Savory - Arachnida 1977

144 Ill. PROLES ARACHNES
FIG. 45. Amblypygi; dorsal aspect. Species, StegojJhrynus dammermani.
a pointed labium. Two more sternites lie behind it. The lower side of
the opisthosoma is characterized by the reduction of the last three
sternites: the small twelfth somite may be described as a pygidium.
The chelicerae are of two segments and are very spider-like both in
form and action (Fig. 46). There are, however, large pointed tubercles
on the first segment, which have no parallel among spiders, and there
are no venom glands within. In some species the basal segment carries
a group of short spines in the inner surface, forming with the opposing
group a stridulating organ.
The pedipalpi are the chief structural characteristic of the order.
They are of six strong segments with a pointed post-tarsus, and the
coxae, femora and tibiae are conspicuous for their length. The femora
and all following segments bear sharp pointed tubercles on their inner
sides, and the pedipalpi can thus be used to secure the prey. This can
17. THE ORDER AMBLYPYGI 145
be done either by flexing one pedipalp against the Yictim, or by bringing
both to bear on it from the sides. Even so, an insect thus seized sometimes
manages to wriggle free; otherwise it is held by the pedipalpi
while the chelicerae cut off pieces and masticate them below the mouth.
The pcdipalpi ha,·c other functions. At the proximal end of the tarsus
there arc two cur\'Cd rows of specialized setae of varying length, which
form a cleaning brush, well adapted for use in the preening of the body
and limbs. These setae are white or cream-coloured, not difiicult to
distinguish from the other "hairs", and those of the lower row are
longer and more cun·ed than those above them.
'Vhen two individuals meet and indulge in combat, they try to grasp
each other with these appendages. The action is rapid, but no harm is
inflicted, and the combatants soon separate.
The femur and tibia of the pedipalp are longer in the males than in
the females, almost the only secondary sexual difference in this order.
The legs of the first pair are characteristically long and thin. They
arc not used in walking, but are stretched out in front of the animal as
tactile organs. Tibia and tarsus are divided into a large number of subsegments,
and the whiplash-like limb is generously supplied with setae.
These first legs are exceptionally mobile. 'Vhile the owner is at rest they
are in constant touch with the surroundings, gently tapping the ground
and anything thereon and bringing different impulses to the central
nervous system. They recall the second legs of Opilioncs, with their
functions exaggerated and specialized.
All legs carry trichobothria on certain segments, and the long first
legs are in particular the first to commence the vital operations of courtship.
The legs of the second, third and fourth pairs are not as peculiar
as the first pair. They consist of the ordinary seven segments followed by
two cun ed claws, and in the family Charontidae a pulvillus as well
(Fig. 4 7). Amblypygi can move very rapidly, and in the family just
named can climb perpendicular surfaces and cling below horizontal
ones.
Two pairs of book-lungs open on the second and third opisthosomatic
sternites. They are of the usual type. In addition there arc in
some families two small ventral sacs near the middle of the third sternite.
They are eversible under pressure from within, and perhaps have an
accessory respiratory function.
FIG. 46. Chelicera of Stegophrynus. After Kaestner.
FIG. 4 7. Tarsus of Charon grayi. After Kraepelin.