves by means of the eyes as well as by the statocysts. They place themselves so that the light falls on their backs. Since, in the water, the light always comes from above, the correct swimming position of the animal is ensured by this attitude.
For this reason, no alteration in behaviour is to be observed in such a crustacean, under normal conditions of light, if the stato cysts only are removed. If, however, the animal is, at the same time, deprived of its sight, it becomes completely disorientated; if the light is caused to fall upon it from below, it swims upon its back instead of upon its ventral side.
In addition to this method of investigating experimentally by operation the function on the statocysts, there is a second which can be applied to Crustacea. At the time of moulting, when the crustacean casts the whole of the chitinous covering of its body, the inner surfaces of the statocysts and the statoliths are also shed. After the moult, the creature, using its pincers, places new statoliths, usually sand-grains, in its statocysts, which are open to the exterior. If the animal is not supplied with sand, but instead is given filings of iron or nickel, it will employ these as statoliths, and thus we get an animal with statoliths which are sensitive to magnetic influences (Kreidl). If, while the creature remains quiet on the ground, we now hold a strong electromagnet over its back, the iron statoliths are attracted to the dorsal region. They are now in exactly the same position as they occupy when a normal animal falls on its back. Just as in such an animal, the reaction to the stimulation of the sensory hairs appears as a regulatory movement through an angle of 18o°. The animal throws itself violently upon its back, the statoliths, attracted by the magnet, come into contact with the ventral surface of the statocysts, and remain in this position, which is the normal one when the creature is in its ordinary attitude.
If one statocyst is left in the animal to be studied, it shows characteristic differences according to the species to which it be longs. Some, such as the freshwater crayfish, or the free-swimming gasteropods, are already very disturbed by this operation ; they revolve about their longitudinal axes in swimming, to the right or to the left, according as the left or the right organ is missing.
Other species, such as certain strong-swimming crustaceans (Mysis, Palaemon) show, on the contrary, no disturbance. We may say, therefore, that in these animals one statocyst alone is sufficient to maintain equilibrium, while, in the others, the co operation of both organs is necessary for normal functioning. This condition has been more definitely analysed. We may imagine that the two statocysts always work the one against the other. When the animal is in the normal position, the two stato cysts function with equal strength, but are opposed the one to the other. In functioning, therefore, the one compensates the other. If, however, the animal is in an oblique position, the lower functions more vigorously than the upper, and brings about a return to the normal position.
The fact that the scallop and its relations possess asymmetrical statocysts is of some theoretical interest. The left one works more strongly than the right. While a normal symmetrical animal, from being in an oblique position finds itself back in the normal one, in which the plane of symmetry is vertical, just the opposite happens in the case of these bivalves, which, strange to say, are able to swim. If they are placed in water so that their median plane is vertical, they turn in swimming back into an oblique position, which for them is the normal one.
The statocysts have a quite different structure in some animals, in jellyfish, among others. These creatures, which float about upon the open sea, bear, on the margin of the umbrella, eight peculiar sense organs, which usually take the form of club-shaped projections, weighted at one end with crystals (auditory concre tions). According to the position which such a tentaculocyst occupies in space, there is a difference in the pressure and pull on the nervous tissue at its base, and, therefore, the effect upon the swimming muscles, which are in connection with the nervous system, also differs. For this reason, the jellyfish Cotylorhiza shows, when swimming, as is usual, in a vertical position, an equal activity in all its swimming muscles, which are drawn up into a ring. If, however, the animal is placed on its side, the tentaculo cysts below and above occupy quite different positions; in the lower there is a stronger movement than in the upper, and the creature rights itself again. In most of the other jellyfish matters are indeed somewhat different, and are not yet fully understood. While in all the cases previously mentioned the statocysts bring about compensatory movements which bring the animal, if it loses its balance, back into the normal position, we know, on the other hand, of instances in which such compensation is com pletely lacking. The common crab, which, as a rule, does not swim, but creeps about upon the floor of the sea, shows a very peculiar connection between the statocysts and the stalked eyes. If such an animal is moved about in the hand, it may be seen that the stalked eyes endeavour to hold their field of vision, and move so that they adjust the effect of the movements of the body upon the field of vision. For example, if the crab is turned about its transverse axis, so that the head is downwards, the stalked eyes are directed upwards ; they are turned downwards if the head region is raised. The meaning of this phenomenon at once becomes clear if we consider the behaviour of human beings.
Man also is able to move his head about in space, at will; to raise it, lower it, move it backwards or forwards, and, in spite of all these movements, is able to keep his eyes fixed on a particular object. • In both cases, in mammals and in crustaceans, it can be proved that the movements of the muscles of the eyes are chiefly de termined by the organs of equilibration. If the statocysts of a crustacean are removed, these movements of the stalked eyes are no longer to be observed. To each position of the body in space there corresponds a single position of the eyes in relation to the body with which it is very closely bound up. The tendency of the eyes is always to adjust their position so that it will corre spond to the position of the body in space.
In many cases, the statocyst does not serve in any way as an organ of equilibration, but makes it possible for the animal to move vertically upwards or downwards (positive and negative geotaxis). This kind of movement is found chiefly in animals which live in sand or marine ooze. It can easily be understood that, for certain functions of life, these creatures must be able to reach the open water above them, for example, for the deposi tion of eggs ; on the other hand, they must also be able to with draw into the sheltering sand when danger threatens. If we consider a worm which bears in its head a pair of such statocysts, we shall see that, in order to make sure of reaching the depths, it has only to place itself so that the statoliths touch the foremost points of the statocysts. The common lug-worm of the European coasts (Arenicola marina) is such an animal, so also are certain Holothurians (Synapta), and, lastly, the mud-dwelling isopods of the genera Anthura and Cyanthura. Negative geotaxis, which directs the animal vertically upwards, can best be observed in our terrestrial gasteropods. If a handful of such creatures is thrown into a vessel full of water, they all creep vertically upwards on the glass wall, and thus escape death by drowning. It is probable that this movement also is connected with the statocysts, but, at present, there is no proof of this.
It is of interest, that an array of cases is known to science in which an active maintenance of equilibrium, or a strong geotactic movement is indeed present, but in which statocysts are absent. The anatomy of the majority of animals is so well-known today that it is very unlikely that organs exist which have been over looked. We are forced to the conclusion that other organs have an auxiliary function as statocysts. In most animals, the internal organs are suspended from the abdominal wall by a number of slings, just as in man. It has been proved in some cases that the internal organs take over, in a certain measure, the role of statocysts. In accordance with the position which the animal occupies in space, the direction of the pull upon the suspensory sheets varies. This enables the animal to perceive its position in space. The starfish affords a fine example of this. If it is placed upon its back, it turns over again. If the stomach is filled with iron filings, the animal lying upon its back, and an electromagnet is brought near the ventral surface, the animal does not turn over, since the suspensory organs are now subjected to the same pull as that which they sustain when the creature is in its normal position. Geotactic movements in the absence of statocysts occur very frequently. They may be observed in sea-anemones, starfish and many other animals. (W. v. BUD.)