Savory - Arachnida 1977

30 II. DE ARACHNIDIS
yet arachnologists have always been loath to admit that spiders can
hear anything or that scorpions and Solifugae produce during stridulation
sounds that are audible to other members of their species. Part of
this reluctance depends on the interpretation of the word "hear", and
part on the strange mechanism of the sense organs im·olved. In
the parallel case of tasting or smelling an acceptable description of the
organs has been found: they are called chemotactic. Similarly, the
setae or the trichobothria, by which Arachnida almost certainly are
caused to respond to vibratory stimuli, might with equal acceptability
be called "sonotactic".
4
Physiology: Internal Organs
As is only to be expected of an animal as highly specialized as an
arachnid, the internal structure of the body is complicated. It is best
described by the conventional method of considering it as made up of a
number of organ systems, each with its own functions; but it is important
for the reader to remember that in the living animal the functioning of
each of these systems is dependent on the rest. The systems are:
The alimentary system
The respiratory system
The vascular system
The glandular systems
The nervous system
The reproductive system
The excretory system
The muscular system
\
The alimentary system follows the pattern common to all Arthropoda
in that it consists of fore-gut, mid-gut and hind-gut, the first and the
last of which are lined with a chitinous invagination of the exoskeleton.
The mouth, whose characteristic position behind the chelicerae has
already been mentioned, usually lies above and between the coxae of
the pedipalpi. It is bordered above by an upper lip, the epistome or
rostrum, and below by the lmver lip or labium, derived from a sternite
(Fig. 7). Either or both of these, as well as the maxillary lobes of the
pedipalpi, may contain glands, the secretion from \Yhich is poured into
the prey. In Scorpiones, Pseudoscorpiones and Solifugae the rostrum
and the pedipalpal coxae form a space or atrium in front of the mouth.
The floor of this atrium is the labium, curved upwards and set with
setae, the whole forming a filter through which fluid nutriment only can
make its way. In L"ropygi, Schizomida and Ricinulei the pedipalpal
coxae meet and fuse in the middle line, forming a plate which is
concave on its dorsal surface. Into this fits the com·ex rostrum. Both
surfaces are set with spines or spikes, which form a filtering device. The
mouth opens into this space, which is known as the camarostome. A
similar fusion of the pal pal coxae occurs in the extinct Kustarachnae.