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.