Savory - Arachnida 1977

12 I. PROLEGOMENA
The following considerations apply to both Acari and Araneae, and
probably also to the Opiliones.
\Vith the increase in the numbers of interested zoologists and with a
consequent increase in the number of collecting expeditions, penetrating
into more and more inaccessible regions, the stream of newly
discovered species has grown from a trickle to a flood. The time may be
not far distant when even full-time specialists will be overwhelmed by
the cascade of "new" species. A consequence of this will be a recurrence
of the state of affairs that existed in the late nineteenth century: the
descriptions become so numerous, the literature so extensive, the
languages used so diverse, that the individual systematist is certain to
miss records of many species unfamiliar to himself. Names, which will
one day be recognized as synonyms, will be given by the hundred
before the end of the present century.
The history of the nomenclature of spiders gi;-es support to this
prophecy. In Bonnet's "Bibliographia Araneorum" there are references
to about 250,000 names, and it is evident that on an average each
species has received five names. This reduces an estimate of a quarter of
a million species to about 50,000. But in his last volume Bonnet
informs us that about 15,000 names have been used once only in all the
literature before 1939: it is safe to assume that nearly all of these have
been described, and are now known, by other names; and this reduces
50,000 to 35,000. The same is true, ifless emphatically, of the names that
have been used twice only, so that critical analysis must put the world's
spiders known in 1940 at about 30,000.
It is axiomatic that the largest orders will have the widest range, but
there is more to the subject of geographical distribution of the Arachnida
than this. A simple survey, which will be elaborated in a later chapter,
shows that the 12 living orders may be described as follows.
(i) Four orders which are widely dispersed all over the world,
including even the frigid regions of the sub-arctic and sub-antarctic:
Araneae, Pseudoscorpiones, Acari and Opiliones.
(ii) Four orders which are confined to the tropical or hot subtropical
belt, but which are widespread within it: Scorpiones,
Amblypygi and Cropygi.
Four orders which are sporadically distributed and found only
in limited and well-separated areas: Cyphophthalmi, Palpigradi,
Ricinulei and Schizomida.
A biological feature that follows immediately on the subject of distribution,
and is connected with it, is that of size. In general terms,
Arachnida are among the small animals of the world, but, by itself
"small" is a rather meaningless adjective. The facts may be summarized
as follows.
2. THE CI~ASS ARACHNIDA 13
(a) There are four living orders in which the length of the largest
species does not exceed 1 cm. These are Pseudoscorpiones, Palpigradi,
Schizomida and Cyphophthalmi.
There are four orders in which the length of the largest species
exceeds 6 cm. These are Araneae, Uropygi, Solifugae and Scorpiones.
(c) This leaves four orders of intermediate or average size: Acari,
Ricinulei, Opiliones and Amblypygi.
It is to be observed that in each group there is an order, named first,
which has a wide or very wide distribution, as well as an order, named
· second, which has a very limited distribution. From this it follows that
no correlation between size and distribution can be detected when a
whole order is considered as an indivisible unit. Geographical barriers
must affect smaller units, sub-orders or families, and will be considered
in the appropriate chapters in Part Ill.
Distribution in space leads to a consideration of distribution in time.
The Eurypterida were aquatic animals, whose fossilized remains
have been found in all the Primary strata from the Cambrian to the
Permian, but in no later rocks. Probably most of them were marine,
but there is evidence that some lived in streams and lakes. The earliest
known scorpion is the Silurian species, Palaeophonus nuncius; Carboniferous
scorpions are fairly numerous: they showed most of the features
that characterize living forms and it appears that the order really
reached its acme during this era.
The De,·onian strata have yielded both spiders and mites, and all
remaining orders, except Cyphophthalmi and Pseudoscorpiones, are
represented in the Carboniferous, many of them, notably Solifugae and
Opiliones, in forms which differed in no essentials from the species living
today.
Five orders which rose to comparative prominence during the
Carboniferous have since disappeared. The Anthracomarti survived
until the Permian, to this extent outliving Kustarachnae, Haptopoda,
Architarbi and Trigonotarbi. All these orders have left no more
evidence of their ancestors than of their descendants.
These facts are remarkable, and the conclusion to which Berland
( 1933) has come is sufficiently striking. Such evolution as can be perceived,
he writes, is not a progressive change, and progress is the essence
of evolution, but a mere replacement of one form by another, apparently
equivalent or comparable to it. This implies that the study of fossil
Arachnida leads to the conclusion that the hypothesis of an evolution
taking place by slow successive degrees is simply not in accordance with
the facts.
The subject of the geological record of Arachnida and its bearing on
the course of their evo.lution are considered more fully in Chapter 11.