Savory - Arachnida 1977

214 Ill. PROLES ARACHNES
The seventh, eighth and ninth somites are much reduced in size.
Their exoskeletal supports are complete rings of chitin and the segments
can be drawn together like a telescope, making them even more
inconspicuous. In this condition, which is probably their normal one,
the eighth and ninth somites can only be seen inside the seventh, their
posterior edges forming concentric circles. The anus is a transverse
slit on the last somite: no glands, like those of U ropygi, open beside it.
The sternum is not normally visible from the outside, since it is
covered by the pedal coxae. If these are removed it is seen as a small
plate, lying longitudinally.
The chelicerae are composed of two segments, and are chelate (Fig.
86). The first, basal, segment is short and stout and provided with a
transverse belt of close setae on the ventral surface and a similar smaller
belt on its dorsal surface. One or two strong processes, with sharp
edges and teeth, rise from the distal end of this segment, and against
them works the second segment. This is a sickle-shaped point, often with
a serrated edge, and closely resembles the corresponding organ in
Araneae.
The pedipalpi are of six segments. They are remarkable in that their
coxal segments are fused together in the middle line instead of acting
independently as gnathites. In this Ricinulei resemble Uropygi. This
common maxillary plate bears the ordinary pair of pal pi, consisting of
two trochanters, femur, tibia and tarsus. The first trochanter moves
slightly up and down, but the second is capable of a complete rotation
2 7. THE ORDER RICINULEI 215
(Fig. 87), turning through 180° and directing the remaining segments
in the opposite way from that in which they usually lie. The femora are
stout, the tibiae long. The joint between these two allows them to touch
each other when fully flexed and, on account of this mobility, the tip of
the limb can reach the mouth from either direction. The distal end of
the tibia carries a small serrate process against which the tarsus moves
and so makes the pedipalpi also chelate on a small scale.
FIG. 87. Pedipalpi of Ricinoides westerrnanni, showing range of rotation. After Hansen
and Sorensen.
FIG. 86. Chelicera of Ricinoides crassipalpe. After Hansen and Sorensen.
The legs of these curious Arachnida also have peculiar characteristics.
The first three pairs of coxae are immovably coalesced and the
fourth pair which is concerned with the linkage of the prosoma and
opisthosoma is freely movable. The legs are not all similarly constituted.
The first has one trochanter and one tarsal segment, the second one
trochanter and five tarsals, the third two trochanters and four tarsals
and the fourth two trochanters and five tarsals. The other segments are
as usual, and hence the total number of segments on the four legs are
seven, 11, 11 and 12. The second pair of legs is always the longest, then
the fourth and the first is always the shortest. All the legs are devoid of
ordinary spines and all end in two simple claws without teeth, situated
in a small excavation at the end of the tarsus.
The male organs of Ricinulei are to be found in a unique situation,
on the metatarsus and tarsus of the third pair of legs. The metatarsus
and first two segments of the tarsus are strangely modified by cavities
and by fixed and movable processes. When the tarsus is bent upwards,