Savory - Arachnida 1977

212 Ill. PROLES ARACHNES
Posteriorly the prosoma seems to end in a transverse ridge, but
actually the end is below this ridge. This appearance is due to a second
characteristic of Ricinulei, the remarkable way in which the prosoma
and opisthosoma are linked or clasped together. On the dorsal surface
of the opisthosoma there is a deep transverse groove between the second
and third tergites, into which the prosomatic ridge fits. Ventrally the
third sternite projects forwards and forms a pair of pocket-like spaces
into which fit processes from the posterior borders of the fourth coxae.
The consequence of this curious arrangement is that the animal does
not appear to be pediculate (Fig. 84), the prosoma and opisthosoma are
27. THE ORDER RICINULEI 213
backward. The second somite is about three times as wide as the pedicel.
Its tergite is a narrow strip, its sternite also crescentic, but with its concavity
forward. Thus there is between the first two sternites an oval
area of membrane in which is the genital orifice, a broad transverse
slit. It will be noticed that this places the genital orifice in front of the
second opisthosomatic somite, whereas in those Arachnida most closely
allied to the Ricinulei it lies on the second somite or on its posterior
margin. It may be, therefore, that an undetected somite exists or existed
in front of the pedicel, or, alternative! y, that the pedicel of Ricinulei is
not homologous with that of Araneae or other Caulogastra.
The third somite is short and wide, being a strip of chitin across the
opisthosoma where this is coupled to the prosoma. Its tergite is generally
divided into a central and two lateral pieces; its sternite is plainly seen
on the lower surface.
The fourth, fifth and sixth somites are the largest of the series and
constitute the bulk of the opisthosoma. The tergite of each is divided
into a large median and smaller lateral areas, separated by softer,
lighter-coloured membrane of great thickness. As the animals grow
older these passages between the tergal elements tend to narrow and
disappear. The sternites of these three somites are not divided longitudinally
(Fig. 85), and, as the spaces between them narrow, they come
to form an almost continuous shield on the ventral side of the body.
Both tergites and sternites of this region are marked each with a pair of
depressions, similar to those often seen on the backs of spiders, and like
them due to the insertion of muscles within.
FIG. 84. Ricinulei; dorsal aspect. Species, Ricinoides crassipalpe. After Hansen and
Sorensen.
securely locked together, and the first two opisthosomatic somites are
hidden. Coupling and uncoupling must be possible to the living animal,
for the genital aperture lies within the enclosed space; hence durir_tg
copulati.on and during egg-laying the attachment must be undone m
order to expose the orifice.
The opisthosoma presents the appearance of four well-defined
somites, but actually nine can with certainty be distinguished. The first
of these is the pedicel. This pedicel has a short narrow tergite, comparatively
feebly chitinized, and surrounded by a quite soft and flexible ·
membrane. Its sternite is crescent-shaped, its concave margin facing
FIG. 85. Ricinulei; ventral aspect. Species, Ricinoides crassipalpe. After Hansen and
Sorensen.