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THE SPIDERS OF EPPING FOREST.
species, three pairs, however, being usually present.
The spinners are more or less cylindrical, conical or
mammiform and each is furnished with numerous small
Spinning Tubes, each of which is capable of emitting a
single silken thread. In some species an extra or super-
numerary spinner, termed a Cribellum, is present. This
organ is of a type different from the normal spinners, in
front of which it is placed. Its form is very constant
in the various families in which it occurs, being a trans-
verse strip often with a central constriction, this latter
indicating, apparently, that it has been formed by the
fusion of a pair of abdominal appendages.
Spiracular organs. Two forms of respiratory organs are
found in the Araneae—Laminal Tracheae and Tube
Tracheae. The laminal tracheae, sometimes termed
sac-tracheae, lungs, lung-sacs, or lung-books, are
situated beneath the integument of the inferior surface
of the abdomen. The organs themselves each consist
of a number of exceedingly delicate laminae plentifully
supplied with very minute vessels and communicating
by a narrow transverse aperture with the atmosphere.
The integument covering each laminal trachea is often
of a different tint from the surrounding portions, and
these parts are termed the Spircuular-plates. The tube
tracheae open sometimes in one aperture and sometimes
in two apertures, usually close to the spinners, but
occasionally upon other parts of the under surface of the
abdomen.
Reproductive organs. The sexual secretory glands of the
male and the entire reproductive system of the female
are situated within the abdomen, their external openings
being placed with few exceptions, between those of the
laminal tracheae. In the male the opening is very
minute and inconspicuous, but in the female it is often
provided with variously formed Epigynal Plates. In
some cases the integument upon the anterior side of the
female genital aperture is chitinised, produced down-
wards and backwards, and more or less modified in
form, the resulting structure being known as the Clavus.
The genital armature of the female is usually a most
important criterion in the differentiation of obscure and
closely allied species.