Savory - Arachnida 1977

18 U. DE ARACHNIDIS
it, move to a forward position. This tendency of the mouth to move back
is frequently to be found in the Arthropoda, and is further mentioned
in Chapter 10.
The second somite carries the pedipalpi, and somites 3 to 6 carry
the four pairs of legs and complete the prosoma.
The seventh somite is of a more diverse character. In Limulus it
marks the posterior border of the mouth and carries the chilaria, organs
which in the Eurypterida are represented by the metastomatic plate.
In the scorpions and the other Arachnida in which the prosoma and
opisthosoma are joined across their whole breadth, the somite is not
present in the adults, but its temporary existence has been observed in
the embryo scorpion. This somite persists in spiders and Ricinulei, and
may normally be recognized wherever a pedicel unites the two portions
of the body. This narrowing of the somite to form a slender pedicel is the
final stage in a process which, like the backward movement of the mouth,
is found to different extents in the different orders of Chelicerata. The
somite is often called the pregenital somite, since it lies next in front of
the genital orifice. The occasional existence of both a tergite and a
sternite, the lorum and the plagula, above and below the pedicel,
clearly indicates its nature as a separate somite, even when it is much
reduced in diameter.
The lower surface of the prosoma presents very different aspects.
Fundamentally it should be expected to show a series of six protective
sternites, a condition which is in fact found in the extinct genus Stenarthron.
In living Arachnida it is modified by the position of the mouth, by
the different extents to which the processes of the coxae or gnathobases
take a share in the mastication of the food, and by the approach of the
coxae of the legs towards the middle. 'When this is complete, as in
Solifugae, there may be no sternites at all, or, as in Opiliones, they
may be covered by the coxae and so be normally hidden. Alternatively,
there may be a broad and conspicuous sternum derived from four
sternites, as in spiders, with the labium in front and the plagula
behind.
The opisthosomatic somites behind the pregenital are not more than
11 in number and are to be reckoned as somites 8 to 18.
The eighth somite invariably carries the genital orifice and one or two
immediately following it bear the respiratory book-lungs. A distinction
between the seven somites of the mesosoma and five somites of the
metasoma is obvious in Euypterida and scorpions; but generally the
outline is a smooth oval. The hindmost somites provide a very interesting
example of comparative morphology.
Hansen and Sjljrensen pointed out that the last three somites of the
opisthosoma show a tendency to form a narrower and semi-independent
3. MORPHOLOGY; EXTERNAL APPEARANCE 19
portion in "Pedipalpi", Palpigradi, Ricinulei and Araneae. In fact,
they suggest the inclusive term Arachnida micrura for these orders.
In Uropygi the relative mobility of the tenth to the twelfth opisthosomatic
somites may well enable the animal to direct the secretion of its
acid-producing glands. In Amblypygi there are no such glands and the
"tail" is less obvious. In Palpigradi the mobility of this part is probably
a help in the use of the flagellum, whatever this may be. In Araneae,
it is supposed that the value of the "tail" is that it separates the anus
from the spinnerets and so prevents the latter from being soiled. The
habits ofRicinulei are so little known that no purpose can be suggested
for the tail in this order. Nevertheless it is undeniable that these four
orders do very definitely resemble each other in having a kind of tail,
typically formed from the last three somites of the body.
Similar comparisons may be made when the presence of a post-anal
telson is considered. In Arachnida post-anal structures are very different
in form and function. The scorpions have a short sharp perforate sting
with a swollen base enclosing a poison sac. The Dropygi and Palpigradi
have long whip-like structures whose function is unknown. In Uropygi
the telson trails behind, but in Palpigradi it is arched and carried over
the abdomen, like the metasoma of a scorpion.
In striking contrast to this there is no trace whatever of either a
three-somite tail or a post-anal telson of any sort in Solifugae, Opiliones
or Pseudoscorpiones. This shows that the latter structure has evidently
been developed independently by the groups which possess it. Its origin
may be sought in the pointed termination to the body or certain of
Eurypterida, but its existence does not necessarily point to a closer
relationship between the telson-bearing orders.
Thus there is a very considerable diversity in the form of the opisthosoma
in the different orders of Arachnida, a diversity which is most
puzzling in the foremost somites. If it is to be assumed that in all orders
the genitalia open on tae same somite, the eighth, then it follows that in
Solifugae, Opiliones and Ricinulei there are not as many as 12 opisthosomatic
somites in all, in the scorpions there are 11 post-genital
somites and in the false scorpions, spiders and some others there are ten
post-genital somites. ,
A general summary of the segmentation of the arachnid body is given
in tabular form on p. 20.
In all ordinary Arachnida there are six pairs of prosomatic
appendages.
•
The chelicerae, the first or pre-oral pair, consist of two or three segments.
In the latter case the t~ird segment meets a prolongation of the
second, forming a chelate organ, useful for picking things up and perhaps
biting their headg..off. The two-segment chelicerae cannot nip but