ARACHNIDA EMBOLOBRANCHIATA (_CONTINUED_)—ARANEAE—EXTERNAL STRUCTURE—
INTERNAL STRUCTURE.
=Order III. Araneae.=
(ARANEIDA,[256] ARANEINA.)
_Arachnida breathing by tracheae and “lung-books.” Cephalothorax and
pedicellate abdomen, the latter usually soft, and only very rarely
showing any traces of segmentation. Two-jointed non-chelate chelicerae,
the distal joint bearing the orifice of a poison-gland. The tarsal joint
of the male pedipalp develops a sexual organ. The abdomen is furnished
with spinning mammillae._
The true Spiders can readily be distinguished from allied Arachnid
groups, with which they are often popularly confounded, by the presence
of a narrow constriction or “waist” between the cephalothorax and
abdomen, and of a group of “spinnerets” or external spinning organs
beneath the hind portion of the body. Thus the so-called
“Harvest-spider” or “Harvestman” is clearly not a Spider, for there is
no constriction of its body into two parts, nor does it possess any
spinnerets. It belongs to the Phalangidea. The same considerations will
exclude the “Red Spider” of popular nomenclature, which must be referred
to the Acarina or Mites.
The Araneae, even as at present known, form a very extensive and
widely-distributed order of animals. Compared with certain insect
orders, they have received little attention from the collector, and the
number of known forms is certain to be very largely increased. They form
an extremely compact and natural group, for though, within the order,
there is an infinite variety of detail, their uniformity in essential
points of structure is remarkable, and they are sharply marked off from
the neighbouring groups of Arachnida.
[Illustration:
FIG. 172.—_Epeira angulata._ ♀.
]
It is perhaps unfortunate that the obtrusiveness of particularly
unattractive specimens of the race has always caused spiders to be
regarded with more or less aversion. This prejudice can hardly fail to
be modified by a wider acquaintance with these animals. There are
certainly few groups which present points of greater interest in respect
to their adaptation to special modes of life and the ingenuity displayed
in the construction of their nests and the ensnaring of their prey.
Spiders are wingless, yet they may often be observed travelling through
the air. They are air-breathing, yet many are amphibious in their
habits, and one species at least spends the greater part of its
existence beneath the surface of the water. On land they may be found in
all imaginable localities which admit of the existence of that insect
life on which they depend for food.
=External Structure.=—The spider’s body consists of two portions, the
cephalothorax and the abdomen.
=Cephalothorax.=—Looked at dorsally (Fig. 173), the cephalothorax is
generally seen to have a depression near the middle, the “median fovea,”
and from this certain lines, the “radial striae,” radiate towards the
sides. These depressions indicate the attachment of internal muscles.
The head region or “caput” lies in front of the foremost of the radial
striae, and is often clearly marked off from the thorax, and different
from it in elevation. It bears the eyes, which, in the great majority of
spiders, are eight in number. Many, however, are six-eyed, while in rare
cases the number is reduced to four (_Tetrablemma_, see p. 404), or even
to two (_Nops_, see p. 395). The number, relative size, and particular
arrangement of these eyes are of considerable systematic importance.
Their disposition varies very greatly, but it is generally possible to
regard them as forming two transverse rows, an anterior and a posterior,
each possessing a pair of median and a pair of lateral eyes.
[Illustration:
FIG. 173.—Diagrammatic dorsal view of a Spider. _ch_, Chelicera; _f_,
median fovea; _n_, normal marking; _o_, ocular area; _p_, pedipalp;
_st_, stria. (The dotted line should reach the radial marking on the
cephalothorax.)
]
In many spiders all the eyes have a dorsal aspect, but in some groups
(Attidae, Lycosidae) the prevailing arrangement is to have the anterior
eyes directed forwards and the posterior upwards. In other spiders,
again, a dorsal view may only show the eyes in profile, all having their
axes directed forwards or sideways, or they may be mounted on turrets,
and thus command a wide range of view. The rows are described as
straight, “procurved” (with the convexity backwards), or “recurved”
(with the convexity forwards). Thus, in Fig. 177, the anterior row is
slightly, and the posterior row considerably “recurved.”
Sometimes there is a marked difference in the colour of the eyes, two or
more being black, while the remainder are pearly white. In other cases
they are homogeneous, either of the black or the white type. Simon
considers the black eyes to be diurnal and the white nocturnal, but the
evidence for this is indirect and not altogether satisfactory. The
portion of the caput occupied by the eyes is often alluded to as the
“ocular area.” The space between the ocular area and the chelicerae,
well shown in Fig. 177, is known as the “clypeus.” It is usually more or
less vertical, but in the Aviculariidae (see p. 386) it is horizontal
and dorsal.
The under surface of the cephalothorax is protected by the “sternum” or
“plastron,” a large plate of variable shape, usually notched at either
side for the reception of the legs, and having in front a small plate,
generally hinged, but sometimes soldered to it, known as the “labium.”
This has no homology with the labium of insects, but is a true sternite,
more correctly described as “pars labialis sterni.”
The labium and the maxillary lobes of the palpi more or less conceal the
under surface of the caput. The shape of the sternum and of the labium,
and the contour and degree of inclination towards one another of the
maxillae, are important considerations in the taxonomy of Spiders.
[Illustration:
FIG. 174.—Diagrammatic ventral view of a Spider. Cephalothorax—_l_,
Labium; _m_, maxilla; _p_, paturon of chelicera; _st_, sternum; _u_,
unguis of chelicera. Abdomen—_a.t_, Anal tubercle; _c_, colulus;
_ep_, epigyne; _s_, stigma; _sp_, spinnerets; _tr_, tracheal
opening.
]
The appendages of the cephalothorax, which are the chelicerae or jaws,
the pedipalpi or feelers, and the four pairs of ambulatory legs, will be
treated separately.
=Pedicle.=—The chitinous investment of the narrow stalk which unites the
thorax with the abdomen is for the most part thin and flexible, with
only slight indurations of various patterns on the dorsal surface, where
it is in most cases more or less protected by the forwardly-projecting
abdomen. Beneath, it is usually quite membranous, guarded only by a sort
of collar formed by the raised border of the anterior portion of the
abdomen at the point of insertion. In some Spiders, however
(Dysderidae), there is a posterior sternal plate, the “plagula,” closely
corresponding with the labium in front, which partly embraces the
pedicle. In _Hermippus_ (Zodariidae) the plagula is detached from the
sternum, and is succeeded posteriorly by two smaller paired plates.
=Abdomen.=—The abdomen differs remarkably in shape in the different
groups of Spiders. In some families the prevailing shape is more or less
globular, and in others cylindrical, while it may be diversified to
almost any extent by prominences or spines. Ordinarily no sign of
segmentation is observable, but in _Liphistius_ it is covered dorsally
by seven well-marked chitinous plates.
In most Spiders the integument of the abdomen is uniformly soft and
flexible all over, but it is not rare to find portions of it thickened
and hardened to form “scuta.” In the Gasteracanthinae and the
Phoroncidinae there is a great dorsal scutum armed with spines, while in
several families there are species characterised by the possession of a
smooth dorsal scutum; and in some a ventral scutum is present.
[Illustration:
FIG. 175.—Spider profiles. 1, _Poltys ideae_; 2, _Phoroncidia
7–aculeata_; 3, _Ariamnes flagellum_; 4, _Stegosoma testudo_; 5,
_Formicinoides brasiliana_.
]
That these scuta are sometimes indicative of an obsolete segmentation
would seem likely from the study of the remarkable species, _Tetrablemma
medioculatum_ (Fig. 176), described by Pickard-Cambridge, from Ceylon.
In addition to large dorsal and ventral scuta, the sides and posterior
extremity are guarded by smaller scuta, the disposition of which is well
seen in the figure.
[Illustration:
FIG. 176.—_Tetrablemma medioculatum_, much enlarged. =A=, Posterior
view; =B=, profile, showing the scuta. (After Cambridge.)
]
The normal smooth abdomen presents dorsally no very striking features.
In species of variegated coloration there is very generally noticeable a
median dentated band (Fig. 173), the “normal marking” of some writers,
which would appear to have some correlation with the underlying dorsal
vessel. Beneath the abdomen are to be seen the orifices of the breathing
and genital organs, the spinnerets, and the anal aperture upon its
tubercle.
The breathing organs are, as will be explained later, of two kinds,
lung-books and tracheae. The great majority of Spiders possess only two
lung-books, and their transverse, slit-like openings (“stigmata” or
“spiracles”) may be seen on either side of the anterior part of the
abdomen. Where, as in the Theraphosae, there are four lung-books, the
second pair open by similar slits a short distance behind the first.
According to Bertkau, pulmonary sacs are entirely lacking in the genus
_Nops_.
The tracheae generally debouch by a single median stigma towards the
posterior end of the abdomen, just in front of the spinnerets. This
opening clearly results from the fusion of two stigmata, which in some
species retain their paired arrangement.
On a level with the openings of the anterior lung-books or pulmonary
sacs there is usually observable a slight transverse ridge, the
epigastric fold (Fig. 174), and in the centre of this is the genital
opening. This is never visible until after the last moult, and in the
male is always a simple inconspicuous aperture. This is also the case
with the females of some groups (Theraphosae, Filistatidae, Dysderidae,
etc.), but in most cases there is a more or less complicated armature,
the “epigyne,” the special design of which is of great specific value.
In its simplest form it is merely a plate, usually of dark colour, with
one or two apertures (Fig. 174, _ep_), but in some families, notably the
Epeiridae, it is more complicated, and is furnished with a hooked median
projection, the “ovipositor” (“clavus” of Menge), which is often
absurdly like a petrified elephant’s trunk in miniature.
The abdomen also presents on its under surface, usually towards the
posterior end or apex, a group of finger-like mammillae or spinnerets.
They are normally six in number, two superior (or posterior), two
median, and two inferior (or anterior). The number is reduced, in most
of the Theraphosae, to four, while a few spiders possess only a single
pair of spinnerets. These organs are described more fully on p. 325.
A small papilla, the “colulus” (Fig. 174, _c_), is often observable,
projecting between the anterior spinnerets. The “anal tubercle” (Fig.
174, _a.t_), on which the vent is situated, terminates the abdomen, and
is generally in close juxtaposition with the posterior spinnerets.
=Appendages.=—The cephalothoracic appendages are the chelicerae, the
pedipalpi, and the four pairs of ambulatory legs. Those of the abdomen
are the mammillae or spinnerets.
=Chelicerae.=—These are two-jointed appendages, articulated immediately
below or in front of the clypeus. They are the “mandibles” of many
authors, but there is good reason for believing that they are not
homologous with the mandibles of Insects. There is little agreement,
moreover, with regard to the names given to the two joints of which they
consist. The term “falx,” often applied to the basal joint, is much more
appropriate to the sickle-like distal joint. Base and fang are tolerably
satisfactory, or we may avoid ambiguity by adopting the terms “paturon”
and “unguis” suggested by Lyonnet.[257]
The paturon is a stout joint of more or less cylindrical or conical
shape. The unguis (the “crochet” of Simon) is hook-like, and can
generally be folded back upon the paturon, which often presents a groove
for its reception. The Theraphosid spiders are distinguished from all
others by the fact that the plane of action of the chelicerae is
vertical and longitudinal. The paturon projects forward in a line
parallel with the axis of the body, and its distal end can be raised or
depressed, but not moved laterally; while the unguis in action has the
point directed downwards, and, at rest, is applied to the under surface
of the paturon.
In other spiders the patura hang more or less vertically, and while to
some extent mobile in all directions, their principal motion is
_lateral_, and the ungues have their points directed towards each other
in action, and are applied to the inner surfaces of the patura in
repose. The plane of action in this case is also more or less vertical,
but _transverse_.
[Illustration:
FIG. 177.—Front view of _Textrix denticulata_. × about 10. 1, Caput;
2, eyes; 3, paturon; and 4, unguis of chelicera.
]
The paturon is always extremely hard and strong. In Theraphosae of
burrowing habits the distal end is furnished with a group of powerful
teeth, the “rastellus.” The groove for the reception of the unguis is
often guarded on one side or on both by rows of teeth, the arrangement
of which is frequently an important specific character. The inner
anterior border is also often furnished with a group of stiff hairs or
bristles. This powerful joint is of use in crushing and expressing the
fluids of insects pierced by the ungues.
The crescent-shaped unguis is tapering and smooth, except for the
presence, on the posterior surface, of one or two feebly dentated
ridges. Near its free extremity there is a small orifice leading to the
poison reservoir and gland.
In the genus _Pholcus_ (see p. 401) the chelicerae may almost be
regarded as chelate, the unguis being met by a spiny projection from the
inner anterior border of the paturon.
=Rostrum.=—On examining a spider, even under a dissecting microscope, it
will not be easy at first to discover the mouth. Indeed, Lyonnet had
almost come to the conclusion that Spiders, like some Myrmelionid
larvae, imbibed the juices of their prey by way of the mandibles, before
he found the orifice and gave a remarkably accurate description of the
adjacent parts.
If a specimen be placed on its back, and the labium raised while the
chelicerae are pushed forward, no orifice is visible, but on careful
examination it will be found that what appears to be a thick and fleshy
labium is, in reality, two organs. The labium is thin and flat, and
closely opposed to its upper surface is a somewhat flattened cone. This
is the “rostrum,” and when it is separated from the labium the buccal
orifice is disclosed. In a few spiders (Archeidae) in which the
chelicerae are far removed from the mouth, the rostrum is tolerably
conspicuous, but in most it is so hidden as to have escaped the
observation of the great majority of observers. Schimkewitsch considers
it homologous with the labrum of insects, but Simon thinks that it
represents all the insect mouth-parts reduced to an exceedingly simple
form. It is more probable that a beak consisting of a simple labrum and
labium was a primitive Arachnid characteristic. If the rostrum be
removed and its inner (or posterior) surface examined, a lance-shaped
chitinous plate, the “palate,” becomes visible. It is furrowed down the
middle by a narrow groove, which is converted into a tube for the
passage of fluids when the rostrum is opposed to the labium.
[Illustration:
FIG. 178.—Pedipalp of _Tegenaria domestica_ ♂, × 5. 1, Coxa; 2,
maxilla; 3, trochanter; 4, femur; 5, patella; 6, tibia; 7, tarsus;
8, palpal organ.
]
=Pedipalpi.=—The pedipalpi are extremely leg-like feelers, and are
six-jointed, the metatarsal joint of the ambulatory legs being absent.
The joints, therefore, are the coxa, trochanter, femur, patella, tibia,
and tarsus (Fig. 178).[258]
In the Theraphosae the coxa resembles that of the ambulatory leg, but in
other spiders it is furnished, on the inner side, with a blade-like
projection, the “maxilla” (Fig. 178). The shape of the maxillae and the
degree of their inclination towards the labium are of considerable
taxonomic importance. The inner border of the maxilla is furnished with
a tuft of hairs, which assist in retaining the juices expressed by the
chelicerae, and its anterior border presents a cutting edge with a
finely dentated ridge called the “serrula.”
In the female, and in the immature male, the remaining joints differ
little from those of the legs, except that the tarsal joint is either
clawless or has a single claw, which is generally smooth, and is never
much dentated.
At the last moult but one the male pedipalp appears tumid at the end,
and after the last moult the tarsus is seen to have developed a
remarkable copulatory apparatus, the “palpal organ,” comparatively
simple in some families, but in others presenting an extraordinary
complexity of structure.
[Illustration:
FIG. 179.—Diagram of palpal organ. 1, Tarsus; 2, bulb; 3, receptaculum
seminis; 4, its aperture; 5, style; 6, haematodocha; 7, alveolus; 8,
tibia.
]
=Palpal Organs.=—Externally the essential parts of the palpal organ are
three, the “haematodocha,” the “bulb,” and the “style.” The spines and
projections, or “apophyses,” which often accompany the palpal organ
proper, are of secondary importance, and in many spiders are entirely
absent; nor is their function when present at all clear; but the
infinite variety of design which they exhibit, and their singular
uniformity in all the males of a species, render them of the utmost
value as specific characteristics.
The “haematodocha” is the portion of the palpal organ attached to the
tarsus, and often received into an excavation, the “alveolus,” on its
under surface. It is a fibro-elastic bag, in its normal collapsed state
usually somewhat spirally disposed round the base of the following
portion, the “bulb.”
The bulb is generally the most conspicuous portion of the organ, and is
a sub-globular sac with firm, though often semi-transparent, integument.
Its base rests upon the haematodocha, and its apex is produced, often
spirally, to a point which bears the seminal orifice. This external
opening leads into a coiled tube within the bulb, ending in a blind sac,
the “receptaculum seminis,” which projects into the haematodocha; and it
is the aperture by which the sperm both enters and leaves the organ. How
the sperm is conveyed to the receptaculum was long a matter for
speculation, after the belief in a direct communication between the
generative glands and the pedipalpi had been abandoned. The process has
been actually observed in the case of a few spiders, which have been
seen to deposit their sperm on a small web woven for the purpose, and
then, inserting the styles of their palpal organs into the fluid, to
suck it up into the receptacula seminis. This is probably the usual
method of procedure, though it may be true, as some have asserted, that
the palp is sometimes applied directly to the genital orifice.
The receptaculum and its tube being thus charged with sperm, it is the
function of the haematodocha to eject it by exerting pressure on its
base. For this purpose the haematodocha is in communication with the
cavity of the tarsus, from which, in copulation, it receives a great
flow of blood, and becomes greatly distended. Bertkau believes that he
has detected very minute pores (meatus sanguinis) communicating between
the haematodocha and the receptaculum, and allowing some of the
blood-plasma from the former to mingle with the semen, but this appears
to be very doubtful.
The =Legs= are uniformly eight in number, and are seven-jointed, the
joints, counting from the body, being the _coxa_, _trochanter_, _femur_,
_patella_, _tibia_, _metatarsus_, and _tarsus_.[259] In a few cases,
through the presence of false articulations, _i.e._ rings of softer
chitin, this number appears to be exceeded. Some of the Palpimanidae
(see p. 398) were at first thought to have only six joints on their
anterior legs, but the tarsus is present, though very small.
In the case of most spiders, the legs take a general fore and aft
direction, the first pair being directed forwards, the second forwards
or laterally, and the third and fourth backwards. In the large group of
“Crab-spiders” (Thomisidae), and in many of the Sparassinae, all the
legs have a more or less lateral direction, and the spider moves with
equal ease forwards, backwards, or sideways. The legs are usually more
or less thickly clothed with hairs, but in some genera the clothing is
so sparse that they appear glossy, while in others they have a
positively shaggy appearance. Stouter hairs or “bristles” are often
present, and some of the joints are also often furnished with “spines,”
which in many cases are erectile.
The tarsi of all spiders are furnished with terminal claws, usually
three in number, though in some families (Drassidae, Thomisidae, etc.)
there are only two. The two principal claws are paired and usually
dentated, though the number of their teeth may be unequal. The third
claw, when present, is always smaller, median, and inferior.
[Illustration:
FIG. 180.—Spider tarsi. 1, Tarsus of _Epeira_ showing three claws and
supplemental serrate hairs (_a_); 2, tarsus of a Thomisid Spider,
with two claws; 3, 3_a_, lateral and dorsal view of tarsus of an
Attid Spider, showing scopula at _b_.
]
In many spiders of climbing habits the place of the third claw is taken
by a remarkable tuft of club-like hairs termed a “scopula” (Fig. 180,
_b_), by means of which they are able to cling to smooth surfaces where
claws would be able to obtain no hold. In some species there is a
special false articulation—the “onychium”—at the end of the tarsus to
bear the claws.
In the Cribellatae the metatarsus is always furnished with a comb-like
organ, the “calamistrum,” correlated with an extra spinning apparatus,
the “cribellum,” but this will be dealt with when we reach the
systematic portion of the subject.
The general direction taken by the legs, the comparative length of the
different joints, their armature of hairs, bristles, and spines, and the
number and conformation of the tarsal claws, are points of great
importance in the classification of Spiders.
Under considerable magnification the legs of all Spiders exhibit a
number of minute organs, arranged with absolute uniformity throughout
the Araneae, and known as the “lyriform organs.” They consist of little
parallel ridges of thickened chitin, the slit between them being covered
by thinner chitin. They are eleven on each leg, and are distributed near
the distal extremities of each of the first six joints. Their function
is unknown, though some authors consider them to be organs of hearing.
[Illustration:
FIG. 181.—Spinnerets of _Epeira diademata_. =A=, Ventral view of
_Epeira_; =B=, spinnerets magnified; =C=, profile.
]
The =Spinnerets= are normally six in number, and, except in rare
instances, are placed beneath the abdomen, near its apex and immediately
in front of the anal tubercle. Their arrangement varies greatly, but
they can generally be recognised as comprising three pairs, a posterior
(or superior) pair, a median pair, and an anterior (or inferior) pair.
In nearly all the Theraphosae the anterior pair are absent, while the
posterior spinnerets are largely developed. In the Palpimanidae only the
anterior spinnerets are present. When all six are found, the usual
arrangement is in the form of a rosette, the median spinnerets being
hidden by the others in repose, but this disposition is widely departed
from. In _Hahnia_ (Agelenidae), for instance, they are ranged in a
transverse row at the end of the abdomen, the posterior spinnerets
occupying the extremities of the row, and the median ones the centre.
These spinnerets are highly mobile appendages, and additional play is
given to their action by the presence of articulations, much resembling
the “false” joints sometimes found on the legs, on the posterior and
anterior pairs. They are always at least bi-articulate, and sometimes
present three or four joints. They are movable turrets on which are
mounted the “fusulae” or projections where the tubes from the spinning
glands open. These are often very numerous, especially in the
orb-weaving spiders, where the spinning powers are most highly
developed. They consist of two portions, a cylindrical or conical basal
part, succeeded by a very fine, generally tapering tube.
In some spiders the fusulae are all much alike, but usually a few very
much larger than the rest are noticeable under the microscope, and these
are often alluded to as “spigots.” The smaller ones are also divisible
into two kinds, a few short conical fusulae being noticeable amongst the
much more numerous cylindrical tubes. We shall treat of the functions of
the various fusulae later (see pp. 335 and 349).
Simon remarks that though the battery of fusulae is most complicated in
those spiders which possess the greatest spinning powers, it is by no
means among them that extremely long spinnerets are developed. The
posterior spinnerets of some of the Hersiliidae are of great length, but
these spiders spin very little except in forming their egg-cocoons.
[Illustration:
FIG. 182.—=A=, Spinnerets of _Amaurobius similis_ ♀. Much enlarged.
_a_, Anus; _cr_, cribellum; _i.s_, inferior spinneret; _m.s_, median
spinneret; _s.s_, superior spinneret. =B=, Part of the 4th leg of
the same Spider, showing the calamistrum (_ca_) on the metatarsus.
]
In addition to the six spinnerets, and just in front of them, there is
to be found in some spiders an extra spinning organ in the form of a
double sieve-like plate, the “cribellum.” This is always correlated with
a comb of curved bristles on the metatarsi of the fourth pair of legs,
the “calamistrum.” Such importance is assigned to these organs by Simon,
that the Araneae Veraeare divided by him according to whether they are
present or absent, into CRIBELLATAE and ECRIBELLATAE. This is probably
an exaggerated view of the importance of these organs, and the spiders
possessing them certainly do not seem to form a natural group.
=Stridulating Organs.=—When Arthropod animals are capable of producing a
sound, the result is nearly always obtained by “stridulation,” that is,
by the friction of two rough surfaces against each other. The surfaces
which are modified for this purpose form what is called a “stridulating
organ.” Such organs have been found in three very distinct Spider
families, the Theridiidae, the Sicariidae, and the Aviculariidae.
Hitherto they have only been observed in three positions—either between
the thorax and abdomen, or between the chelicerae and the pedipalpi, or
between the pedipalpi and the first legs.
In the Sicariidae and the Aviculariidae, the sounds have been distinctly
heard and described. Those produced by the Theridiidae would appear to
be inaudible to human ears.
[Illustration:
FIG. 183.—Stridulating apparatus of _Steatoda bipunctata_, ♂. Much
enlarged. =A=, Ridged and toothed abdominal socket; =B=, striated
area on the cephalothorax; =C=, profile of the Spider, × 5.
]
Westring[260] was the first to discover (1843) a stridulating organ in
the small Theridiid spider _Asagena phalerata_. The abdomen, where the
pedicle enters it, gives off a chitinous collar, which projects over the
cephalothorax, and has the inner surface of the dorsal part finely
toothed. When the abdomen is raised and depressed, these teeth scrape
against a number of fine striae on the back of the posterior part of the
cephalothorax. A similar organ has been since found in various allied
spiders, of which the commonest English species is _Steatoda
bipunctata_. In this group it is generally possessed by the male alone,
being merely rudimentary, if present at all, in the female.
In 1880 Campbell[261] observed that in some of the Theridiid Spiders of
the genus _Lephthyphantes_, the outer surface of the chelicera and the
inner surface of the femur of the pedipalp were finely striated at the
point, where they were rubbed together when the palps were agitated, but
though the appropriate motion was frequently given, he could hear no
sound.
[Illustration:
FIG. 184.—_Chilobrachys stridulans_ in stridulating attitude. After
Wood-Mason. Natural size.
]
Meanwhile the noise produced by a large Theraphosid spider in Assam
(_Chilobrachys stridulans_) had attracted attention, and its
stridulating apparatus was described in 1875 by Wood-Mason.[262] The
sound resembled that obtained by “drawing the back of a knife along the
edge of a strong comb.”
Subsequently certain Sicariid spiders of a genus confined to the
southern hemisphere were heard to produce a sound like the buzzing of a
bee by the agitation of their palps, and both sexes were found to
possess a very perfect stridulating organ, consisting of a row of short
teeth on the femur of the pedipalp, and a striated area on the paturon
of the chelicera.
Pocock has recently discovered that all the large kinds of Theraphosidae
in the countries between India and New Zealand are, like _Chilobrachys_,
provided with a stridulating organ. In these spiders also it is between
the palp and the chelicera, and consists of a row of teeth or spines
constituting a “pecten,” and a series of vibratile spines or “lyra,” but
whereas in _Chilobrachys_ and its near relations the lyra is on the palp
and the pecten on the paturon, in other spiders the positions are
reversed. The lyra is a very remarkable organ, consisting of
club-shaped, often feathery bristles or spines, which lie parallel to
the surface to which they are attached, and which is slightly excavated
for their reception.
Lastly, many African Theraphosids possess a similar organ, not between
the palp and the chelicera, but between the palp and the first leg.
Various suggestions have been hazarded as to the use of these organs,
but they partake largely of the nature of conjecture, especially in
connexion with the doubt as to the possession of a true auditory organ
by the Araneae. They may be summarised as follows. The Theridiid spiders
are among those which show most indication of auditory powers, and the
stridulating organs, being practically confined to the male, may have a
sexual significance. _Chilobrachys_ stridulates when attacked, assuming
at the same time a “terrifying attitude,” and its stridulating organ may
serve the purpose attributed to the rattle of the rattlesnake, and warn
its enemies that it is best let alone. If this be the case, there is no
need that it should itself hear the sound, and, indeed, there is no
evidence that the Aviculariidae possess the power of hearing. In the
inoffensive stridulating Sicariid spiders the sounds could hardly serve
this purpose, and the presence of the organ in both sexes, and in
immature examples, precludes the idea that its function is to utter a
sexual call. Instead of trying to escape when disturbed, the spider
starts stridulating, and Pocock suggests that the similarity of the
sound produced to the buzzing of a bee may be calculated to induce its
enemies to leave it in peace.
=Internal Anatomy.=
=Alimentary System.=—The alimentary canal of the Spider is divided into
three regions, the “stomodaeum,” the mid-gut or “mesenteron,” and the
hind-gut or “proctodaeum.”
=The Stomodaeum= consists of the pharynx, the oesophagus, and the
sucking stomach. As we have said, the mouth is to be found between the
rostrum and the labium. It opens into the pharynx, the anterior wall of
which is formed by a chitinous plate on the inner surface of the
rostrum, sometimes called the palate. As the inner surfaces of the
rostrum and labium are practically flat, the cavity of the pharynx would
be obliterated when they are pressed together, were it not for a groove
running down the centre of the palate, which the apposed labium converts
into a tube, up which the fluids of the prey are sucked. In the
Theraphosidae there is a corresponding groove on the inner surface of
the labium.
At the top of the pharynx, which is nearly perpendicular, the canal
continues backwards and upwards as a narrow tube, the oesophagus,
passing right through the nerve-mass, which embraces it closely on all
sides, to the sucking stomach. At the commencement of the oesophagus is
the opening of a gland, probably salivary, which is situated in the
rostrum.
[Illustration:
FIG. 185.—Diagram showing the anatomy of the cephalothorax of a
Spider. The right alimentary diverticulum has been removed. _a_,
Aorta; _c_, left diverticulum with secondary caeca; _e_,
endosternite; _oes_, oesophagus, descending to the mouth; _s_,
sucking stomach; _sh_, dorsal shield of sucking stomach.
]
We now reach the sucking stomach, which occupies the centre of the
cephalothorax. It is placed directly over a skeletal plate, the
“endosternite” (Fig. 185, _e_), to which its lower surface is connected
by powerful muscles, while its upper wall is protected by a hard plate
or “buckler,” which is similarly attached to the roof of the
cephalothorax in the region of the “fovea media.” The walls of the
stomach are not themselves muscular, but by the contraction of the
muscles above mentioned its cavity is enlarged, and fluids from the
pharynx are pumped up into it.
The canal thus far is lined by chitin, like the exterior of the body,
and forms a sort of complicated mouth-apparatus.
The =Mesenteron= lies partly in the cephalothorax and partly in the
abdomen. The thoracic portion, shortly behind the sucking stomach, sends
forward on either side a large branch or “diverticulum,” from each of
which five secondary branches or “caeca” are given off (Fig. 185). Of
these the anterior pair sometimes join, thus forming a complete ring;
but usually, though adjacent, they remain distinct. The other four pairs
of caeca curve downwards, protruding into the coxae of the legs, where
they often terminate, but sometimes (_Epeira_) they continue their curve
until they meet, though they never fuse, under the nerve-mass. Behind
the origin of the diverticula the mesenteron continues as a widish tube,
and shortly passes through the pedicle and enters the abdomen, where,
curving slightly upwards, it proceeds along the middle line till it ends
in the proctodaeum.
In the abdomen it is surrounded by a large gland, the so-called liver,
and is dilated at one spot (Fig. 186) to receive the ducts from this
gland. The fluid elaborated by this large abdominal gland has been shown
to have more affinity with pancreatic juice than with bile.
The =Proctodaeum= consists of a short rectum, from the dorsal side of
which protrudes a large sac, the “stercoral pocket.” At its origin, the
rectum receives the openings of two lateral tubes which reach it after
ramifying in the substance of the liver. These have been called
“Malpighian tubules,” but their function is unknown. Loman[263] has
shown that they open into the mid-gut and not into the rectum, and there
is reason to believe that true Malpighian tubules homologous to those of
Insecta are absent in Arachnida, where their place seems to be taken by
the coxal glands, which are considered to be the true excretory organs.
In most spiders they open near the third coxae. Like the stomodaeum, the
proctodaeum has a chitinous lining.
=Vascular System.=—The earlier investigations on the circulation of the
blood in Spiders were made by direct observations of the movements of
the blood corpuscles through the more or less transparent integuments of
the newly-hatched young. Claparède’s[264] results were arrived at by
this method. It is invaluable for demonstrating roughly the course taken
by the blood, but in these immature spiders the blood-system has not
attained its full complexity, and other methods of research have shown
the spider to possess a much more elaborate vascular system than was at
first suspected.
The tubular heart lies along the middle line in the anterior two-thirds
of the abdomen, sometimes close up against the dorsal wall, but
occasionally at some little distance from it, buried in the substance of
the liver. It is a muscular tube with three pairs of lateral openings or
“ostia,” each furnished with a simple valve which allows the entrance,
but prevents the exit, of the blood. It is contained in a bag, the
“pericardium,” into which the ostia open. Both heart and pericardium are
kept in place by a complicated system of connective tissue strands, by
which they are anchored to the dorsal wall of the abdomen. Eight
arteries leave the heart, the principal one, or “aorta,” plunging
downward and passing through the pedicle to supply the cephalothorax.
Besides this, there is a caudal artery at the posterior end, and three
pairs of abdominal arteries, which proceed from the under surface of the
heart, and the ramifications of which supply, in a very complete manner,
the various organs of the abdomen. The heart is not divided up into
compartments. The anterior aorta passes through the pedicle, above the
intestine, and presently forks into two main branches, which run along
either side of the sucking stomach, near the front of which they bend
suddenly downwards and end in a “patte d’oie,” as Causard[265] expresses
it—a bundle of arteries which proceed to the limbs (Fig. 185). Where the
downward curve begins, a considerable artery, the mandibulo-cephalic,
runs forward to supply the chelicerae and the head region. We have
omitted certain minor branches from the main trunks which supply the
thoracic muscles. The nerve-mass receives fine vessels from the “patte
d’oie.”
[Illustration:
FIG. 186.—Diagram of a Spider, _Epeira diademata_, showing the
arrangement of the internal organs, × about 8. 1, Mouth; 2, sucking
stomach; 3, ducts of liver; 4, so-called Malpighian tubules; 5,
stercoral pocket; 6, anus; 7, dorsal muscle of sucking stomach; 8,
caecal prolongation of stomach; 9, cerebral ganglion giving off
nerves to eyes; 10, sub-oesophageal ganglionic mass; 11, heart with
three lateral openings or ostia; 12, lung-sac; 13, ovary; 14,
acinate and pyriform silk-glands; 15, tubuliform silk-gland; 16,
ampulliform silk-gland; 17, aggregate or dendriform silk-glands; 18,
spinnerets or mammillae; 19, distal joint of chelicera; 20,
poison-gland; 21, eye; 22, pericardium; 23, vessel bringing blood
from lung-sac to pericardium; 24, artery.
]
There are no capillaries, but the blood is delivered into the tissues
and finds its way, by irregular spaces or “lacunae,” into certain main
venous channels or “sinuses.” There are three such in the cephalothorax,
one median and the others lateral, considerably dilated in front, in the
region of the eyes, and connected by transverse passages. By these the
blood is brought back through the pedicle to the lung-books. In the
abdomen also there are three main sinuses, two parallel to one another
near the lower surface, and one beneath the pericardium. These likewise
bring the blood to the lung-books, whence it is conducted finally by
pulmonary veins (Fig. 186) back to the pericardial chamber, and thus, by
the ostia, to the heart.
The Spider’s blood is colourless, and the majority of the corpuscles are
“amoeboid,” or capable of changing their shape.
=Generative System.=—The internal generative organs present no great
complexity, consisting, in the male, of a pair of testes lying beneath
the liver, and connected by convoluted tubes, the “vasa deferentia,”
with a simple aperture under the abdomen, between the anterior stigmata.
The ovaries are hollow sacs with short oviducts which presently dilate
to form chambers called “spermathecae,” which open to the exterior by
distinct ducts, thus forming a double orifice, fortified by an external
structure already alluded to as the “epigyne.” The eggs project from the
outer surface of the ovary like beads, connected with the gland by
narrow stalks, and it was not at first clear how they found their way
into the interior cavity, but it has been ascertained that, when ripe,
they pass through these stalks, the empty capsules never presenting any
external rupture.
The palpal organs have already been described. The spermatozoa, when
received by them, are not perfectly elaborated, but are contained in
little globular packets known as “spermatophores.”
=Nervous System.=—The Spider’s central nervous system is entirely
concentrated in the cephalothorax, near its floor, and presents the
appearance of a single mass, penetrated by the oesophagus. It may,
however, be divided into a pre-oesophageal portion or brain, and a
post-oesophageal or thoracic portion.
The brain supplies nerves to the eyes and chelicerae, while from the
thoracic mass nerves proceed to the other appendages, and through the
pedicle to the abdomen. The walls of the oesophagus are closely invested
on all sides by the nerve-sheath or neurilemma.
=Sense-Organs.=—Spiders possess the senses of sight, smell, and touch.
Whether or not they have a true auditory sense is still a matter of
doubt. Since sounds are conveyed by vibrations of the air, it is never
very easy to determine whether responses to sounds produced near the
animal experimented upon are proofs of the existence of an auditory
organ, or whether they are only perceived through the ordinary channels
of touch. In any case, the organs of hearing and of smell have not yet
been located in the Spider. M‘Cook considers various hairs scattered
over the body of the spider to be olfactory, but from Gaskell’s
researches upon allied Arachnid groups it would seem that the true
smelling organ is to be sought for in the rostrum.
=Eyes.=—Spiders possess from two to eight simple eyes, the external
appearance and arrangement of which have already been briefly explained.
They are sessile and immovable, though often so placed as to command a
view in several directions. In structure they are essentially like the
ocelli of Insects. Externally there is a lens, succeeded by a mass of
transparent cells, behind which is a layer of pigment. Then come the
rods and cones of the retina, to which the optic nerve is distributed. A
comparison of this with the arrangement in the Vertebrate eye will show
a reversal of the positions of the retina and the pigment-layer. The
lens is part of the outside covering of the animal, and is cast at the
time of moulting, when the spider is temporarily blind. It is stated,
however, that the eyes do not all moult simultaneously. There is often a
considerable difference between the various eyes of the same spider,
especially with regard to the convexity of the lens and the number of
rods and cones.
Though most spiders possess eight eyes, the number is sometimes smaller,
and in some groups of eight-eyed spiders two of the eyes are sometimes
so reduced and degenerate as to be practically rudimentary. As might be
expected, Cave-spiders (e.g. _Anthrobia mammouthia_) may be entirely
sightless.
=Touch.=—The sense of touch would appear to be extremely well developed
in some spiders, and there is reason for believing that the Orb-weavers,
at all events, depend far more upon it than upon that of sight.
Among the hairs which are distributed over the spider’s body and limbs,
several different forms may be distinguished, and some of them are
undoubtedly very delicate sense-organs of probably tactile function.
=Spinning Glands.=—Spiders vary greatly in their spinning powers. Some
only use their silk for spinning a cocoon to protect their eggs, while
others employ it to make snares and retreats, to bind up their prey, and
to anchor themselves to spots to which they may wish to return, and
whence they “drag at each remove a lengthening chain.”
All these functions are performed by the silk-glands of the Orb-weavers,
and hence it is with them that the organs have attained their greatest
perfection. We may conveniently take the case of the common large
Garden-spider, _Epeira diademata_. The glands occupy the entire floor of
the abdomen. They have been very thoroughly investigated by
Apstein,[266] and may be divided into five kinds.
[Illustration:
FIG. 187.—Spinning glands. =A=, Aciniform; =B=, tubuliform; =C=,
piriform gland.
]
On either side of the abdomen there are two large “ampullaceal” glands
debouching on “spigots,” one on the anterior, and one on the middle
spinneret; there are three large “aggregate” glands which all terminate
on spigots on the posterior spinneret; and three “tubuliform” glands,
two of which have their orifices on the posterior, and one on the middle
spinneret. Thus, in the entire abdomen there are sixteen large glands,
terminating in the large fusulae known as spigots. In addition to this
there are about 200 “piriform” glands whose openings are on the short
conical fusulae of the posterior and anterior spinnerets, and about 400
“aciniform” glands which debouch, by cylindrical fusulae, on the middle
and posterior spinnerets. Thus there are, in all, about 600 glands with
their separate fusulae in the case of _Epeira diademata_.
The great number of orifices from which silk may be emitted has given
rise to the widespread belief that, fine as the Spider’s line is, it is
woven of hundreds of strands. This is an entire misconception, as we
shall have occasion to show when we deal with the various spinning
operations.
A few families are, as has already been stated, characterised by the
possession of an extra spinning organ, the cribellum, and the orifices
on this sieve-like plate lead to a large number of small glands, the
“cribellum glands.”
=Respiratory Organs.=—Spiders possess two kinds of breathing organs,
very different in form, though essentially much alike. They are called
respectively “lung-books” and “tracheae.” The Theraphosae (and
_Hypochilus_) have four lung-books, while all other spiders, except
_Nops_, have two. Tracheae appear to be present almost universally, but
they have not been found in the Pholcidae.
The pulmonary stigmata lead into chambers which extend forwards, and
which are practically filled with horizontal shelves, so to speak,
attached at the front and sides, but having their posterior edges free.
These shelves are the leaves of the lung-book. Each leaf is hollow, and
its cavity is continuous, anteriorly and laterally, with the blood-sinus
into which the blood from the various parts of the Spider’s body is
poured.
The minute structure of the leaf is curious. Its under surface is
covered with smooth chitin, but from its upper surface rise vast numbers
of minute chitinous points whose summits are connected to form a kind of
trellis-work. The roof and floor of the flattened chamber within are
connected at intervals by columns. The pulmonary chamber usually
contains from fifteen to twenty of these leaves, and the two chambers
are always connected internally between the stigmata.
The tracheae are either two or four (Dysderidae, Oonopidae,
Filistatidae) in number, and their stigmata may be separate or fused in
the middle line. Each consists of a large trunk, projecting forwards,
and giving off tufts of small tubes which lose themselves among the
organs of the abdomen, but do not ramify. In the tracheae of
_Argyroneta_[267] a lateral tuft is given off immediately after leaving
the stigma, and another tuft proceeds from the anterior end.
Histologically the main trunk of the trachea is precisely like the
general chamber of the pulmonary sac, and differs greatly from the
trachea of an insect.
=Cephalothoracic Glands.=—In addition to the generative glands and the
so-called “liver” which occupy so large a portion of the abdomen, there
are, in Spiders, certain glandular organs situated in the cephalothorax
which call for some notice. These are the coxal glands and the
poison-glands.
The COXAL GLANDS are two elongated brownish-yellow bodies, situated
beneath the lateral diverticula of the stomach, and between it and the
endosternite. They present four slight protuberances which project a
short distance into the coxae of the legs. The glands appear to be
ductless, but their function is thought to be excretory. They were first
observed in the Theraphosae.
All Spiders possess a pair of POISON-GLANDS, connected by a narrow duct
with a small opening near the extremity of the fang of the chelicerae.
The glands are sac-like bodies, usually situated in the cephalothorax,
but sometimes partially (_Clubiona_) or even entirely (_Mygale_) in the
patura, or basal joints of the chelicerae. Each sac has a thin outer
layer of spirally-arranged muscular and connective tissue fibres, and a
deep inner epithelial layer of glandular cells. The cavity of the gland
acts as a reservoir for the fluid it secretes. The virulence of the
poison secreted by these glands has been the subject of much discussion,
and the most diverse opinions have been held with regard to it. The
matter is again referred to on p. 360.