Savory - Arachnida 1977

150 III. PROLES ARACHNES
chelicerae work in two different ways. Among the Liphistiomorphae
and Theraphosomorphae they strike downwards and parallel to each
other and arc described as paraxial; among other spiders they strike
transversely, tending to meet in the middle of the victim and are known
as adaxial.
The pedipalpi consist of six segments (Fig. 50). Except in the more
primitive sub-orders the coxa carries a maxillary lobe which has a
compressing, masticatory function. The femur is sometimes used in
stridulation. The remaining segments are used by young and female
spiders as tactile organs, but in the mature male the tarsus is modified
18. THE ORDER ARANEAE 151
becomes di\·isible externally into three regions, containing the fundus,
reservoir and duct respectively. The apical division is usually called the
embolus.
In the next stage the embolus becomes divided into two. One of these
is the ejaculatory duct or embolus proper; the other, called the conductor,
protects it when the organ is at rest (Fig. 51). The tarsus, too,
is sometimes di\'ided into two parts, the smaller of which is called the
paracymbium.
FIG. 50. Labium, maxillae and pedipalpi of a spider.
so that it can be used as an intromittent organ, carrying the sperm to the
female spider. The male palpal tibia is often shorter than that of the
female, and it often carries on its outer side a short process or apophysis
which, during mating, is fitted into a groove in the female epigynum.
This use of the pcdipalp is one of several advances on the device of the
spermatophore for insemination that occur among the Arachnida.
The male palpal organ lies near the tip cf the tarsus in a cavity, the
alveolus. In its simplest form it consists of a coiled tube, or receptaculum
seminis, of three parts. These are a basal or proximal swollen bulb, the
fundus, an intermediate reservoir, and, distally, a dark elongated
ejaculatory duct.
The first advance from this simple condition is the migration of the
whole genital bulb to the lower side of the tarsus and an increase in the
size of the alveolus. The tarsus thus becomes more or less cup-like and is
often n:named the cymbium. At the same time the palpal organ
' ....... .....
Fw. 51. Pcdipalpal tarsus of male spider, sjJ.. palpal organ expanded.
(i) cymbium; fundus; (iii) reservoir; embolus; (\") conductor.
In the most complicated type of pal pus thne are elaboratious of these
parts consisting chiefly in the addition of blood cavities or haematodochas,
protecting rings of chitin and extra apophyses. Its numerous
variations form a valuable means of characterizing the males of every
speCies.
The are always composed of seven segments (Fig. . The coxae
lie round the sternum and never exercise a masticatory function. The
femora of some male mygalomorph spiders have small hooks on tht>ir
inner surfaces; these are used in mating, when the male thrusts them
against the chelicerae of the female, her temporarily and reducing
any possible risks to himself. The tarsi carry the paired claws which
terminate the legs of all spiders. Sometimes an extension of the tarsus,
the empodium or post-tarsus, extends between the paired claws, either
as a pad, or as a bearer of adhesive hairs, or as a third, median, claw.
The claws, whether two or three in number, are hard and sharp,
curved, and set with a row of teeth on the inside of the curve. Nielsen
iv