Savory - Arachnida 1977

106 II. DE ARACHNIDIS
Here are three different attempts to find a satisfactory way of expressing
on paper the relationships between 16 orders. There are points in all
of them that may be criticized, and there are points of resemblance that
are ob,·ious.
Petrunkevitch's system \\'as largely the result of prolonged study of the
fossil from palaeozoic strata. It is therefore based almost entirely
on external features and depends chiefly on the breadth of the first
opisthosomatic somite, which may or may not be reduced to a pedicel,
on the difference between the number of detectable opisthosomatic
somites and the primiti\·e number 12, and on the configuration of the
mouth parts,
The dependence on the pedicel brings together in the Latigastra
orders that are as different in most other respects as are the Scorpiones
and the Opiliones; and in the Caulogastra, the Solifugae and Ricinulei.
This is a consequence, familiar to taxonomists in other classes, of basing
a classification on an insufficient number of characteristics; for this
may leave a residue of aberrant groups, to be conveniently consigned to
a "dustbin" taxon. In this case the Caulogastra supply the accommodating
receptacle.
Dubinin's system was intended to be a C'Omplete revision, which
should take account of embryonic and ontogenetic development, as well
as on morphology. This, it was hoped, '.Vould override the many apparent
differences between the existing orders and replace a seemingly
heterogeneous by a more homogeneous, not to say a more logical
grouping. As a result, the four orders of spiders are, by implication, as
different from one another as are the Ricinulei and Scorpiones; and the
relationships between the Opiliones and the Acari, and between the
Araneae and Amblypygi are hidden. "Moreover, the concept of "class"
and "ordcr" has been wholly changed.
The third system tried to take account of as large a number of characters
as was possible, but again was almost wholly concerned with externals.
It represents no more than a step towards the system about
to be suggested and used here. Its basic concept is that the orders may be
brought together in groups or "convenient assemblages", each of which
represents a distinct evolutionary pedigree, and it is based on the arguments
following.
Any attC'mpt to construct a classification of the Arachnida highlights
the problem that faces every systematist. He has to choose whether he is
going to devise a plan that will enable a taxonomist to place any given
specimen in its correct group; or alternatively to produce an acceptable
scheme that will show a phylogenist the course that evolution has followed
as the different taxa have come into existence.
For breaking the class Arachnida into orders, the first of these
12. TAXONOMY: CLASSIFICATION 107
choices is scarcely worthwhile. The orders are so distinct that thev can
be recognized at once. It would be very difficult to mistake a sco~pion
for a spider or a Ricinuleid for a mite, or a harvestman for a solifugid.
The second choice presents a virtual impossibility, since the paths of
arachnid evolution are virtually unknown, are obscure and are lacking
in guiding principles. In such circumstances evidence must be drawn
from details, the merest hint must be sympathetically considered, and,
above all, speculation must not be forbidden.
There are two interlacing yet independent features of the orders of
the Arachnida, which affect fundamentally any attempt to propose a
logical order for their arrangement. These are, first, the sporadic distribution
of certain characteristics among the orders. For example,
(a) there are enlarged pedipalpi in scorpions and false scorpions, (b)
a pedicel may form the first abdominal somite in Araneae, Amblypygi,
U ropygi, Schizomida and Ricinulei, (c) the last three abdominal
somites are compressed in Araneae, Palpigradi, Ricinulci, Amblypygi,
Uropygi and Schizomida, (d) there is a terminal ftagcllum in Schizomida,
Uropygi and Palpigradi, (e) there is a six-legged phase in Acari
and Ricinulei.
Secondly, and in addition to this, the features emphasized as important
in our diagnoses tend conspicuously to occur in alternative
forms. A least ten may be mentioned.
( 1) The carapace may be uniform or in part segmented.
(2) The opisthosoma may be uniform or segmented.
(3) The pedicel may be present or absenL
( 4) The sternum may be uniform or segmented.
(5) The chelicerae may be of two or three podomeres.
(6) The pedipalpi may be leg-like or pincer-like.
(7) The coxae of legs or pedipalpi may bear gnathobases or may
not.
(8) The first leg may be used as a leg or like an antenna.
(9) The legs may be of seven podomeres or may be sub-segmented
anywhere.
( l 0) The coxae may meet and hide the sternum, or may be separated.
Here are ten pairs of alternatives, and ten alternatives may theoretically
be arranged in 2 10 or 1,024 combinations. Thus, even with a limit of
ten pairs, we might expect 1,024 orders of Arachnida. But this is not so,
and we may pause to ask why we have been robbed of 1,007 orders and
left with a mere 17.
Three answers are possible. One is that perhaps many of the 1,024
have at some time actually come into existence, but were ill-adapted to