Savory - Arachnida 1977

62 11. DE ARACHNIDIS
of the exoskeleton prevents expansion of the internal fluids, so that
vessels and ducts are fatally closed by pressure. This is normally
avoided by the adoption of a cryptozoic life, so that the animal, under
stones, leaves or logs, is always in the shade and is active only at night.
Scorpions, however, have been shown by Alexander ( 1958) to meet
a rise of temperature by a remarkable habit which she has described
as stilting. The scorpion straightens its legs, so that its body is raised
above the ground and free circulation of air around it is made possible.
Experiment made it clear that heat is the stimulus for this action and
that control of the body temperature does, in fact, follow it.
A widespread method of protection and of escape from predatory
enemies, which is common among both Arachnida and Crustacea, is
the quick dropping of a captured limb. This permits the animal to
escape, purchasing life and freedom at the temporary expense of a
limb. The habit was closely studied by Wood ( 1926), who divided the
circumstances in which it occurs into the following.
(i) Autotomy is the reflex self-mutilation or automatic severance of a
limb from the body. This does not exist in the Arachnida, but is found
among Crustacea.
(ii) Autospasy is the casting of a limb when pulled by some outside
agent, like the forceps of an investigator.
(iii) Autotilly is the pulling off of a limb by the animal itself, as
when an injured leg is seized in the chelicerae and severed from the
body.
(iv) Autophagy is the act of eating a part of the body after severance
from the rest.
In addition to these a fifth term, autosalizy, has been coined by
Pieron ( 1924) with much the same meaning as autotilly, but apparently
involving an element of choice or decision on the part of the animal.
According to most views of arachnid behaviour, this phenomenon could
not be exhibited by them. It is impossible to imagine that the arachnid,
as it pulls on its imprisoned leg, shall come to a decision to run away on
seven legs rather than perish.
Wood's experiments showed conclusively that the shedding of a limb
is not the result of a reflex action or of a special mechanism, as had
previously been believed. It is simply the inability of the skeletal and
muscular components of the leg to resist more than a certain force. The
leg, when pulled, parts at its weakest point. Consequently, all spiders
cast their legs at the coxa-trochanter joint, all "Pedipalpi" at the
patella-tibia joint and Opiliones at the trochanter-femur joint. On the
other hand, in the scorpions, king-crabs, and some of the mites there is
no predetermined locus of fracture. The muscles are evenly arranged
throughout the legs and the chitin is well provided with longitudinal
7. BIONOMICS: GENERAL HABITS 63
fibres at all the interarticular membranes. These factors, combined
with a lack of response on stimulation, make autotomy and autospasy
impossible and autotilly very unlikely.
The spider Tidarrenfordum, however, provides a remarkable example
of autotilly. The palpal organ of the mature male is large; it is almost
halfthe size of the spider's small body, and Chamberlin and lvie (1933)
found that while all young specimens of both sexes had two palpi, the
mature and sub-adult males had all lost either the right or the left
palpus. Bonnet ( 1935) subsequently discovered that during the last
instar the sex organs inside the palp cause it to swell to a considerable
size. To the small owner one such organ was an inconvenience, but two
were intolerable, and the spider therefore pulled off, with its chelicerae,
one of its overgrown appendages. It seemed to be a matter of indifference
whether the right or the left was removed, but the self-mutilation
was invariable. After the last ecdysis the remaining palpus was fully
developed and functionaL It is clear that this autotilly is habitual in this
species and the loss of one palp is not the result of an accident.
A method of defence that is interesting because it is unusual is
adopted by several species of the spider family Theraphosidae. Specialized
"hairs" of four different kinds grow on the dorsal of the
abdomen and from here they can be scraped off in numbers by the
metatarsal spines of the spider's fourth legs. Under threat a shower of
these hairs is directed towards the predator. They produce intense
irritation of the human skin, and seem to be used chiefly against small
mammals (Cooke, 1974).
Among the Uropygi defence by secretion of a nauseous vapour is
more fully developed than among the familiar Opiliones. Glands at the
base of the flagellum can produce a spray of acidic droplets, which,
owing to the mobility of the abdomen can be directed over a wide
range. The method is said to be most against birds and
mammals.
An arachnid's constant occupation, like that of every other animal,
is to provide itself with food.
The average arachnid, if there is such a thing, is a predator, and the
picture that rises in the mind as we think of its feeding is one of a
nocturnal wanderer, which does not so much seek its prey as come upon
it by chance, detecting it by the sense of touch and seizing it forthwith.
No doubt this is broadly accurate for many and there is little in such
casual collecting that is worth attention. It seems reasonable to regard
it as the primitive method of obtaining food, to be contrasted with more
specialized methods evolved later. The most obvious of these improvements
is perhaps the acquisition of greater speed and strength, or, as in
spiders, the highly specialized and unique method of web spinning.