Savory - Arachnida 1977

28 11. DE ARACHNIDIS
3. MORPHOLOGY: EXTERNAL APPEARANCE 29
work at Munich has shown that the organs are strictly mechanoreceptors,
and that the minute deformations of the membrane, on
which they depend, may be produced by:
(i) vibration of the substrate;
( ii) air-borne sounds;
(iii) movements of parts of the body;
(iv) the motion of walking.
FIG. 5. Lyriform organ in section.
"slit sense organs" they occur on the carapace and on the sternum and
on the pedipalpi and legs, especially near the joints. Earth and Seyfarth
( 1972) estimate as many as 3,000 on the surface of a single spider.
During a long period of uncertainty several functions have been
attributed to these objects. They have been described as auditory
organs, chemoreceptors, detectors of heat and of atmospheric moisture.
Pringle ( 1955), developing the idea that they are similar to the campaniform
organs of insects, described them as modifications of the
cuticle near the limb joints, with nerves or "sense cell processes"
attached to the centre of a thin membrane. Thus he introduced the
conception of mechanoreceptors, affecting orientation. Working with
amputated legs from scorpions and Amblypygi, he attached to the
bases of the nerves platinum electrodes connected to an amplifier and
an oscilloscope. He found that pressure on the limb produced large
impulses, and concluded that the lyriform organs supply a kinaesthetic
sense, comparable to the muscle-tendon senses of vertebrates. Such a
sense would play an important part in the phenomenon of Netzstarrheitstaxis
of Holzapfel ( 1933).
Similar results were obtained by Edgar (1963), using the legs of six
species of Opiliones.
Earth and Seyfarth ( 1972) have obtained striking results in favour of
the orientation theory. In their apparatus a live spider, Cupiennius salei,
was attracted by the buzzing of a bluebottle at distances ofup to 30 cm.
On reaching the fly the spider was chased away and the fly removed.
In normal circumstances and even with its eyes covered, the spider
soon returned to the site of the insect. By immobilizing the lyriform
organs on the femur or tibia the authors proved that these organs
controlled kinetic orientation, a term that implies the determinaton of
an animal's direction by its previous movements. Continuation of this
(i) (ii) (iii)
FIG. 6. Tarsal organs of spiders: (i) Myro; (ii) Bomis; (iii) Synaema.
The tarsal organs (Fig. 6) are chemoreceptors and their use has
been satisfactorily demonstrated by Blumenthal ( 1935). These organs
can be seen under the microscope as small round holes in the upper
surface of the tarsus, leading into a depression at the bottom of which
lies a small projection. They are used in the testing of drinking water
and the examination of edible prey. If the tarsus of one of the anterior
legs of a thirsty spider is touched with a drop of water, the spider moves
forward and drinks; if one of the posterior legs be so touched, the spider
turns round and drinks. If the tarsal organ is sealed these responses
disappear, and if a drop of water touches any other joint of the leg, the
spider merely moves away. Blumenthal also showed that the organ
enabled a spider to distinguish between water, brine, sugar solution and
quinine.
There is abundant evidence that most Arachnida react to sounds,