Savory - Arachnida 1977

20 11. DE ARACHNIDIS
can only pierce. By their position in the front of the body the chelicerae
are well placed to meet a variety of needs and perform a variety of
functions, and indeed the versatility of the arachnid chelicera is
equalled by but few arthropodan appendages. They are enormously
deYeloped in Solifugae, where they also carry out the vital operation of
placing the packet of sperm in the vagina of the female, they are used
by trap-door spiders for digging and by scorpions for making a noise.
In scorpions they contain silk glands and in spiders, poison.
Somite
I.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
SEGMENTATION OF THE ARACHNID BODY
Chelicerae
Pedipalpi
Legs I
Legs 2
Legs 3 (Mesopeltidium)
Legs 4 (Metapeltidium)
Pre-genital or Pedicel
Genital: Book-lungs I
Book-lungs 2: Pectines
Book-lungs 3: Spinnerets I
Book-lungs 4: Spinnerets 2
Tergite 6
Tergite 7
Tergite 8
Tergite 9
Tergite 10}
Tergite 11 Pygidium or post-abdomen
Tergite 12
Telson Sting; Flagellum
The pedipalpi are appendages of six
the first of which,
the coxae, most frequently have extensions, called maxillae or gnathobases,
which function as mouth parts with or without contribution
from the coxae of the anterior legs. The limbs themselves may be
simple tactile organs outwardly resembling the legs, as in spiders, or
chelate weapons of great size, as in scorpions and false scorpions. They
may be specialized in different ways, as in spiders where they act as
accessory male organs, and in Solifugae, where they terminate in suckers.
It is to be noticed that the conspicuous and actively functional limbs are
sometimes the chelicerae and sometimes the pedipalpi, but that both
are not found enlarged in the same order.
The legs are of seven segments: coxa, trochanter, femur, patella,
tibia, metatarsus and tarsus. It is the metatarsus that is missing from the
pedipalpi. They may be all alike (and this is the general rule), but in
3. MORPHOLOGY; EXTERNAL APPEARANCE 21
some orders, notably Amblypygi and Solifugae, the first pair are not used
for walking but are carried aloft and directed forwards as tactile organs.
Some confusion exists in the names applied to the segments of the
legs in Scorpiones, Amblypygi, Lropygi and Palpigradi. The problem
is a dual one. In some instances it is merely the use of a different word to
describe the same thing, as for example the alternative of praetarsus or
transtarsus to describe the extreme terminal portion of a limb.
In other instances there is a divergence of opinion as to the category
to which a segment really belongs. For example, the fourth of
the scorpion's leg is named the "tibia" by Kraepelin and the "patella"
by Petrunke\·itch. This is a more serious problem. A solution could be
reached only by a rather lengthy discussion of the exact meaning of
each of the words applied to leg segments, and would involve a discussion
of the internal musculature and also a comparison with other classes
of Arthropoda. This extremely interesting piece of work it must be
admitted, disproportionate to the design and scope of this book. The
solution seems to be that the most useful purpose will be served if the existing
nomenclature be tabulated so that divergences are made evident.
This will reduce the confusion that otherwise is likely to arise p. 22).
Legs also differ in other ways among themselves. The number of
claws is not always the same on all, and there arc differences in the
number that bear gnathobases on the coxae. In Limulus and some of the
Opiliones all the coxae assist in mastication; in scorpions only the first
two pairs; and in spiders this duty is confined to the pedipalpi and the
legs take no share in the work.
Like the other appendages, legs have several functions, even when
climbing, seizing prey, digging and
have been added to their
fundamental purpose of walking. are richly provided with sense
organs in the form of spines of different types, mentioned below, as well
as tarsal organs \·vhich are chemoreceptors, and lyriform organs of
uncertain function. It is thus no that in some forelegs the sensory
information-value has superseded the value as organs of transport,
and by members of some orders they are carried clear of the ground. Of
Opiliones it has been said that "the study of harnstmen is the study of
legs", and of the spider that "while it is sufficiently true to the traditions
of the animal kingdom to see with its eyes and taste with some part of its
mouth, it hears and it feels and it smells with its legs".
In Arachnida the legs almost always, and the pedipalpi often, end in
claws: curved, sharply pointed pieces of chitin which may be smooth or
toothed. There may be one, two or three claws. The number of teeth
varies over a wide range. Some of the claws on the legs of the spider
Liphistius, for have one or t\vo teeth, while the legs of the
spider Philaeus may h'ave 29. The number is not constant throughout