Art. II.—On the Senses of Insects.
[Presidential Address to the Wellington Philosophical Society, delivered 25th June, 1901.]
The few remarks I propose to offer to-night contain but little original matter, and probably many members will find that they are quite familiar with most of the facts about to be related. My primary object in recounting these observations is, however, to show the large amount of valuable work which may be done by any one who is endowed with a fair amount of leisure, and has a taste for observing natural-history objects. Unlike other branches of entomological field-work, the study of the senses and intelligence of insects can be pursued without the observer leaving his own home, and this, no doubt, will be a recommendation to many whose health, and other considerations perhaps, do not permit of prolonged visits into the wilds of New Zealand.
Observations on senses and habits, &c., do not require that minute and technical knowledge of species and genera the acquisition of which is often regarded as dry and laborious. A few of the commonest and most easily recognised insects will suffice for this class of work, and it is only necessary for the student to know the names of these in order that he may place his observations on record.
I should state that most of the observations here given are taken from the works of Sir John Lubbock (now Lord Avebury), who has done so much to encourage the investigation of living insects, and has so clearly shown that the systematic collection and classification of dead insects is not the whole science of entomology, as some entomologists appear to imagine.
Regarding the senses of insects, there seems to be little doubt that they have in some degree all the senses possessed by man; but in certain cases these senses are considerably modified. There are also some reasons for supposing that insects may be endowed with other senses which we do not possess, and of which we can consequently have no conception The most primitive of the senses, that of touch, is undoubtedly possessed by insects in a very marked degree. It was formerly supposed to reside chiefly in the antennæ and in the palpi, but more modern investigations tend to show that the sense of touch in insects is chiefly situated in certain special hairs which occur on various parts of the body and appendages. The bases of these hairs penetrate the horny integument of
the insect, and are connected with special nerve-fibres. They are much more numerous on some points of the creature than on others. They occur in very large numbers, for instance, on the proboscis of the common house-fly. The skin of insects is so much harder and more insensible to outside impressions than the covering of most other animals that special tactile organs are necessary, and it appears that these hairs perform the needed function.
The possession of the sense of taste in insects cannot be questioned. No one who has ever watched a bee or a wasp can entertain the slightest doubt on the subject. It is, again, probably by taste that caterpillars recognise their food-plant. Moreover, this is partly the effect of individual experience, for when first hatched caterpillars will often eat leaves which they would not fouch when they are older and have become accustomed to a particular kind of food. Special experiments have, moreover, been made by various entomologists, particularly by Forel and Will. Forel mixed morphine and strychnine with some honey which he offered to his ants. Their antennæ gave them no warning. The smell of the honey attracted them and they began to feed; but the moment the honey touched their lips they perceived the fraud. Will tried wasps with alum, placing it where they had been accustomed to be fed with sugar. They fell into the trap and ate some, but soon found out their error, and began assiduously rubbing. their mouth-parts to take away the taste.
Will found that glycerine, even if mixed with a large proportion of honey, was avoided, and to quinine they had a great objection. If the distasteful substance is inodorous and mixed in honey the ant or bee commences to feed unsuspiciously, and finds out the trick played on her more or less quickly according to the proportion of the substance and the bitterness or strength of its taste. The delicacy of taste is doubtless greater in bees and ants than in omnivorous flies or in carnivorous insects. At the same time the sense of taste in ants is far from perfect, and they cannot always distinguish injurious substances. Forel found that if he mixed phosphorus in their honey they swallowed it unsuspectingly and were made very unwell. It cannot, then, be doubted that insects possess a sense of taste. The seat of it can hardly be elsewhere than in the mouth or its immediate neighbourhood; and in all the orders of insects there are found on the tongue, the maxillæ, and in the mouth certain minute pits, which are probably the organs of taste. In each pit is a minute hair, or rod, which is probably perforated at the end.
Passing to the sense of smell, we find that there are good reasons for supposing that most insects are well endowed in this respect. The seat of the sense is supposed to be situated
in a number of minute cylinders and pits which are placed on the antennæ and on the palpi. The evidence obtained from experiments is somewhat conflicting as between the antennæ and the palpi; but, if the sense of smell is supposed to reside partly in each of these appendages, the results of the various observations are brought into accord. In connection with this subject the following observation, which I made on the 4th September, 1882, on a specimen of one of our common butterflies (Vanessa gonerilla), may be of some interest: At 9 a.m. I placed some moistened sugar in a small china colour-pan about¼ in. square. The butterfly was rather torpid owing to the low temperature, and I therefore removed it from the cage and placed it on the edge of the vessel containing the sugar. Almost at once it began to uncoil its proboscis, and whilst doing so it steadily elevated and depressed its antennæ, the tips of which frequently touched the sides of the vessel. Sometimes each antenna was elevated and depressed singly; at other times both organs were moved together. These remarkable movements of the antennæ were, I think, merely indicative of the insect's pleasure, and this explanation is supported by the observations of Sir John Lubbock respecting the movements of the antennæ in ants. When the butterfly bad completely uncoiled its proboscis it felt all round the vessel with the sensitive extremity of that organ, which certainly appeared to be endowed with the sense of smell. Soon it found the liquid sugar, which it eagerly sucked for about three minutes; and during the whole of this time the butterfly continued to move the antennæ in the manner above described. As soon as the insect ceased feeding, however, the antennæ were restored to their normal position—i.e., almost perpendicular to the main axis of the insect's body. After this I made several very sharp noises—whistling, and ringing on a tumbler—but the butterfly did not appear to hear them, and the antennæ were held perfectly motionless throughout. This experiment appears to indicate, I think, that in butterflies the sense of smell is situated in or near the extremity of the proboscis, and that the sense of hearing is absent or but little developed. I have often tried to frighten butterflies in the field by shouting at them, but have never succeeded in making one of these insects rise from its perch in this way, although the slightest movement on the part of the observer would at once have caused the insect to take flight.
The following are some of the experiments related by Lord Avebury in connection with testing the organs of smelling in insects. He says, “I myself took a large ant (Formica ligniperda) and tethered her on a board by a thread. When she was quite quiet I tried her with tuning-
forks, but they did not disturb her in the least. I then approached the feather of a pen very quietly so as almost to touch first one and then the other of the antennæ of, which, however, did not move. I then dipped the pen in essence of musk and did the same; the antenna was slowly retracted and drawn quite back. I then repeated the same with the other antenna. I was, of course, careful not to touch the antennæ. I have repeated this experiment with other substances with several ants, and with the same results. Perris also made the same experiments with the palpi, and with the same result; but if the palpi were removed the rest of the mouth gave no indications of perceiving odours.”
Graber also made a number of experiments, and found that in some cases (though by no means in all) insects which had been deprived of their antennæ still appeared to possess the sense of smell. But if, as we have, I think, good reason to suppose, the power of smell resides partly in the palpi, this would naturally be the case. He also tested a beetle (Silpha thoracica) with oil of rosemary and assafætida. It showed its perception by a movement in half a second to a second in the case of the oil of rosemary, and rather longer—one second to two seconds - in the case of the assafœtida. He then deprived it of its antennæ, after which it showed its perception of the oil of rosemary in three seconds, on an average of eleven trials; while in no case did it show any indication of perceiving the assafœtida, even in sixty seconds.
This would seem to indicate a further complication—not only that both the antennæ and the palpi may possess the sense of smell, but also that certain odours may be perceived by the former, and others by the latter. As regards flies (Musca), Forel removed the wings from some bluebottle flies and placed them near a decaying mole. They immediately walked to it, and began licking it and laying eggs. He then took them away and removed the antennæ, after which, even when placed close to the mole, they did not appear to perceive it.
Plateau also put some food of which cockroaches are fond on a table, and surrounded it with a low circular wall of cardboard. He then put some cockroaches on the table. They evidently scented the food, and made straight for it. He then removed their antennæ, after which as long as they could not see the food they failed to find it, even though they wandered about quite close to it.
On the whole, then, the experiments which have been made seem clearly to prove that in insects the sense of smell resides partly in the antennæ and partly in the palpi. This distribution would be manifestly advantageous. The palpi are more suited for the examination of food, while the an-
tennæ are more conveniently situated for the perception of more distant objects. The remarkable power possessed by the males of many species of moths which enables them to discover anewly emerged female of the same species, even when enclosed in a box situated perhaps a considerable distance from the insect's natural haunts, is well known to collectors; and its truth is sufficiently proved by the very much higher prices charged by dealers in entomological specimens for the female specimens of all such species. This faculty of finding the female at a distance is in all probability resident in the antennæ of the male, which are always very amply pectinated in all those species possessing abilities of the kind. It is not, however, by any means certain that the sense involved is that of scent only. In the mosquitoes, at any rate, it has been practically proved that the ability to discover the location of the female is due to a sense nearly akin to that of hearing, and that this sense is situated in the extensive pectinations of the antennæ possessed by the male of that insect.
In connection with this subject the following ingenious experiment made by Mayer is of interest: He fastened a male mosquito down on a glass slide, and then sounded a series of tuning-forks. With an Ut4 fork of 512 vibrations per second he found that some of the hairs were thrown into vigorous movement, while others remained nearly stationary. The lower (Ut3) and higher (Ut5) harmonics of Ut4 also caused more vibration than any intermediate notes. These hairs, then, are specially tuned so as to respond to vibrations numbering 512 per second. Other hairs vibrated to other notes, extending through the middle and next higher octave of the piano. Mayer then made large wooden models of these hairs, and, on counting the number of vibrations they made when they were clamped at one end and then drawn on one side, he found that it “coincided with the ratio existing between the numbers of vibrations of the forks to which co-vibrated the fibrils.” It is interesting that the hum of the female gnat corresponds nearly to this note, and would consequently set the hairs in vibration. 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 will 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 little 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.
The auditory organs of insects, then, are situated, in different insects, in different parts of the body; and there is
strong reason to believe that, even in the same animal, this is not necessarily confined to one sensitiveness to sounds is not necessarily confined to one part.
From the above it will be seen that there are good reasons for supposing that the organs of hearing are situated in the antennæ of insects, and there is much to support this view, though modern investigation has revealed the existence of well-developed organs of hearing on other parts of the body. Before relating a few of the experiments which have been made in connection with the power of hearing in insects it will, perhaps, be well to very briefly describe the curious ears which have been discovered on the tibiæ of the anterior legs in many grasshoppers, and which are very well developed and easily seen in the specimens of one of our native wetas, which I have pleasure in exhibiting this evening. The researches of Muller, Siebold, Leydig, Hensen, Graber, and Schmidt conclusively prove that these drum-like organs are veritable ears. In grasshoppers and crickets the auditory organ lies in the tibia of the anterior leg, on both sides of which there is a disc generally more or less oval in form, and differing from the rest of the surface in consisting of a thin, tense, shining membrane, surrounded wholly or partially by a sort of hame or ridge.
If now we examine the interior of the leg, the trachea, or air-tube, will be found to be remarkably modified. Upon entering the tibia it immediately enlarges and divides into two branches, which reunite lower down. To supply air to this wide trachea the corresponding spiracle, or breathinghole, is considerably enlarged, while in the dumb species it is only of the usual size. The enlarged trachea occupies a considerable part of the tibia, and its wall is closely applied to the tympanum, which thus has air on both sides of it, the open air on the outer the air of the trachea on its inner surface. In fact, 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 the atmospheric vibrations.
On the 17th January, 1890, I made the following experiment on a female specimen of weta (Deinacrida megacephala), an insect possessing well-developed ears on the tibiæ of its anterior legs. 11.20 a.m.: I placed the insect on a board suspended from the ceiling, where no vibrations except those of sound could reach it. First I tried a piano, but insect did not appear to hear either treble or bass notes. Then tried beating a kerosene-tin with an iron rod, but apparently insect could not hear noise except when the sounds were very rapidly made and of loud pitch. At this stage the insect seemed to wake up and put out its antennæ and palpi.
When I ceased making the noises the insect again reposed, but on my resuming it jumped off the board and ran away. The creature appeared alarmed at the sounds. The palpi were spread out whilst it was listening, and the antennæ moved up and down. 11.40 a.m.: Placed insect in a cage also suspended, and allowed her to settle down. Repeated the loud jarring sounds. The insect started, and then squeezed itself into a corner of the cage as though in great fear. After this it took no further notice although sounds were continued for about half a minute. Five minutes later I repeated the sounds, but no notice was taken of them, and I think that the insect was asleep, as in the first instance. 11.50: Again repeated sounds, very loudly this time. The insect trembled and moved its antennæ, but five flies perched on the string supporting the cage took no notice. The female D. megacephala appears to only regard the very discordant sounds made with the kerosene-tin and poker. It does not appear to hear the piano at all. 12 noon: Repeated, with same result; insect started, put out palpi, and moved antennæ. Feel sure that when she does not hear she is asleep.
The following experiments are related by Lord Avebury: “Kirby states that ‘once a little moth was reposing upon my window. I made 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.’ And again, ‘I was once observing the motions of an Apion (a small weevil) under a pocket microscope. On seeing me it receded. Upon my making a slight but distinct noise its antennæ started. I repeated the noise several times, and invariably with the same effect.’”
Will has made some interesting observations on some of the Longicorn beetles which appear to confirm the view that the antennæ are the organs of hearing. These insects produce a low shrill sound by rubbing together the prothorax and the mesothorax. The posterior edge of the prothorax bears a toothed ridge and the anterior end of the mesothorax a roughened surface, and when these are rubbed together a sound is produced something like that made by rubbing a quill on a fine file. Will took a pair of Cerambyx (beetles), and put the female in a box and the male on a table at a distance of about 4 in. They were at first a little restless, but are naturally calm insects, and soon became quiet, resting as usual with the antennæ half extended. The male evidently was not conscious of the presence of the female. Will then touched the female with a long needle and she began to
stridulate. At the first sound the male became restless, extended his antennæ, moving them round and round as if to determine from which direction the sound came, and then marched straight towards the female. Will repeated this experiment many times, and with different individuals, but always with the same result. As the male took no notice of the female until she began to stridulate, it is evident that he was not guided by smell. From the manner in which this Cerambyx was obviously made aware of the presence of the female by the sound, Will considered it clearly proved that in this case he was guided by the sense of hearing. Will has also repeated with these insects the experiments Lord Avebury made with ants, bees, and wasps, and found that they took no notice whatever of ordinary noises; but when he imitated their own sounds with a quill and a fine file their attention was excited—they extended their antennæ as before, but evidently perceived the difference, for they appeared alarmed, and endeavoured to escape.
Hicks, in 1859, justly observed that “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, and how carefully the insect proceeds in its course when it conceives that no danger threatens it from the unusual noise.”
Passing now to the organs of vision in insects, we find no difficulty in exactly locating their position. The eyes of these animals, as is well known, are of two distinct kinds—Firstly, the compound eyes, which are made up of an immense assemblage of minute hexagonal eyes usually collected into two large hemispheres situated on each side of the insect's head; secondly, simple eyes, or ocelli, of which there are a variable number usually situated on the top of the head. The compound eyes are present in almost all fully developed insects, but the ocelli are very frequently absent. In insect larvæ and in spiders ocelli are the only organs of sight.
With regard to the actual power of vision possessed by insects little is known with certainty at present. There is no doubt that some species see much better than others. I remember specially noticing this when the European blowfly (Calliphora erythrocephala) first appeared in New Zealand during 1888. At that time both the native and introduced species were to be seen resting on fences in the Wellington Botanical Gardens. I experienced considerable difficulty in capturing the European species, owing to its great agility, but could capture the native insect with comparative ease. This circumstance was undoubtedly due to the superior power
of vision possessed by the European insect. I afterwards proved this by experiment. When a number of both insects were resting close together I would gradually move my hand towards them, and I noticed that the introduced blowflies invariably saw my hand and took wing before the native species. I verified this by numerous experiments and always obtained the same results.
Ordinary observation in the field proves that some insects undoubtedly have very keen sight, and almost every one must have noticed that dragon-flies and butterflies are especially well endowed in this respect.
Another interesting question in connection with the vision of insects is the relative functions of the simple and compound eyes. A large number of experiments have been made, and it appears probable that the ocelli are useful in dark places and for near vision. Lord Avebury adds: “Whatever the special function of the ocelli may be, it seems clear that they must see in the same manner as our eyes do—that is to say, the image must be reversed. On the other hand, in the case of the compound eyes it seems probable that the vision is direct, and the difficulty of accounting for the existence in the same animal of two such different kinds of eyes is certainly enhanced by the fact that, as it would seem, the image given by the medial eyes is reversed, while that of the lateral ones is direct.”
The modern theories of the evolution of flowers through the agency of insect visitors is now very generally accepted amongst naturalists, but it is obvious that these ideas would be effectually disproved if it could be demonstrated that insects were unable to distinguish colours. The following experiments conducted by Lord Avebury prove, I think, that bees, at any rate, possess the faculty of distinguishing colours: “I brought a bee to some honey which I placed on a slip of glass laid on blue paper, and about 3 ft. off I placed a similar drop of honey on orange paper. With a drop of honey before her a bee takes two or three minutes to fill herself, then flies away, stores up the honey, and returns for more. My hives were about 200 yards from the window, and the bees were absent about three minutes, or even less. When working quietly they fly very quickly, and the actual journeys to and fro did not take more than a few seconds. After the bee had returned twice I transposed the papers; but she returned to the honey on the blue paper. I allowed her to continue this for some time, and then again transposed the papers. She returned to the old spot and was just going to alight when she observed the change of colour, pulled herself up, and without a moment's hesitation darted off to the blue. No one who saw her at that moment
could have the slightest doubt about her perceiving the difference between the two colours. I also made a number of similar observations with red, yellow, green, and white.”
The remarkable power which many animals have of finding their way back after having been carried a long distance from home has been explained by some persons as due to a special faculty which has been termed a “sense of direction.” In connection with this subject M. Fabre made a number of interesting and amusing experiments. “He took ten bees belonging to the genus Chalicodoma, marked them on the back with a spot of white, and put them in a bag. He then carried them half a kilometre in one direction, stopping at a point where an old cross stands by the wayside, and whirled the bag rapidly round his head. While he was doing so a good woman came by, who was not a little surprised to find the professor standing in front of the old cross solemnly whirling a bag round his head, and, M. Fabre fears, strongly suspected him of some satanic practice. However this may be, M. Fabre, having sufficiently whirled his bees, started off back in the opposite direction, and carried his prisoners to a distance from their home of three kilometres. Here he again whirled them round and let them go one by one. They made one or two turns round him, and then flew off in the direction of home. In the meanwhile his daughter Antonia was on the watch. The first bee did the mile and three-quarters in a quarter of an hour. Some hours after two more returned; the other seven did not reappear. The next day he repeated this experiment with ten other bees; the first returned in five minutes, and two more in about an hour. In this case again seven out of ten failed to find their way home. In another experiment he took forty-nine bees. When let out a few started wrong, but he says that, ‘while the rapidity of the flight allows me to recognise the direction followed,’ the great majority flew homewards. The first arrived in fifteen minutes. In an hour and a half eleven had returned, in five hours six more, making seventeen out of forty-nine. Again, he experimented with twenty, of which seven found their way home. In the next experiment he took the bees rather further—to a distance of about two miles and a quarter. In an hour and a half two had returned, in three hours and a half seven more; total, nine out of forty. Lastly, he took thirty bees. Fifteen, marked rose, he took by a roundabout route of over five miles; the other fifteen, marked blue, he sent straight to the rendezvous, about a mile and a half from home. All the thirty were let out at noon, by 5 in the evening seven ‘rose’ bees and six ‘blue’ bees had returned, so that the long detour had made no appreciable difference. These experiments seem to M. Fabre conclusive. The
demonstration is sufficient. Neither the bewildering movements of a rotation like I have described; neither the obstacle of hillocks to pass over and of woods to cross; neither the snares of a track which starts, goes back, and comes again by a very circuitous way can confuse the Chalicodomas on their homeward way and hinder them from coming back to the nest.” When these experiments are summarised, however, it appears that only forty-seven bees out of 144 actually found their way home, which is a very small proportion when the question of a special unerring instinct is involved.
The following experiment, conducted by the late Mr. Romanes, conclusively proves that it is by sight, and sight alone, that bees find their way home: “In connection,” he says, “with Sir John Lubbock's paper at the British Association, in which this subject is treated, it is perhaps worth while to describe some experiments which I made last year. The question to be answered is whether bees find their way home merely by their knowledge of land - marks, or by means of some mysterious faculty usually termed a ‘sense of direction.’ The ordinary impression appears to have been that they do so in virtue of some such sense, and are therefore independent of any special knowledge of the district in which they may be suddenly liberated; and, as Sir John observes, this impression was corroborated by the experiments of M. Fabre. The conclusions drawn from these experiments, however, appeared to me, as they appeared to Sir John, unwarranted by the facts, and therefore, like him, I repeated them, with certain variations. In the result I satisfied myself that the bees depend entirely upon their special knowledge of the district or land-marks, and it is because my experiments thus fully corroborate those which were made by Sir John that it now occurs to me to publish them. The house where I conducted the observations is situated several hundred yards from the coast, with flower-gardens on each side and lawns between the house and the sea. Therefore bees starting from the house would find their honey on either side of it, while the lawns in front would be rarely or never visited, being themselves barren of honey and leading only to the sea. Such being the geographical conditions, I placed a hive of bees in one of the front rooms on the basement of the house. When the bees became thoroughly well acquainted with their new quarters by flying in and out of the open window for a fortnight I began the experiments. The modus operandi consisted in closing the window after dark when all the bees were in their hive, and also slipping a glass shutter in front of the hive-door, so that all the bees were doubly imprisoned. Next morning I slightly raised the glass shutter, thus enabling
any desired number of bees to escape. When the desired number had escaped the glass shutter was again closed, and all the liberated bees were caught as they buzzed about the inside of the shut window. These bees were then counted into a box, the window of the room opened, and a card well smeared over with birdlime placed upon the threshold of the beehive, or just in front of the closed glass shutter. The object of all these arrangements was to obviate the necessity of marking the bees, and so to enable me not merely to experiment with ease upon any number of individuals that I might desire, but also to feel confident that no one individual could return to the hive unnoticed; for whenever a bee returned it was certain to become entangled in the birdlime, and whenever I found a bee so entangled I was certain it was one I had taken from the hive, as there were no other hives in the neighbourhood. Such being the method, I began by taking a score of bees in the box out to sea, where there could be no land-marks to guide the insects home. Had any of these insects returned, I should next have taken another score out to sea (after an interval of several days so as to be sure that the first lot had become permanently lost), and then before liberating them have rotated the box in a sling for a considerable time, in order to see whether this would have confused their sense of direction. But as none of the bees returned after the first experiment it was clearly needless to proceed to the second. Accordingly I liberated the next lot of bees on the sea-shore; and as none of these returned I liberated another lot on the lawn between the shore and the house. I was somewhat surprised to find that neither did any of these return, although the distance from the lawn to the hive was not above 200 yards. Lastly, I liberated bees in different parts of the flower-garden, and these I always found stuck upon the birdlime within a few minutes of their liberation; indeed, they often arrived before I had had time to run from the place where I had liberated them to the hive. Now, as the garden was a large one, many of these bees had to fly a greater distance in order to reach the hive than was the case with their lost sisters upon the lawn, and therefore I could have no doubt that their uniform success in finding their way home so immediately was due to their special knowledge of the flower-garden, and not to any general sense of direction. I may add that, while in Germany a few weeks ago, I tried on several species of ant the same experiments as Sir John Lubbock describes in his paper as having been tried by him upon English species, and here also I obtained identical results—in all cases the ants were hopelessly lost if liberated more than a moderate distance from their nest.” Mr. Romanes's experiments, therefore, as he
himself says, entirely confirm the opinion Lord Avebury expressed—that there is not sufficient evidence among insects of anything which can justly be called a “sense of direction.”
In conclusion, I should like to allude to the very remarkable discovery made by Lord Avebury some years ago in connection with the limits of vision in ants—i.e., the power possessed by those insects of perceiving the ultra-violet rays of the spectrum, which are invisible to human eyes. This fact was elicited by means of a very exhaustive series of experiments, during which the ants were placed under variously coloured glasses, and also under two distinct, though exactly similarly coloured, chemical solutions, one of which intercepted the ultra-violet rays, whilst the other allowed these invisible rays to pass through it The results of these numerous experiments are very conclusive, and are recounted at length in Lord Avebury's delightful book on “Ants, Bees, and Wasps” I will not now repeat the details of these experiments, but the following general reflections suggested by this discovery are of more than passing interest, and may well conclude this address: “Again, it has been shown that animals hear sounds which are beyond the range of our hearing, and that they can perceive the ultra-violet rays, which are invisible to our eyes. Now, as every ray of homogeneous light, which we can perceive at all, appears to us as a distinct colour, it becomes probable that these ultra-violet rays must make themselves apparent to the ants as a distinct and separate colour (of which we can form no idea), but as different from the rest as red is from yellow or green from violet. The question also arises whether white light to these insects would differ from our white light in containing this additional colour. At any rate, as few of the colours in nature are pure, but almost all arise from the combination of rays of different wavelengths, and as in such cases the visible resultant would be composed not only of the rays we see, but of these and the ultra-violet, it would appear that the colours of objects and the general aspect of nature must present to animals a very different appearance from what it does to us.
“These considerations cannot but raise the reflection how different the world may—I was going to say must—appear to other animals from what it does to us. Sound is the sensation produced on us when the vibrations of the air strike on the drum of our ear. When they are few the sound is deep; as they increase in number it becomes shriller and shriller; but when they reach forty thousand in a second they cease to be audible. Light is the effect produced on us when waves of light strike on the eye. When four hundred millions of millions of vibrations of ether strike the retina in a second they produce red, and as the number increases the colour
passes into orange, then yellow, green, blue, and violet. But between forty thousand vibrations in a second and four hundred millions of millions we have no organ of sense capable of receiving the impression. Yet between these limits any number of sensations may exist. We have five senses, and sometimes fancy that no others are possible. But it is obvious that we cannot measure the infinite by our own narrow limitations.
“Moreover, looking at the question from the other side, we find in animals complex organs of sense richly supplied with nerves, but the function of which we are as yet powerless to explain. There may be fifty other senses as different from ours as sound is from sight; and even within the boundaries of our own senses there may be endless sounds which we cannot hear, and colours, as different as red from green, of which we have no conception. These and a thousand other questions remain for solution. The familiar world which surrounds us may be a totally different place to other animals. To them it may be full of music which we cannot hear, of colour which we cannot see, of sensations which we cannot conceive. To place stuffed birds and beasts in glass cases, to arrange insects in cabinets and dry plants in drawers, is merely the drudgery and preliminary of study; to watch their habits, to understand their relations to one another, to study their instincts and intelligence, to ascertain their adaptations and their relations to the forces of nature, to realise what the world appears to them—these constitute, as it seems to me at least, the true interest of natural history, and may even give us the clue to senses and perceptions of which at present we have no conception.”