Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

ARACHNIDES.
genera of Phalnngides are these—Trogulus, Siro, Pha-
langium.
Section II.—Body divided into three or four distinct seg¬
ments. This section is likewise composed of two families,
Pycnogonides and Pseudo-scorpio7ies. Of the first family
the genera are, Nymphum, Phoxichilus, Pycnogonum;
of the second, Galeodes, Chelifer.
Order III.—Non-antemiated branchial Arachnides. No
antennae. Branchial cells or pouches for respiration.
From six to eight smooth eyes.
Section I.—Pedipalpi or Scorpionidce. Palpi very large,
in the form of projecting arms, terminated by claws or
pincers. Abdomen distinctly ringed, and not furnished
with a spinning apparatus. Genera—Scorpio, Thelypho-
nus, Phrynus.
Section II.—Araneides or Spinning Arachnides. Palpi
simple, in the form of small feet; those of the male
bearing the sexual organs. Mandibles terminated by
a movable crotchet. Abdomen without rings, and
furnished at its termination with a web-making appara¬
tus, consisting of from four to six spinners. Genera—
Aranea, divided into numerous tribes or sub-genera;
Atypus, Mygale, Avicularia.
In his Considerations Generates, M. Latreille founds the
orders of the class Arachnides on two principal points as
the first basis. These animals are either masticators or
suckers. The jaws of the former are simple, and fitted
for cutting and triturating the substances on which they
feed; those of the latter, when they exist at all, serve
only to seize their prey, and are terminated by a movable
piece, either solitary and hooked, like a claw or crotchet,
or accompanied by a fixed projection like a small finger.
In the latter case the mandibles have the appearance of a
pair of pincers. The Arachnides of this division compress
with their pincers the small animals on which they prey,
and so force the alimentary juices to pass by degrees into
the oesophagus. The body of their prey having under¬
gone this operation, is thrown aside. In spiders the claw
of the mandible seems to execute an additional function.
It distils a poisonous liquid, analogous in its nature to that
which exudes from the mouth of the scolopendra, and
the tail or sting of the scorpion. Moreover, all the mas¬
ticating Arachnides are furnished with antennae, while the
suctorial tribes, with the exception of two genera, Ricinus
and Pulex, are unprovided with these organs. Thus the
primary divisions of this class by Latreille nearly corre¬
spond to Lamarck’s two orders, the Antennistes and Pal-
pistes.
M. Latreille then subdivides the masticating Arachnides
into three orders— Tetracera, Myriapoda, and Thysanoura.
The last alone present a thorax distinct from the abdo¬
men, and have no more than six feet. In the two first-
named orders the organs of movement amount at least
to the number of fourteen, disposed along the sides of the
body, each segment of which, with the exception at most
of the last three, carry one or two pair. The Tetracera
(genus Oniscus of Linnaeus) are distinguished by four an¬
tennae ; there are only two in the Myriapoda. The
Tetracera have besides several jaws, and are furnished with
plates or foliaceous appendages on the inferior surface of
the posterior extremity of their bodies. The number of
their feet is invariably fourteen. They are related in
several particulars to the Crustacea. De Geer observed,
that in Assellus and Idotea there is a kind of membranous
cavity, frequently filled with air, beneath the plates of the
tail; and similar parts are observable in the true crustace-
ous Squillae of Fabricius. In Oniscus the air penetrates
the body by openings, which are covered by the first folds
of the tail. The other Tetracera probably receive it in a
similar manner: at all events they are not provided with
distinct external stigmata, like the Arachnides of the suc¬
ceeding orders. We still find in this first division species
which inhabit the sea; but as soon as we enter the order
Accra (scorpions, spiders, &c.) we find none occurring in
that element.
The structure of suctorial Arachnides is reducible to
three principal forms or combinations of organs ; Lf, Those
possessed of antennae, and of which the head is distinct
from the thorax—Parasita ; 2d, Those which are unpro¬
vided with antennae, each segment of the body furnished
with a pair of legs, and the head distinct—Pycnogonides;
3(/, Those which are unprovided with antennae, but of
which the head and thorax are confounded as it were in a
single segment, which alone is provided with feet—Acei'a.
This last-mentioned order, according to Latreille, would
form the first, if the organs of the mouth alone were con¬
sidered, as these are more complicated than in any other
suctorial Arachnides. The Acera have two palpigerous
maxillae, and frequently a lip, with two strong mandibles.
The generality of the Pycnogonides have also mandibles,
which have been taken for palpi, and true palpi, which
have been erroneously regarded as antennae; but their
mouth consists of a tunnel or syphon of a single piece. It
is still more simple in the Parasites, being nothing more
than a very short projection, containing a small sucker, or
a cavity of which the sides are dilatable, and accompanied
by two crotchets.
In regard to the families of the Arachnides, the Tetra¬
cera offer two principal groups. Some dwell in salt or
fresh water, are usually fixed upon other animals, of which
they suck the blood, and have, with the exception of the
genus Bobyra, four very obvious and distinct antennae.
They constitute the first family, that of Others
are more terrestrial than aquatic. They wander from place
to place, love obscure and sombre situations, and live upon
putrid substances ; their two intermediate antennae are
but slightly developed. They form the second family,
called Oniscides. According to Latreille, the structure of
the mouth in Scolopendra is so different from that of Julus,
that it is difficult to conceive the motive which influenced
Fabricius to unite these Arachnides into a single order,
that of Mitosata. In Julus, the maxillae and the lip are
soldered together, forming one transverse plate, without
distinct palpi, or their place supplied by tubercles. The
Scolopendrae have their maxillae separated, four projecting
palpi, and the labia in the form of hooks, subservient to
the same uses as the mandibles of spiders. These consi¬
derations, and the figure of the antennae, have required
the establishment of two families, which compose Latreille’s
order Myriapoda, viz. Chilognatha and Syngnatha. Pro¬
ceeding upon the same principles, he divides the order
Thysanoura likewise into two families, the Lepismenae and
the Podurellcc. The next orders, Parasita and Pycnogo¬
nides, contain few genera, and do not admit of subdivision
into families. The last order, Acera, is composed of eight
families, viz. Scorpionides, Pedipalpi, Aranides,Phalangita,
Acaridiae, Riciniae, Hydrachnellae, and Microphthira.
Before commencing our systematic exposition of this
class, we shall lay before our readers the sentiments of
Mr Kirby. “ I must next say something on the orders of
the Araehnida. Every one at first sight sees that spi¬
ders and scorpions are separated by characters so strongly
marked, that they look rather like animals belonging to
different classes, than to the same. These form the two
primary orders of the Arachnida, and they appear to be
connected by two secondary or osculant ones,—on the one
side by Galeodes, and on the other by Thelyphonus and
Phrynus. This class, although there is an appearance of
Arach.
nides.
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