Savory - Arachnida 1977

108 IL DE ARACHNIDIS
survive and have left no trace of their short lives. Secondly, some of the
combinations of chromosomes may have generated a lethal gene, and,
like the homozygous yellow mouse, never have seen the light. Thirdly,
some may be in existence, lurking unsuspected as sub-orders or
super-families.
Be this as it may, there are 17 orders among which all the characteristics
just mentioned are to be found. To the question "To what do these
characteristics owe their existence?" the answer today will presumably
be "To the presence of certain chromosomes, or to certain genes in the
chromosomes, or to certain nucleotides in the DNA spiral". Since chromosomes
separate, divide, cross-over and are variously distributed at
meiosis, one is inclined, if not forced, to visualize groups, which may
well be called stable gene complexes, each responsible for a particular
characteristic in the adult arachnid.
There is much that this hypothesis can explain or make intelligible.
For example, why do the Amblypygi seem to resemble the spiders
while the Uropygi seem to recall the scorpions; why do pseudoscorpions
merely look like scorpions while their true relations seem to be the
Solifugae; why have the {; ropygi a flagellum and the Amblypygi
none and yet a short flagellum is found on salticid spiders of the genus
I\lantisatta; why is there a mysterious resemblance between the
Cyphophthalmi, the Notostigmata and the Architarbi? Why do we
never agree on an acceptable system of classification; and >vhy are all
the orders so different from one another?
Finally, where did all the orders of Arachnida come from? The
answer is that their origin was enshrined in the gene pool drifting about
the waters of the prehistoric oceans, carried in the gonads of the early
Eurypterida. From here they have emerged in a set of combinations
determined by chance.
Too much emphasis cannot be laid on the fortuitous nature of the
whole process. This implies that the orders of Arachnida are the consequences
of chance combinations of stable gene complexes, which were
not only viable but were able to maintain themselves in the environment
in which they were produced. Here is the basis of a hypothesis
that may be described as aleatory evolution.
In the light of this hypothesis an attempt must now be made to arrange
the orders of Arachnida in as logical a manner as is possible, to
be tested by illustrating it in the form of a dendrogram (Fig. 27).
(I) A start may obviously be made with the Scorpiones, which are
placed on the left of the diagram, as suitable to their primitive position.
Their separation from the rest reflects their occurrence in the Silurian
strata before any other order is known to have existed.
12. TAXONOMY: CLASSIFICATION
.r-----------------Eurypterida
_,.------------------- Scorpiones
..------------Palpigradi
Uropygi
Schizomida
Amblypygi
Araneae
'-------Kustarachnae
..------ Trigonotarbi
~--------l
Anthracomarti
Haptopoda
..------------ Ricinulei
Opiliones
Cyphophthalmi
'------Acari
~._ ______ Architarb1
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, r------------ Pseudoscorpiones
~._ ___________ Solifugae
'------------------ Pycnogonida
FIG. 27. Dendrogram of the orders of Arachnida.
(2) \Vhat I have called "the scorpion pattern" (Chap. 17) is repeated
in the Uropygi. Based on an overall similarity, this is emphasized
and confirmed by their general bionomics. The mode of life, the conduct
of "bloodless battles" between half-hearted disputants, the
character of the courtship and the type of spermatophore are all features
which suggest that the "whip scorpion assemblage" should follow
the Scorpiones. The first of these, however, must be the Palpigradi,
which show a larger proportion of primitive characteristics than any
other living order. Alone of this grouping they retain the primitive
three-jointed chelccrae, but their terminal ftagellum may be taken as
evidence for their association with the Uropygi.
No relation is indicated between the Palpigradi and Scorpiones other
than the features common to all Arachnida. The Schizomida and
Uropygi are more closely allied, so that a common ancestry is shown,
while the deeper origin of the Palpigradi denotes their phylogenic
position.
(3) The Amblypygi manifestly follow. Though the flagellum, atshorter
in the Schizomida, has disappeared, their relation to the
Uropygi is beyond question. In other respects, the Amblypygi recall the