Savory - Arachnida 1977

90 11. DE ARACHNIDIS
Autecology concentrates attention on the lives of the separate species
and on their reactions to physical conditions. These include the
preYalent temperature and its range of variation, the humidity of the
atmosphere, the intensity of the light, the velocity of the winds and
the chemical constitution of the soil; all of which are together described
as the edaphic factors. Synccology is concerned more with the community
or population than with the individual. Thus it takes account
of predators and parasites, of food chains, and of changes in the density
of population during the year. These arc known collectively as the
biotic factors. Clearly it is less elementary than autecology, a characteristic
of which is that the effects of physical conditions can often be most
precisely determined by experimental study in the laboratory.
The synecologist faces a more complex situation, which is most
readily approached by a convenient division of the biosphere into three
layers.
(i) The Gound Layer: the moss and litter layer, up to about
15 cm above the surface.
(ii) The Field Layer: the plant and shrub layer, up to about l m
above the surface.
(iii) The Canopy: the tree layer of trunk and branches, as
well as walls and the outsides of buildings.
The lowest of these layers is the home of many more individual
animals than either of the others, for in there are to be found not only
those that are actually underground, but also the vast assembly known
collectively as the Cryptozoa. These are the animals that lead lives in
conditions that are continuously dark and damp, and in which conditions
are less changeable than elsewhere. This constancy is favourable
to most Arachnida, and in fact Lawrence ( 1953) estimated that about
half the cryptozoa belong to this class. Every field naturalist knows that
the sifting of leaf-litter yields spiders, mites, false scorpions and harvestmen,
while in the right parts of the world there may also be Palpigradi,
Schizomida and Ricinulei.
A conspicuous feature of all cryptozoa is the primitive nature of many
of their number, relating them to their distant ancestors, who long ago
crept from the primaeval ocean. They found in the cryptosphere an
environment in which survival was possible for small animals with thin
exoskeletons, and tactile and chemotactic sense organs that made
blindness of small consequence. Life in the cryptosphere was serene
and there was little temptation to leave it and to evolve into more
specialized forms. From this arises the statement that whereas the
vertebrate palaeontologist looks for fossils the invertebrate palaeontolo-
10. ECOLOGY: MIGRATION AND DISPERSAL 91
gist looks for cryptozoa. The cryptozoic lives of the Palpigradi and
Schizomida are examples of this.
On the other hand, spiders are among the most highly specialized of
the Arachnida, and among the cryptozoic spiders the families Linyphiidae
and Erigonidae are prominent. They illustrate the advantages of
small size. In addition there are many spiders that may be found in
the soil itself. Estimates of their number have ranged from 11,000 to
600,000 spiders per acre. And there are some spiders which, though
normally living on the surface, have adopted a habit of burying themselves
and thus securing protection. Reiskind ( 1965) has described the
steps by which a species of Sicarius digs a hole in sand with its forelegs,
enters, and throws sand over itself until it is covered.
An environment that may be considered as an extension or modification
of the cryptosphere is that of the cave. A cave offers the same
conditions of darkness and humidity, and the further a cave may extend
from its entrance, the less are these conditions likely to change.
The occupants of caves include a proportion of Arachnida, which share
with members of other classes the features that may be attributed to the
environment.
The high relative humidity, which often reaches lOO%, results in a
slower metabolic rate. Since oxidation helps to produce melanin, cavedwelling
fauna are often pale or colourless. For the same reason, they
are often sluggish in movement and less aggressive in behaviour; and
this is associated with the reduction in the proportion of chitin in the
exoskeleton and also with an increased length of body and legs. The
blindness, which is common, is sometimes attributed to the loss of eyes
owing to disuse, or alternatively to the fact that organisms, eyeless
from some other cause, are at no consequent disadvantage in a cave.
Probably the most interesting relation between caves and Arachnida
is shown by the Ricinulei. Long regarded as the rarest representatives
of their class, these animals have been found by l'vlitchell ( 1970) to be
living in their thousands in the caves of Mexico. In the words of one
arachnologist, "They were so numerous that it was hard to avoid treading
on them".
To this there should be added the report by Mitchell ( 1968) of the
discovery of three eyeless scorpions, also in Mexican caves. For these, the
first blind scorpions recorded, a new genus, Typhlochactas, and a new
sub-family were necessary. The species are described as being some
15 mm long, smooth and weakly sclerotized, the exoskeleton so translucent
that the alimentary canal could be seen within. The occurrence
of such unusual scorpions, at the considerable height of about l ,400 m
poses interesting problems in the evolution of the order.
The Arachnida that live in the upper, field and canopy layers,