Savory - Arachnida 1977

76 11. DE ARACHNIDIS
Telotaxis is an orientation of the animal followed by movement
towards the source of the stimulus, which is acting as if it were a goal
or an end to be sought. Telotactic movements haYe therefore an outward
appearance of purpose, since there is a minimum of deviation in
the path taken.
It has been asserted that true telotaxis is shown only by movements
towards or away from a source oflight, but there can be no doubt that
vibrotaxis, which is so conspicuous a part of the behaviour of spiders,
is an equally good example. It is readily to be seen whenever a web
spider runs across its web to seize its prey. At the same time it should be
remembered that when vibrotaxis was first recognized by Barrows
( 1915) he also described the result of simultaneous oscillation of the
web at two foci, when the spider took a path between them. This is
typical tropotaxis.
Vibrotaxis is not confined to web spiders, for spiders that hunt on the
surface of water respond in the same way to the ripples produced either
by struggling insects or by the prong of a vibrating tuning fork. It can
readily be seen that this tropism is a direct consequence of the flexor
reflex. Let it be supposed that the spider is standing with all eight legs
on the surface, as shown in Fig. 25. The ripples passing under the right
Frc. 25. Diagram to illustrate the mechanism of vibrotaxis.
legs will have a smaller amplitude than those passing under the left
legs, and thus there is a more intense stimulation of the right legs,
followed by a greater degree of flexion of the femoral muscles. The consequence
of this forced movement is that the extended left legs exert a
turning effect relatively to the less extended right legs so that as the
spider runs it is automatically directed towards the origin of the
disturbance.
The distinction between kineses and taxes is well illustrated by considering
movements determined by the force of gravity. An arachnid
which habitually climbs upwards or downwards shows a discrimination
between "up" and "down", which is a response to the direction of the
stimulus. This is pure geotaxis. As it climbs up (or down) the force of
gravity does not alter by an appreciable amount. Therefore the animal
does not move more and more quickly as it approaches the top (or the
8. ETHOLOGY: BEHAVIOUR 77
bottom) of a tree trunk. If it did so, it would be exhibiting geokinesis.
Because it does not, there is no geokinesis in arachnid behaviour.
The essential difference between the tropism theory and the taxis
theory is a difference in interpretation, the former being mechanistic,
the latter teleological. If a biologist can convince himself, in spite of the
impossibility of obtaining direct evidence of the animal's subjective
state, that the animal has a conscious appreciation of sensations, then
the taxis theory will afford an acceptable interpretation of many facts.
For instance, Ki.ihn states that flies move towards the window if they are
chased and that on the theory of tropisms such a sudden change in the
sign of the reaction is inexplicable, as in any other such change when it
appears as a result of suddenly approaching danger. As an innate
reaction to "the simple sensation of danger", such behaviour may be
intelligible.
The present position with regard to the interpretation of the behaviour
of invertebrates like Arachnida may be summarized as follows.
The mechanistic theories, such as the tropism theory, were valuable
because they pointed the way to a great deal of experimental work, most
if not all of which may be broken down into measurement of the reactions
of the sense organs and the response of the muscles. It may be
useful-perhaps it may be necessary-to know the facts about these
actions, but their relation to any psychic events is obscure and experiment
tells us nothing about these. The tactic view of behaviour may lead
a biologist to ask whether a creature which, for example, lurks under
stones does so for some conscious reason, and then force him to give a
negative answer simply because the taxes which guided it have been
recognized and named.
Mechanistic theories of behaviour carry us very little further than
this. Despite the persuasiveness of the mechanists, an animal does not
seem to be a mechanical device; it seems to be much more obviously a
sentient being, trying, now in one way, now in another, to survive amid
the perils of existence.
The position is no more than a logical consequence of a very simple
dilemma. Either there is nothing in animals that corresponds to the
origin of thoughts in man, or there is something in animals in which
originate thoughts, or such homologues of thought as may occur in
cerebral ganglia. The nature of these thoughts and of the mind in which
they are developed is a fundamental problem of biopsychology, to which
this chapter can do no more than serve as one path along which that
problem may be approached.
Reflexes, kineses and taxes may well be looked upon as patterns of
mechanical behaviour, evolved and retained in response to the environmental
conditions in which Arachnida can survive. One is inclined to