Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

388 ARACHNIDES.
Arach- Genus Erythr^eus, Latr.—Mandibles and palpi as in
nides. Trombidium, but the body is undivided, and the eyes
are not mounted on a pedicle.
Sp. Phalangioides.—Legs very long, the last joint
broad, compressed. Body obscure red, with a dorsal band
of orange yellow. Inhabits most European countries,
running on the ground with great rapidity.
Genus Gamasus, Latr.—Mandibles didactylous; palpi
projecting, distinct, filiform.
Some species of this genus have the upper surface of
the body clothed, in whole or in part, with a scaly skin,
while others are entirely soft.
Sp. Coleoptratorum.—Anterior pair of legs somewhat
longer than the others. Coriaceous parts of the back
fuscous.
Inhabits the excrements of horses and cattle, and is
frequently found adhering in great numbers to the bodies
of coleopterous insects of the genus Scarabaeus, Hister, &c.
To this genus belongs the Acarus marginatus of Her¬
mann, which is sometimes found in the brain (corpus cal¬
losum) of the human race.
Genus Cheyletus, Latr.—Mandibles didactylous. Palpi
thick, arm-shaped, falcated at the extremities.
Sp. Eruditus. Acarus eruditus, Schrank.—Colour brown¬
ish. Inhabits books and museums.
Genus Oribata.—Mandibles didactylous. Palpi short
and concealed. Body covered by a coriaceous or scaly
skin, in the form of a shield or buckler. Legs long, or of
medium length.
The body in this genus is prolonged anteriorly in the
form of a muzzle. There is sometimes an indication of
a thorax. The ends of the tarsi are terminated by a
single crotchet in some, by two or three in others, with¬
out any vesicular ball or cushion. The species are found
beneath stones, among moss, and on trees. Their move¬
ments are slow.
Sp. Geniculata.—Of a brownish chesnut colour, shin¬
ing, hairy. Legs pale brown, thighs sub-clavate. Com¬
mon in Sweden, Germany, and England.
Genus Uropoda, Latr.—Mandibles pincer-shaped; palpi
not conspicuous. Body covered by a scaly skin, and ter¬
minated by a slender filament, by means of which the
species adhere to the bodies of coleopterous insects.
Sp. Vegetans. (PL XLVII. fig. 8.)—Brown, smooth, and
shining. Inhabits J ranee and England, attaching itself
to the legs and other parts of insects by its pedunculated
anus.
Genus Acarus, Fab. Latr.—’Furnished like the preced¬
es genera with didactylous mandibles, and very short
or concealed palpi; but the body is soft, and unfurnish¬
ed with a scaly crust. The tarsi are provided at their
extremity with a vesicular tuft. Many species live on
the substances used as aliments by the human race.
Sp. Siro. The cheese-mite. (PI. XLVIL fig. 9.)
Whitish, with two brown spots. Body ovate, the^middle
coarctate, with long hairs. Legs of equal length. Inha¬
bits houses, feeding on flour and long-kept cheese.
Sp. Scabiei. (PI. XLVII. fig. 10.)—A microscopical spe¬
cies, which inhabits the skin of man in a diseased state. It
appears, from the observations of Bonnani and others
that this insect usually accompanies the disease called
the itch.
Genus Bdella, Latr.—Palpi elongated, bent, terminat¬
ed by hairs or bristles. Four eyes, posterior feet the Aracli.
longest. Sucker prolonged in the form of a conical or nides.
awl-shaped beak. Found in moss, beneath stones, and'k“'"VN-/
under the bark of trees.
Sp. Rubra. La Bdelle rouge of Latreille. (PI. XLVII.
fig. 11.)—Rostrum longer than the thorax. Colour coccine-
ous, legs paler than the body. Dwells beneath stones.
This is the pince rouge of Geoffrey, and the Acarus longi-
cornis of Linnaeus. It is a minute insect, measuring scarcely
half a line in length.
Genus Smaridia, Latr.—Distinguished from the preced¬
ing genus by the palpi, which are scarcely longer than
the sucker; straight, and without bristles at the extre-
mity. Eyes two. Anterior pair of legs longer than the
others.
Sp. Sambuci. Acarus Sambuci, Schrank.—Colour red,
the body slightly haired. Movements slow. Dwells be¬
neath the bark of trees, more especially that of the elder,
observed by Latreille in the south of France. This genus
is represented in PI. XLVII. fig. 12., by a figure of Sma¬
ridia passerina.
Genus Ixodes.—Palpi inclosed in the sucker, along with
which they form a short projecting beak, truncated, with
a slight expansion at the end.
The animals of this genus occur among bushes and
underwood, from which they detach themselves to fasten
on dogs, sheep, cattle, and other quadrupeds, to which they
adhere with remarkable tenacity. Their eggs, of which
they lay a prodigious quantity, are, according to M. Cha-
brier, obtruded by the mouth.
Sp. Reduvius. Acarus reduvius, Linn. (PI. XLVII.
fig. 13.)—The colour and appearance of this species vary
according to its state of repletion. The legs are black.
Genus Argas.—Palpi conical, composed of four articles,
not inclosed in the sucker.
Sp. Reflexus. Acarus marginatus. Fab. (PI. XLVII.
fig. 14.)—Pale yellowish, or flesh-coloured, with deeper an¬
astomosing lines. Inhabits houses in France, sucking the
blood of doves.
A species of this genus found in Persia (the malleh de
mianeh) is considered to be extremely poisonous. It ap¬
pears, however, from the observations of M. Szovits, a na¬
turalist recently employed by the Russian government to
explore the Caucasus, that the bite of these bugs ofMiana,
as they are sometimes called (Argas Persicus of Fischer),
is in reality by no means dangerous.
The three following genera correspond to the genus
Hydrachna of Muller. Their habits are aquatic, and
their forms oval, or nearly globular, and of a soft consis¬
tence. The number of their eyes varies from two to four,
and even to six, according to Muller. They were usual¬
ly confounded with the mites till the time of the last-
named observer, who inferred that as they lived habitual¬
ly in a different element, they ought to form a separate
division. They resemble small spiders, and probably on
that account received the name of Flydrachna, which
signifies water-spider. Fabricius, who only employed in
the formation of his groups characters drawn from the
structure of the mouth, has united Hydrachna with Trom¬
bidium. The observations of Latreille have led not only to
the distinct separation of these two genera, but to the sub¬
division of the genus Hydrachna into at least three dis¬
tinct groups of species, all of which may be readily distin¬
guished from the various kinds of mites (Acari) by their
ciliated or natatorial legs.
I he minute beings now under consideration occur