Animal Life and Intelligence

by C. Lloyd Morgan

Observation seems to point to the fact that in most insects the sense of hearing is lodged in the feelers, or antennæ. Kirby made the following observation on a little moth: "I made," he says, "a quiet, not loud, but distinct noise; the antenna nearest to me immediately moved towards me. I repeated the noise at least a dozen times, and it was followed every time by the same motion of that organ, till at length the insect, being alarmed, became more agitated and violent in its motions." Hicks wrote, in 1859, "Whoever has observed a tranquilly proceeding capricorn beetle which is suddenly surprised by a loud sound, will have seen how immovably outward it spreads its antennæ, and holds them porrect, as it were, with great attention, as long as it listens." The same observer described certain highly specialized organs in the antennæ of the hymenoptera (ants, bees, and wasps), which he thus describes: "They consist," he says, "of a small pit leading into a delicate tube, which, bending towards the base, dilates into an elongated sac having its end inverted." Of these remarkable organs, Sir John Lubbock says there are about twelve in the terminal segment, and he has suggested that they may serve as microscopic stethoscopes.

Mayer, experimenting with the feathered antenna of the male mosquito, found that some of the hairs were thrown into vigorous vibration when a note with 512 vibrations per second was sounded. And Sir John Lubbock, who quotes this observation, adds,[EZ] "It is interesting that the hum of the female gnat corresponds nearly to this note, and would consequently set the hairs in vibration." The same writer continues, "Moreover, those auditory hairs are most affected which are at right angles to the direction from which the sound comes. Hence, from the position of the antennæ and the hairs, a sound would act most intensely if it is directly in front of the head. Suppose, then, a male gnat hears the hum of a female at some distance. Perhaps the sound affects one antenna more than the other. He turns his head until the two antennæ are equally affected, and is thus able to direct his flight straight towards the female."

It is difficult to determine the range of hearing in the lower organisms. But it is quite possible, nay, very probable, that the superior limit of auditory sensation is much more extended in insects than it is in man. We know that many insects, such as the cicadas, the crickets and grasshoppers, many beetles, the death's-head moth, the death-watch, and others, make, in one way or another, sounds audible to us. But there may be many insect-sounds—we may not call them voices—which, though beyond our limits of hearing, are nevertheless audible to insects. At the other end of the scale, on the other hand, slow pulsations may be appreciated—for example, by aquatic creatures—by means of what we term the auditory organs, in a way that is not analogous to the sensation of sound in us. It may be noted that auditory organs are dotted about the body somewhat promiscuously in the various invertebrates. We have seen that auditory organs, or what are generally believed to be such, are found in the foot of bivalves, in the antennules of lobsters, in the fore legs of crickets and ants, in the abdomen of locusts, in the balancers of flies, and in the tail of Mysis. But when we come to consider the matter, there is no reason why the organ of hearing should be in any special part of the body. The waves of sound rain in upon the organism from all sides. There is no great advantage in having the organs of hearing in the line of progression, as with sight, where the rays come in right lines; nor in having them in close association with the mouth, as in the case of the organ of smell.

Closely connected with the organ of hearing in vertebrates is the organ of another and but recently recognized sense. In briefly describing the auditory apparatus in man, mention was made of three curved membranous loops, the so-called semicircular canals. A few more words must now be said about them and the membranous sac with which they are connected.

The sac lies in a somewhat irregular cavity in a bone at the side of the head, in the walls of which are five openings leading into curved tunnels in the bone in which lie the membranous loops. The planes in which the three semicircular canals lie are nearly at right angles to each other, and they are called respectively the horizontal, the superior, and the posterior. The two latter unite at one end before they reach the sac; hence there are five, and not six, openings into the cavity. At one end of each semicircular canal is a swelling, or ampulla, in each of which is a ridge, or crest, abundantly supplied with hair-cells. And in a little recess in the sac there is, occupying its floor, its front wall, and part of its outer wall, a patch of hair-cells covered by a gelatinous material with numerous small crystalline otoliths. The only other point that calls for notice is that the membranous sac does not fit closely in the bony cavity in which it lies, while the diameter of the membranous semicircular canals is considerably less than that of their bony tunnels, except at the ampullæ, or swellings, where they fit pretty closely. Both the bony cavity and the membranous labyrinth (as it is called) are filled with fluid.

From its close connection with the organ of hearing, this apparatus was for long regarded as in some way auditory in its function, and it was surmised that it enabled us to perceive the direction from which the sound came. But how it could do so was not clear. In 1820 M. Flourens made the observation that the injury or division of a membranous canal gave rise in the patient to rotatory movements of the animal round an axis at right angles to the plane of the divided canal; and he, therefore, suggested that the canals might be concerned in the co-ordination of movement. They are now regarded as the organs of a sense of rotation or acceleration.

That we have such a sense of rotation has been proved experimentally.[FA] Let a man, blindfolded, sit on a smooth-running turn-table. When it begins to rotate he feels that he is being moved round, but if the rotation be continued at the same rate, this feeling quickly dies away. If the rotation be increased, he again feels as if he were being moved round, but this again soon dies away. Further increase gives a fresh sensation, which in turn subsides, and the man may then be spinning round rapidly, and be perfectly unconscious of the fact. He is only aware that he has been gently turned round a little two or three times. Now let the speed of rotation be slackened. He has a sensation of being gently turned round a little in the opposite direction. Each time the speed is lessened he has this sense of being turned the reverse way. From these experiments we see that what we are conscious of is change of rate of rotation, or, in technical language, acceleration, positive or negative.

Fig. 30.—Diagram of semicircular canals.

A. bony labyrinth of human ear (after Sömmering). c, c., the cochlea; s.c., superior semicircular canal; p.c., posterior semicircular canal; h.c., horizontal semicircular canal; a, a, a, their swellings, or ampullæ; f.o., f.r., fenestra ovalis and rotunda (oval and round windows) in the vestibule.