Savory - Arachnida 1977

26 II. DE ARACHNIDIS
3. MORPHOLOGY; EXTERNAL APPEARANCE 27
(i)
Fw. 3. Setae: (i) from Biton (Solifuge) after Bernard; (ii) from Anomalobuthus
(Scorpion) after Pawlowsky.
(il)
Setae
Setae (chaetae) are finer than spines and often lie closer to the surface
instead of standing at right angles to it, and unlike spines, which form
rows, setae grow in patches, covering an area. The overall hairiness in
the appearance of an arachnid leg is due to its clothing of setae, which
may be plumose. Setae are not erectile, their bases lie inside rings
surrounding pores in the cuticle. lVIicrosetae resemble them but are
smaller.
Trichobothria
These are the finest of all. They are more or less erect, and are much
longer than setae, among which they sometimes grow. They are sensory
receptors, and are as characteristic of certain orders as are the spider's
web and the scorpion's sting. They arise from two or three cupules in
the hypodermis (Fig. 4).
First described and named Horhaare by Dahl ( 1911) they have not
attracted intense research until recently, it being generally believed that
they were stimulated by the slightest movements of air, including infrasonic
and ultrasonic sound waves. This supported the contemporary
belief that spiders appreciate music; an idea that suffered from the
observations of .\1cCook. He had detected a reaction to the notes of a
flute, and later found the same response to a silent puff of air.
An important function follows from their response to air movement
such as is produced by the passing of other animals nearby. A false
scorpion can, by their means, detect the presence of prey at a distance of
15 mm. A scorpion such as Centruioides sculpturatus sits motionless in the
dark, prepared by the same stimulus from its trichobothria to pounce
upon an approaching insect. Each trichobothrium has been shown to
oscillate in one plane only, so that a set of them collectively indicate the
direction from which air disturbance is coming. Thus they have an
Fra. 4. Trichobothrium. After Gossel.
orientation function. A further suggestion is that they react to the light
of the sun and the moon, and so are described as astrotactic. This is in
addition to their sonotactic function, now well established.
These remarkable organs occur on the legs and pedipalpi of spiders,
on the pedipalpi offalse scorpions and scorpions, on the tarsi and patellae
of the three orders of "Pedipalpi", and on the bodies of certain mites.
They are not found on Solifugae, Opiliones or Ricinulei.
Gabbutt ( 1972) has shown that the trichobothria of pseudoscorpions
form a valuable guide to the genera and species in this order; and further
that they help to distinguish between the nymphal stages of immature
specimens. .\Iost recently the trichobothria of scorpions have been
exhaustively studied by Vachon ( 1973). Among his conclusions is the
important fact that the numbers and positions of all trichobothria are
established among the scorpions at the early stage of the first nymph,
and that this is a distinction from the state among pseudoscorpions.
Each trichobothrium is to be regarded as an individual sense organ, to
be given and described by its own symbol. Thus there develops a
trichobothrial nomenclature. It is clear that trichobothriotaxy has great
taxonomic and phylogenetic significance.
The exoskeleton of an arachnid also carries other sense organs of
types that are less familiar because they have no analogues among the
vertebrates. The most plentiful and the most mysterious of these are the
lyriform or slit sense organs, the outward appearance of which is shown
in Fig. 5. Under a microscope they look like slits in the cuticle.
They may be very numerous, for, either as single slits or as compound