Savory - Arachnida 1977

152 Ill. PROLES ARACHNES
FIG. 52. Leg of spider. (i) Coxa; (ii) trochanter; (iii) femur; (iv) patella; (v) tibia;
(vi) metatarsus; (vii) tarsus.
( 1932) has described the action of the claws and claw-tufts on the
threads of the web. The set of the claws gives a twist to the thread, and
the claw-tuft forms a springy pad which releases the claws as the spider
runs. In this unexpectedly elaborate way the spider avoids entanglement.
The first leg is often but not always the longest, and only rarely
is it held stretched out in front of the animal. In some male spiders it is
decorated with tufts of setae or with black or coloured patches, which
are displayed before the female during courtship.
The underside of the prosoma is formed from two unequal plates of
chitin, the labium and the sternum. The former, which is the sternite of
the second body somite, is situated between the maxillary lobes of the
pedipalpi, and is variable in shape: square or elongated, semicircular or
oval. Directly above it is a flattened cone of tissue, the rostrum. Below
the rostrum is a chitinous plate, the epipharynx, and above the labium
is a similar, corresponding plate, the hypopharynx. The epipharynx is
marked with a longitudinal groove, the stomodaeum, up which the
liquid food rises into the oesophagus, partly by surface tension and partly
by the sucking action of the stomach within.
The ste~num is generally oval or heart-shaped, but many variations
are to be found in different families. It is slightly convex, and generally
marked on each side by four bays or acetabula, which receive the coxae
of the legs. There are also lyriform organs on its surface. The sternum
probably represents the fused sternites of the third to the sixth somites,
and Giltay has reported the existence in certain young specimens,
18. THE ORDER ARANEAE 153
suitably fixed, of three transverse striations, dividing it into the four
regions expected. But this appearance cannot be seen in any adult.
The opisthosoma of spiders is normally a cylindrical or oval sac,
with no outward sign of segmentation. Sometimes it has no pattern,
but often there is a longitudinal mark above the heart, as well as small
depressed points due to internal muscle attachments. Sometimes, however,
there is an elaborate and even beautiful pattern. Segmentation
persists in the sub-order Liphistiomorphae, where tcrgites indicate the
existence of 12 opisthosomatic somites, while a smaller number of
sternites protect the opisthosoma below. In some families an unsegmented
dorsal plate is found.
The most remarkable feature of spiders' opisthosomas is the way in
which they may, in some families, be strangely modified and developed
into bizarre and fantastic forms. No other order of Arachnida shows
anything like this, and the phenomenon cannot be readily explained.
It may reasonably be said that a prickly abdomen is a discouragement to
the tender mouths of predators, that a short rounded one may be helpful
because the owner may be mistaken for a mollusc or a beetle, or that a
long, thin one may mimic a caterpillar; but the peculiar shapes that are
to be found seem to be merely awkward to control and valueless in
themselves.
The underside of the opisthosoma shows more features than the upper.
The region near the pedicel is more convex than the rest and is called
the epigastrium: it is separated by the epigastric furrow. Two booklungs,
or the two anterior book-lungs of four-lunged spiders, lie in the
epigastrium, and are conspicuous as pale patches. Behind them are
either the posterior book-lungs or a pair of spiracles leading to tracheal
tubes. In many species these spiracular tracheae lie at the ends of a
transverse furrow, not easy to see and often far back, near the spinnerets.
The genital orifice lies in the middle of the epigastric furrow. In the
male the vas deferens has a very small aperture, difficult to discern and
unprotected by any epiandrium. The oviduct of the female has a larger
aperture, in close association with the single or paired opening of the
spermathecae, the whole surrounded by a complex epigyne (or epigynum)
(Fig. 53). In this the actual vulva is protected by an operculum,
the scape, and in the most elaborate forms there is a downward projection
from the anterior side of the scape, called the crochet or clavus.
This is occasionally accompanied by a second, posterior, process, the
parmula: the two together act as a short ovipositor. Again, the unique
character of Araneae is emphasized by the fact that in no other order is
the female system provided with so elaborate an exit, which is different
in every species, and, like its counterpart, the male palp, is the invariable
method of identifying a species.