Savory - Arachnida 1977

154 Ill. PROLES ARACHNES
FIG. 53. Section through the genital orifice of a female spider, Holocnemus hispanicus.
After Wiehle. (i) Uterus internus; (ii) receptaculum seminis; (iii) vagina; (iv)
perforated plate of chitin.
In many spiders, but not in the Theraphosomorphae, nor in those
possessing a cribellum, there is a small pointed appendage just in front
of the spinnerets. This is the colulus. It is probably without function,
being merely derived from the anterior median spinnerets by degeneration.
Behind the spinnerets a small tubercle, not always very obvious,
carries the anus at its tip. This is sometimes called the anal tubercle,
sometimes the post-abdomen. It is a vestigial structure, representing the
remains of the last seven of the 12 opisthosomatic so mites.
The only opisthosomatic appendages persisting in the adult spider
are those of the fourth and fifth somites, where they function as spinning
organs, namely, the cribellum, when this is present, and the six spinnerets
(Fig. 54).
FIG. 54. Cribellum and spinnerets of spider, Ciniflo.
18. THE ORDER ARANEAE 155
The number of spinnerets is not constant. In Liphistiomorphae the
primitive number, eight, is found, occupying the middle of the ventral
surface, but only the four expodites are said to be active. In most
Theraphosomorphae there are two pairs only, belonging to the fifth
somite, and in a few genera there is only one of these pairs. The relative
lengths of the spinnerets in different families are also variable, and seem
to be related to the method by which the silk is distributed. Where a
sheet-web is made and the opisthosoma is swayed from side to side, the
anterior spinnerets are very long.
The spinnerets are not the actual tubes through which the silk is
secreted. The tip of a spinneret is covered with a number of minute
tubes, through which the fluid silk passes. The smallest of these are
called spools and are numerous. Those on the anterior spinnerets produce
the attachment disc or transverse sweep of short threads which
anchor a spider's line to the ground and those on the posterior spinnerets
produce the broad ribbon or swathing band which is wrapped
round a victim. The larger tubes or spigots produce the drag lines and
foundation lines of the web; also the soft wadding found in egg cocoons
and the viscid fluid which makes the spiral thread of a web adhesive.
The cribellum is an oval plate found just in front of the anterior
spinnerets in certain families. It is perforated with a large number of
minute pores, each of which is the orifice of the duct from a gland. Thus
the cribellum produces a broad strip of silk composed of some hundreds
of threads. This is combed out from the cribellum by the calamistrum
(Fig. 55) on the fourth metatarsus and laid upon the plain silk strand
which the spinnerets are simultaneously producing. The effect is to
render the web more adhesive, or at any rate a harder entanglement
from which to escape;it also gives it a characteristic bluish appearance.
The sense organs of spiders include the spines or setae with which
their bodies and legs are covered, and the lyriform organs. Probably all
the setae on a spider are more or less developed as sense organs, but
some of those on the legs are useful accessories to the spinning organs.
On examination it is easy to distinguish at least three different kinds.
The most conspicuous are the stout sharp spines on the legs and pedipalpi,
generally described as tactile. The most difficult to distinguish,
even under the microscope, are the long delicate acoustic setae, believed
to be receptors of sound waves. Intermediate between these extremes
are many others, vaguely termed protective, and found in different
forms, some club-shaped, some spatulate, some branched and some like
smaller spines. In many cases spiders of a particular family are
characterized by a special arrangement of leg spines (Fig. 55). The socalled
acoustic setae are situated on the upper surface of the leg
segments, either alone or in a series (Fig. 52).