Savory - Arachnida 1977

7
Bionomics: General Habits
Every animal is constrained throughout its life to protect itself from
unfavourable circumstances and from living enemies, to feed itself,
and finally to reproduce itself; and among Arachnida the ways in
which success is achieved in this triple problem are no less diverse than
among the animals of any other class.
The dependence of an invertebrate on the physical conditions of its
immediate surroundings is always manifest. Fundamentally it underlies
much of that side of biology known as ecology, and as such is discussed
in Chapter 9. \\Then, however, an arachnid has reached an inhabitable
neighbourhood and has discovered an area in which it can remain, it
must be able to continue to respond appropriately to all the stimuli to
which it will be subject. In other words, it depends on maintaining the
efficiency of its sense organs, whether they be eyes, setae or
chemoreceptors.
Vpon this follows the always surprising fact that Arachnida constantly
preen themselves, cleansing and at the same time anointing their
cuticles with products of their own making.
The preening of spiders is the easiest of these actions to see, for a
common house spider can be made to exhibit them by dropping it into
a basin of water. It struggles and, in effect, swims to the side of the
vessel, and if it is then picked up and put back in its web, it dries itself
very thoroughly.
The legs of the second and third pairs are pulled slowly through the
opening and shutting chelicerae, and finished by sucking the water off
the tarsus. The pedipalpi are dried in the same way. The first and fourth
pair of legs are cleaned a little by this method, but mostly by rubbing
against the second or third pairs. The sternum and abdomen are rubbed
with the terminal segments of a leg. The separate actions do not take
place in any orderly fashion; a little of one is followed by a little of
another, and often two are simultaneous. The spider dodges from limb
7. BIONOMICS: GENERAL HABITS 61
to limb and from side to side without sequence, and the whole operation
may take half-an-hour.
The drying of the limbs and body may not be the sole purpose or
result of these actions. Holding the tip of a leg in the chelicerae may
allow secretions from the maxillary glands in the gnathobases to collect
on the tarsus, which then spreads the fluid over abdomen, sternum and
elsewhere.
This alternative view is supported by Shulov's full account of cornparable
behaviour in scorpions. He has described how a scorpion
puts the end of a pedipalp between its chelicerae, and adds, "At the
same time a liquid, which probably comes from the alimentary canal, is
secreted on the pincer". The legs pick up this liquid from the pedipalpi
and spread it over the lower surfaces of the body. The end of the tail
is then placed between the chelicerae and similarly coated with liquid­
Shulov calls it varnish-which it then smears over the carapace and
rnesosoma. The process may take more than an hour; it may be repeated
several times, and it very often occurs when the animal has been drinking
or has been wet.
False scorpions show a peculiar modification of the same habit, which
can often be seen in specimens, and has been fully described
by Gilbert ( 1951). He records the fact that a false scorpion starts to
brush its chelicerae and palpi against one another when the prey is
observed: the victim may be seized in a pedipalp while the other pedipalp
is being passed through the chelicerae. This has the appearance of
cleaning the pedipalp, but in actual fact it is the pedipalp which is
cleaning the chelicera. It has been observed that the pedipalp may
often be allowed to remain dirty, but the cleaning of the chelicerae is
agreement with the actions of spiders when engaged in the same way.
Harvestrnen also pass their long legs through their chelicerae at
frequent intervals, and the spectacle is the more remarkable because
of the length of the leg. By the time that the tarsus has reached the jaws
the rest of the leg is bowed into an almost complete circle, and when
released it shoots out like a spring. A most significant feature of the
preening of harvestmen is the fact that it is almost continuous when
the animal is warmed, as when they cannot escape from the rays of
the sun.
For Amblypygi this cleaning of the long antenniform legs also appears
to be difficult, but it is important because of the animals' dependence
on the trichobothria which these limbs carry. The sub-segmentation of
the tibia is a helpful adaptation, making it possible so to bend this joint
that the leg can be passed to its tip between the chelicerae.
. Excessive heat is a constant menace to all Arachnida, partly because
It tends to increase the rate of water loss and partly because the firmness