MENTAL POWERS OF SPIDERS

Before leaving the garden-spider let us undertake some little investigation of its mental powers—if it possesses any. The commonest mistake with regard to all animals is to interpret their actions from the human standpoint, and to credit them with emotions and with deliberate forethought of which there is in reality no proof whatever. The power to spin such a complicated snare as we have just described predisposes us to attribute a high order of intelligence to a creature capable of such an achievement, and when it “shams death” on being disturbed we immediately pronounce it “cunning.” The wildest conclusions are sometimes arrived at. One author, for instance, states that he has seen an Attid spider “instructing its young ones how to hunt” and adds that “whenever an old one missed its leap, it would run from the place and hide itself in some crevice as if ashamed of its mismanagement.” Such inferences, of course, were entirely unwarranted from the facts observed. Now the fact that a newly-hatched garden-spider can make a complete snare without ever having seen the operation performed immediately relegates that action to the realm of instinct,—not less wonderful than intelligence perhaps, but certainly quite distinct from it. With the much discussed origin of instinct we are not here concerned, but a pure instinct differs from intelligence in this: that it is due to inherited nervous mechanism and results in actions the object of which may be quite unknown to the actors. There is no conscious adaptation of means to an end. When a young spider spins a web there is not only no evidence that it does so with the deliberate purpose of catching flies, but many known facts go to prove that it performs the feat, “because it feels as if it must,” and is quite ignorant of the purpose to be subserved.

It is no doubt quite beyond our power to ascertain accurately the mental condition of a spider, but it is perfectly easy to make a few illuminating experiments on two points which have a very decided bearing on intelligence:—the development of the senses, and the degree of what has been called educability, or the power of learning from experience. To what extent can the spider see, hear, smell, feel, taste? How far is it capable of varying its action as the result of experience? The senses, as far as we know, are the principal—if not the only—avenues by which external impressions can reach the seat of intelligence, and there is no surer indication of the intelligence of an animal than the degree to which it is susceptible of education. Probably most readers know the immortal story of the pike cited by Darwin in the Descent of Man. The pike was in an aquarium, separated by a sheet of glass from a tank in which were numerous small fish. Not till three months had expired did the pike cease to dash itself against the glass partition in its attempts to seize the fish in the neighbouring tank. It then desisted and had evidently learnt something—but what? After three months, the glass partition was removed, but the pike refused to attack those particular fish, though it immediately seized any new specimens introduced to the tank. All that it had apparently learnt was that an attack on a particular fish resulted in a violent blow on the nose. Some degree of intelligence must be conceded to the pike, but it can hardly be considered of a high order.

Now the garden-spider possesses eight eyes, and might be expected to see fairly well, but the experimenter will very soon come to the conclusion that the habitual use it makes of them—at all events in day-light—is very slight. Touch a web with a vibrating tuning-fork and the spider will rush to the spot and investigate the instrument with its fore-legs before distinguishing it from a fly. Remember, however, that this is only true of what are sometimes called sedentary spiders; species which hunt their prey have much better vision. Yet even among sedentary spiders the power of sight is not negligible, for a most trustworthy observer states that he has several times seen Meta segmentata, a very common small Epeirid, drop from its web to secure an insect on the ground beneath, and return with it by way of the drop line, and the same action has been observed in the case of Theridion, which spins an irregular snare.

There are peculiar difficulties attending experiments on the subject of hearing. An absolutely deaf person may be aware of the sounding of a deep organ note through the sense of feeling, and a well-known experimenter was on the point of drawing interesting conclusions from the behaviour of a spider in response to the notes of a flute, when he found that precisely the same results were obtained by a soundless puff of air. It seems hardly possible to make sure, in the case of a spider in a snare, that the sound vibrations are not felt, apart from any sense of hearing, and it is a remarkable fact that it is only the snare-spinning spiders that make any response to sounds:—free-roving spiders are apparently quite deaf.

In experimenting with sound we must take two precautions: the instrument used must not necessitate any marked action which may be visible to the spider, nor must it give rise to palpable air-currents. These requirements are best met by a tuning-fork of not too low a pitch. We cannot feel the air vibrations emanating from it, but can only perceive them by the ear, but we have no proof that the spider’s sense of touch ceases precisely at the same point as our own. However, no better instrument for experiment seems to be available, so we take a tuning-fork, and approach it cautiously—in the quiescent state—towards the spider, stationed, we will suppose, in the centre of its snare. No notice is taken, and we carefully withdraw it, set it vibrating, and approach it again in the same manner. There is now generally a response, the spider raising its front legs and extending them in the direction of the fork, or, if the sound is loud, dropping suddenly by a thread and remaining suspended some inches below the snare. The experiment should be repeated several times with the fork sometimes still, sometimes vibrating, and the conclusion arrived at will be that the spider is aware of the vibrating fork—but by which sense? It is noteworthy that a fork giving a low note is always most effective.

Now here is a very remarkable fact. In two widely different groups of spiders—the Theraphosidae or so called “bird-eating spiders” and the Theridiidae—there are species with a stridulating or sound-making apparatus, and we should hardly expect a deaf creature to evolve an elaborate mechanism for the production of sound. This is a matter, however, that we shall discuss later.

No amount of research has succeeded in localising the sense of hearing in spiders, supposing it to exist. The creature may lose any of its five pairs of limbs (four pairs of legs and one pair of pedipalps) without alteration in its response to sound. If the front legs are missing the second pair are raised when the vibrating fork is approached.

It is fairly easy to test the sense of smell in these creatures, the only necessary precaution being that no acid or pungent substances capable of having an irritating effect on the skin, such as vinegar or ammonia, must be employed. Such perfumes as lavender or heliotrope are free from this defect. Take a clean glass rod and present it to the spider as before, and no notice is taken. Now dip it in oil of lavender, allow it to dry, and present it again. Most spiders respond to such a test, Epeirids generally raising the abdomen, and rubbing one or other of the legs against the jaws, while jumping spiders generally raise the head and back away from the rod. Different essences produce different effects, but there is seldom any doubt that the creature is aware of their presence; it is not deficient in the sense of smell, but its localisation has hitherto baffled research.

The sense of taste does not seem to have been made the subject of any definite experiments among spiders, though such experiments might well lead to interesting conclusions, and the reader might do worse than undertake some on his own account. It would be easy, for instance, to supply a garden-spider with various insects which are generally rejected by other insectivorous animals, and to note its behaviour. It might refuse to have anything to do with them, or it might sample them and turn away in disgust. In the first case the explanation might be that it was warned of their probably evil taste by their coloration or smell, but in any case here is an interesting little field for research. It is the general belief among arachnologists that the sense of taste is well developed among spiders, and it is highly improbable that a sense so necessary for the discrimination of suitable food should be lacking in animals with so respectable a sensory equipment.

There is no doubt at all that the sense of touch is extremely well developed in spiders, especially perhaps, in the sedentary groups, and it is probable that, under ordinary circumstances, the garden-spider works almost entirely by its guidance. Whether in the centre of the web or in its retreat under a neighbouring leaf it is in direct communication with every part of its snare by silken lines, and the least disturbance usually suffices to bring it to the spot; and then, as we have said, it will generally touch the disturbing object, however unpromising in appearance, before deciding on its line of action. There is little doubt that many of the numerous hairs and bristles with which its limbs are furnished are distinctly sensory in function.

So much, then, as to the senses of spiders; but what about their “educability”—their power of learning from experience? Here is evidently a wide subject, and a difficult one full of pit-falls for the unwary, but we may nevertheless draw some inferences from the quite elementary experiments on the senses which have been outlined above. A spider drops on account of the sounding of the tuning-fork in its neighbourhood; can it be educated to take no notice of the sound after repeatedly finding that no evil consequences follow? It will perhaps be most instructive to give in a condensed form the results of an actual experiment selected from many performed by two American arachnologists, George and Elizabeth Peckham, whose researches have thrown more light than any others upon the mental equipment of spiders. They had an individual of the small Epeirid species Cyclosa conica under observation for a month, and tested it almost daily with the tuning-fork. At the sound of the fork the spider would drop; when it had recovered itself and returned to the snare the fork would be sounded again, and so on. Now on July 20 the spider fell nine times successively—the last three times only an inch or two—and then took no further notice of the vibrating fork. On subsequent days, until August 5, she fell either five, six or seven times, except on two occasions when a day’s test had been omitted, and then eleven successive falls occurred before the spider ceased to respond. On August 5 she seemed startled at the sound but did not fall, though the fork was sounded nine times. During the remainder of the experiment she generally remained perfectly indifferent to the fork, though on one or two occasions she partially forgot her lesson and dropped a very short distance, immediately recovering herself.

Observe that the basis of educability is memory. For a fortnight, in the case of this particular spider, the lesson learnt on one day seemed to be entirely forgotten the next morning, but thereafter a definite change of habit seemed to result. This does not appear a very great intellectual achievement, but it is by no means despicable, for it must be borne in mind that the habit of dropping when alarmed is almost the only means of defence such a spider possesses, and the instinct which prompts it must be very strongly ingrained. In the words of the experimenters—“Taking this into consideration, it seems remarkable that one of them should so soon have learned the sound of the vibrating fork, and should have modified her action accordingly.”

This single experiment has been here described in some detail largely for the purpose of impressing the reader with the importance of reducing the problem to its simplest terms before any inferences are drawn, and it may well act as a model for any which he may be inclined to undertake on his own account. The more complicated the action, the more likely is the experimenter to read into it motives and mental operations which exist only in his own imagination, and with this warning we must take leave of a subject which might tempt us to encroach too much on an allotted space.