Savory - Arachnida 1977

42 II. DE ARACHNIDIS
fibres run upward from the endosternite to the carapace, and, in spiders,
a median band in addition. A varying number of compensating muscles
join the sternum to the endosternite, round which a series of lateral
muscles run horizontally.
This full set of muscles effect the movements of the appendages, the
gut and the prosomatic glands.
In the opisthosoma the most significant muscles are those which, in
narrow strips, run vertically between the dorsal and ventral surfaces.
They are manifestly metameric, and must be presumed to be vestiges of
muscles which formerly united all tergites and sternites. The posterior
pairs have vanished from modern Arachnida, and the number remaining
is closely related to the visible segmentation of the opisthosome. Thus
there are eight pairs in Amblypygi, seven in Schizomida and six in
Solifugae and Palpigradi. Other muscles in the opisthosoma are
concerned with the moving of the alimentary canal and the opening and
shutting of the different orifices.
The muscles of a spider's leg, which may be taken as an example of
4. PHYSIOLOGY: INTER1'\AL ORGA.NS 43
an arachnid appendage, are shown in Fig. 15. Their most interesting
feature is the extreme development of a delicate muscular sense, shown
so dramatically by the habits of web spiders. Their responses to the
varying tensions of the threads of their \vebs arc among: the most
amazing items in arachnid behaviour, and are responsible for the
vibrotaxis and Setzstarrheitstaxis described later.
The work of Parry and Brown ( 1959 1
has shown that in jumping
thesC' muscles are not solely responsible for the spieler's leap, but that
much of the motive power is supplied in the form of hvdraulic pressure
from within. Thus there is no exaggeration of one pair of legs, such as
is found in many other invertebrates that jump, and a unique method
of using all pairs of legs simultaneously has taken its place.
vii
FIG. 15. The muscles of a spider's leg. After F. D. Wood. (i) Extensor trochanteris;
(ii) flexor trochanteris; (iii) flexor longus femoris; (iv) flexor bilobatus femoris; (v)
fkxor bilobatus patellae; (vi) flexor patellae; ( vii) protractor tibiae; ( viii) flexor
tibiae; (ix) flexor metatarsi; (x) extensor tarsi; (xi) flexor tarsi.