Savory - Arachnida 1977

24 II. DE ARACHNIDIS
modes of life and by the existence of a layer of wax in the epicuticle.
The result is a considerable tolerance of unfavourable conditions, but a
far from perfect adaptation, so that to be too dry is worse for an
arachnid than to be too wet.
The exoskeleton is also the site of sense organs, such as the eyes,
which are of limited use, and setae, which are of the greatest value as
organs of touch, as well as special chemotactic organs and others which,
like the lyriform organs, are mysterious and puzzling to us.
The eyes or ocelli are nearly always of the smooth or simple type, with
a lens which is a curved and transparent portion of the cuticle. Their
number ,·aries from two to 12, and they are usually sessile, or level
with the surface. Stalked eyes are unknown, but in several families,
especially among the spiders and harvestmen, they are raised on a
turret or ocular tubercle. The general structure of an arachnid eye is
shown in Fig. 2.
The eye consists of the following parts: the lens, described above,
a curved portion of exoskeleton free from setae and pigment; (ii) the
vitreous body, lying beneath the lens; (iii) a pre-retinal membrane, a
prolongation of the basal membrane of the hypodermis; (iv) a retina of
visual cells, processes from which make contact with the optic nerve;
(v) a post-retinal membrane, sometimes pigmented, forms the back of
the eye. There may also be pigment cells or rhabdomes among the
Cul1cular lens
Cuticle
3. MORPHOLOGY: EXTERNAL APPEARANCE 25
cells of the retina and a tapetum or reflecting layer below them.
Considerable variation of these parts, and especially in the shape of
the lens and the extent of the vitreous body, are to be found in different
orders and families, but the chief variation is in the position of the
visual cells of the retina. These cells may be likened to a pear with a long
stalk. In post-bacillar eyes the swollen base, which contains the nucleus,
lies on the post-retinal membrane with the stalk directed outwards
towards the vitreous body; in pre-bacillar eyes the pear is the other way
round and the nucleus is nearer to the lens than the stalk. These·are the
eyes which normally have a tapetum, reflecting the incident light so
that it passes through the visual cells a second time.
Both kinds of eyes are found in most orders of Arachnida. Their value
as organs of distinct vision is limited; only jumping-spiders, and to a
lesser extent wolf spiders, can form anything like a clear image. The eyes
have different outward appearances, which have caused them to be
given a number of contrasting names, such as diurnal and nocturnal
eyes, direct and indirect, principal and secondary, median and lateral
and so on.
On the surface of the body and limbs of Arachnida a number of
important structures are visible and may be described as follows.
Tubercles
These are hollow outgrowths of the cuticle, such as occur on the hoods
ofTrogulidae and on the pedipalpi of many other harvestmen. They are
not sense organs. Similar outgrowths, but solid throughout, are often
known as denticulae, and when very short may be distinguished as
spicules.
The remaining structures are sensory in function.
lentigeo
layer
50p..m
OptiC
nerve
membrane
Fw. 2. Vertical section through the eye of a harvestman. After Curtis.
Spines
These are solid bristles, usually black in colour and very obvious on
the legs of nearly all Arachnida. They may arise directly from the
cuticle, but they often originate from the top of a hollow tubercle. The
arrangement of these spines is constant for a species and is often used in
taxonomy. Under high magnification it can be seen that the surface is
sculptured with a spiral marking, which sometimes produces the
appearance of a saw-edge. The chitin below is in layers and often the
centre is palest, so that a spine seems to be hollow. Spines arise from a
colourless area of the exoskeleton, surrounded by a darker ring and the
shape of their bases is always a characteristic (Fig. 3). Each spine is in
communication with a nerve ending, and it has long been known that
they are erectile. In male spiders they can be seen moving during mating.