Savory - Arachnida 1977

10 I. PROLEGOME);A
one time so rare that every fresh specimen was something of a zoological
triumph, but now better known and obtainable by the hundred.
Pseudoscorpiones
False scorpions or book-scorpions: fascinating arachnids, like tiny tailless
scorpions, the very largest about one-third of an inch long. They are
ubiquitous and quite inexplicably neglected by zoologists until recently.
Two dozen species occur in Britain, and the sifting of a handful of fallen
leaves seldom fails to produce one or two.
Solifugae
\Vind-scorpions, camel-spiders or sun-spiders: desert arachnids,
amongst the most powerfully armed animals in the world, some with
jaws as long as their bodies, some able to climb and run at great
speed.
This quick review serves to emphasize the diversities in general
familiarity, in distribution and in the sizes of the species that are to be
found among the living Arachnida. Two additional comments will
illustrate a fundamental problem of theoretical arachnology.
The first of these is the fact that for many of the orders there exists a
special and obvious feature, peculiar to one order only, and distinguishing
it immediately from all the others. Thus:
Acari
Araneae
Palpigradi
Pseudoscorpiones
Ricinulei
Schizomida
Scorpiones
Solifugae
Uropygi
alone have vegetarian and parasitic forms
alone have opisthosomatic silk glands
alone have a projecting proboscis
alone have chelicera! silk glands
alone have a cucullus in front of the carapace
alone have a short telson
alone have pectines
alone have malleoli
alone secrete acetic acid.
The second comment concerns the sporadic distribution among the
orders of special modifications of the basic structure. For example:
(i) The carapace is segmented in Palpigradi, Schizomida and
Solifugae.
No sternum separates the coxae, which meet in the middle
line, in Cyphophthalmi, Opiliones, Pseudoscorpiones and
Solifugae.
(''\
\_11)
( iii)
Pedal tarsi are composed of many pieces on Opiliones, Pseudoscorpiones,
Ricinulei and Scorpiones.
2. THE CLASS ARACHNIDA 11
Chelicerae are pointed, not chelate, in Amblypygi, Araneae
and L'ropygi.
Pedipalpi are chelate in Pseudoscorpiones, Ricinulei and
Scorpiones.
(Yi) Gnathobases are present on the first pedal coxae in Cyphophthalmi,
Opiliones and Scorpiones.
( vii) Offensive fluids are secreted by Cyphophthalmi, Opiliones and
L'ropygi.
( viii) The first legs are used as tactile organs, rather than for walking
in Amblypygi, Palpigradi and Solifugae.
(ix) Trichobothria are absent from Cyphophthalmi, Opiliones
Ricinulei and Solifugae.
'
(x) A long m~bile telson is present in Palpigradi and Uropygi.
(xi) Venom 1s secreted by Araneae, Pseudoscorpiones and
Scorpiones.
(xii) Silk is secreted by Acari, Arancae and Pseudoscorpiones.
A di~tribution
of various important features as irregular as this
emphaslZes a fundamental fact about the orders of Arachnida; namelv
that they are all so different from one another that their mutual rel;­
tions, wherever they exist, are obscured. Attempts to find a path through
the resultant maze will be found in Chapters 11 and 12.
The number of species that constitute any order can be given accurately
in very few classes of animals, and it is impossible to give more
than an estimate of the numbers of Arachnida. For many years what
may be called traditional totals were associated with the different
orders, and a comparison of the numbers for 1939, a date that chooses
itself for any discussion of this kind, and the estimated numbers for
1974, 35 years later, is interesting.
1939 1974
Acari 6,000 10,000
Amblypygi 60 770
Araneae 50,000 35,000
Cyphophthalmi 20 65
Opiliones 1,600 4,000
Palpigradi 20 50
Pseudoscorpiones 1,000 2,000
Ricinulei 15 35
Schizomida 30 35
Scorpiones 600 750
Solifugae 600 800
Cropygi 70 85