Savory - Arachnida 1977

72 II. DE ARACHNIDIS
possession of the nervous system, supporting the body on its eight legs
against the unceasing gravitational force. For animals like the harvestman
Leiobunum rotundum, with its tiny body and long symmetrically
arranged legs, the tensions in the muscles of all legs are probably almost
equal, and for spiders like the Linyphiidae which hang inverted below
their hammock-like webs, the tensions are probably not very unequal.
It can scarcely be a coincidence that these types are particularly sensitive
to surrounding disturbances. In the common garden spider, which
hangs head-downwards in its lovely orb-web, the weight must clearly
be disproportionately supported by the two hind pairs of legs and the
same is true of almost any arachnid that stands on the ground. The
heavy opisthosomatic region has no legs of its own and the greater part
of its support must come from the posterior pair of thoracic legs. It
should not be surprising, therefore, that when the equilibrium of the
tonic reflexes is upset, the readiest response is a flexion of the femora, so
that the patellas tend to meet over the middle of the animal, and especially
a flexion of the forelegs. This simple automatic response, which may
be called the flexor reflex, is easily witnessed whenever a spider is, as
we would say, frightened, and several valuable consequences follow
from its operation.
One of these is the flash-colouring method of protection, which most
naturalists would associate with the tree frogs of tropical forests, but
which is also well shown by several British spiders. A good example is
Segestria senoculata, a common species with diamond-shaped marks on its
abdomen and bright tawny femora. As it runs, the moving femora are
conspicuous, but when it suddenly stops and draws in its legs this
brightness vanishes and the observer finds the spider invisible.
Without any help from flash-colours, the flexion assists in producing
invisibility in other spiders. If on the beach in certain parts of the coast
the heaps of dried seaweed are turned over, there runs out a sombrelycoloured
active spider, Philodromus fallax. As it runs it may easily be
mistaken for a grain of sand, rather bigger than the average, rolling
down a slope, and, in fact, such grains of sand are often mistaken for
spiders. But there is this difference. \Vhen the pellet of sand comes to
rest it can be seen; when the spider stops moving it is invisible. It does
not bury itself nor leap suddenly aside; it simply stops and flexes its legs.
And it is gone. One may stare straight at one of these spiders and be
quite unable to distinguish it until it moves again.
This leads naturally to the obvious and probably the most important
of all the consequences of the reflex, the action metaphorically described
as "feigning death" and less inaccurately as the cataleptic reflex. In
many Arachnida a sudden disturbance causes them to draw in their
legs and fall motionless, and in some the caresses of the opposite sex
8. ETHOLOGY: BEHAVIOUR 73
produce the same condition. Various considerations make it clear that
the animal is not in a true state of catalepsis. Often after first falling
motionless it will re-arrange its legs slightly, and in some species a
periodical tremor runs through the limbs. In addition to these movements,
which would not be seen in true catalepsy, there is not the complete
insensibility which would be expected. If the motionless spider be
gently touched it will often get up and run away. If the spider is lying
low at the end of a thread attached to its web and the web be touched
with a vibrating tuning-fork, the spider at once awakes and returns to
its web.
The whole action is nothing but a sustaining of the flexor reflex and
the more closely it is studied, the more clearly it shows the characteristics
of reflexes in general. Many reflexes continue to discharge after
the stimulus ceases, and in the "cataleptic" arachnid we are witnessing
the retention of the position produced by the flexion of the femoral
muscles. All reflexes, however, are subject to fatigue which causes them
to give up possession of the nerve-paths in favour of some other impulse,
previously inhibited. Thus the quiescent spider awakes and a response
to a probably new situation occurs.
In several families of spiders the flexor reflex operates with a different
result. The common house spiders, Tegenaria, are familiar examples
of spiders which rest in the corner of a sheet-web. Their forelegs
are outstretched and their claws grasp the silk so that often the sheet
can be seen to be drawn up into small cones. If an insect brushes against
the web, the legs are jerked inwards, plucking sharply at the sheet;
and the result is not invisibility for the spider but a further entanglement
of the insect. Orb-web spiders perform exactly the same action,
the web shakes and the spider plucks at it. Very often the spider
then turns about and repeats the jerking in another direction, an action
which, despite its intelligent appearance, is only a consequence of
unequal tensions in the threads of the web. The inequality may be due
to the recently added weight of the captive and it is this which turns the
spider round, and not an attempt to discover whereabouts in the web
the arrival has landed. Like beauty, which lies in the eye of the beholder,
purposiveness lies in his interpretation of what he beholds.
A consideration of a second reflex action furnishes support for this
view. If a reflex is found to have no manifest result or no meaning in an
anthropomorphic sense, the observer refrains from imputing purpose w
the animal and is content to describe the action itself as a mystery.
Spiders often exhibit one such action. If the flexor muscles raise the
femora while the extensor muscles straighten the patella, the result will
be that the spider lifts its leg into the air, stretched up at right ~ngles
to its body. This is most often seen in orb-web spiders, for which it