ARACHNIDA (_CONTINUED_)—EMBOLOBRANCHIATA—SCORPIONIDEA—PEDIPALPI
SUB-CLASS II.—EMBOLOBRANCHIATA.[238]
=Order I. Scorpionidea.=
_Segmented Arachnids with chelate chelicerae and pedipalpi. The abdomen,
which is broadly attached to the cephalothorax or prosoma, is divided
into two regions, a six-jointed mesosoma and a six-jointed tail-like
metasoma, ending in a poison-sting. There are four pairs of lung-books,
and the second mesosomatic segment bears a pair of comb-like organs, the
pectines._
The Scorpions include the largest tracheate Arachnid forms, and show in
some respects a high grade of organisation. It is impossible, however,
to arrange the Arachnida satisfactorily in an ascending series, for
certain primitive characteristics are often most marked in those Orders
which on other grounds would seem entitled to rank at the head of the
group. Such a primitive characteristic is the very complete segmentation
exhibited by the Scorpions. They are nocturnal animals of rapacious
habit. In size they range from scarcely more than half an inch to eight
inches in length. In the northern hemisphere they are not found above
the fortieth parallel of latitude in the Old World, though in the New
World they extend as high as the forty-fifth. A corresponding southward
limit would practically include all the land in the southern hemisphere,
and here the Order is universally represented except in New Zealand,
South Patagonia, and the Antarctic islands.
Fossil scorpions are rarely found. The earliest examples known occur in
the Silurian rocks, and belong to the genus _Palaeophonus_. In the
Carboniferous _Eoscorpius_ is found, and in the Oligocene _Tityus_.
Much remains to be discovered with regard to the habits of scorpions,
and most of the isolated observations which have been recorded lose much
of their value through the uncertainty as to the species concerned. The
brief accounts given by Lankester and by Pocock,[239] and the more
recent and elaborate studies of Fabre,[240] are free from this defect
and contain almost the only trustworthy information we possess.
All are viviparous, and the females carry the newly-hatched young on
their backs. They are predaceous, feeding for the most part on insects
and spiders. These are seized by the chelate pedipalps, and, if small,
are simply picked to pieces by the chelicerae and devoured, but if large
the tail-sting is brought into play and the victim quickly paralysed.
The process of eating is a slow one, and a Cape scorpion in captivity
took two hours to devour a cockroach.
In walking, scorpions carry their pedipalps horizontally in front, using
them partly as feelers and partly as raptorial organs. As regards the
body the attitude varies considerably. In some cases (_Parabuthus_,
_Prionurus_, etc.) it is raised high upon the legs, and the “tail” or
metasoma is curved forward over the back, but in others (_Euscorpius_)
the body is held low, and the “tail” is dragged along behind, the end
only being slightly curled. In the daytime they hide away under wood or
stone, or in pits which they dig in the sand. _Parabuthus capensis_ was
observed to dig a shallow pit by means of its second and third
ambulatory legs, resting on its first and fourth legs aided by the
chelae and the metasoma. Those that hide under wood are usually found
clinging to their shelter ventral side uppermost. In captivity the
creatures, though supplied with water, were never observed to drink;
indeed, they are characteristic inhabitants of arid steppes and parched
wastes. Like most Arachnids they can endure prolonged abstinence from
food.
The only sense well developed seems to be that of touch. Notwithstanding
the possession of several eyes their sight is poor. A moving object
within the range of a few inches is certainly perceived, but it has to
be touched before its nature is recognised. Some writers have attributed
to scorpions a keen sense of hearing, and so-called “auditory hairs” are
described on the tibia of the pedipalp, but Pocock came to the
conclusion that _Parabuthus capensis_ and _Euscorpius carpathicus_ were
entirely deaf, and Lankester could obtain no indication of auditory
powers in the case of _Prionurus_. The sense of touch is extremely
delicate, and seems to reside in the hairs with which the body and
appendages are more or less thickly clothed. The pectines are special
tactile organs. That they are in some way related to sex seems probable
from the fact that they are larger in the male and sometimes curiously
modified in the female, but they appear to be of use also in determining
the nature of the ground traversed by the animal, being long in such
species as raise the body high on the legs, and short in those that
adopt a more grovelling posture. Pocock noticed that a scorpion which
had walked over a portion of a cockroach far enough for the pectines to
come in contact with it immediately backed and ate it.
[Illustration:
FIG. 167.—_Buthus occitanus_ in the mating period. (After Fabre.)
]
As is the case with most poisonous animals, their ferocity has been much
exaggerated; they never sting unless molested, and their chief anxiety
is to slink off unobserved. The fables that they kill their young, and
that when hard pressed they commit suicide by stinging themselves to
death, perhaps hardly deserve serious consideration. The latter
accusation is disproved by the fact that a scorpion’s poison has no
effect upon itself, or even upon a closely allied species. Some writers
think that in the frantic waving of the “tail,” which is generally
induced by strong excitement, a scorpion may sometimes inadvertently
wound itself with the sharp point of its telson.
Fabre gives a fascinating account of the habits of _Buthus occitanus_,
which occurs in the south of France. He found these scorpions
plentifully in arid, stony spots exposed to the sun. They were always
solitary, and if two were found under the same stone, one was engaged in
eating the other. Their sight is so poor that they do not recognise each
other without absolute contact.
Fabre established colonies in his garden and study, providing them with
suitable soil and sheltering stones. They dug holes by reducing the
earth to powder by means of the three anterior pairs of legs—never using
their pedipalpi in the operation—and sweeping away the débris with the
tail. From October to March they ate nothing, rejecting all food offered
to them, though always awake and ready to resent disturbance. In April
appetite seemed to awaken, though a very trifling amount of food seemed
to suffice. At that time, too, they began to wander, and apparently
without any intention of returning, and they continued daily to escape
from the garden enclosure until the most stringent measures were taken
to keep them in. Not till they were surrounded by glass and the
framework of their cages covered with varnished paper were their
attempts to climb out of their prison frustrated. Fabre came to the
conclusion that they took at least five years to attain their full size.
[Illustration:
FIG. 168.—The “_promenade à deux_” of _Buthus occitanus_. (After
Fabre.)
]
His most interesting observations were concerned with their mating
habits, in connection with which he noted some extraordinary phenomena.
After some very curious antics, in which the animals stood face to face
(Fig. 167) with raised tails, which they intertwined—evidently with no
hostile intention—they always indulged in what Fabre calls a “promenade
à deux,” hand in hand, so to speak, the male seizing the chelae of the
female with its own, and walking backwards, while the female followed,
usually without any reluctance. This promenade occupied an hour or more,
during which the animals turned several times. At length, if in the
neighbourhood of a suitable stone, the male would dig a hole, without
for a moment entirely quitting its hold of the female, and presently
both would disappear into the newly-formed retreat.
After mating, the male was often devoured by the female. Moreover, after
any combat with an enemy, such as a _Lycosa_ or a _Scolopendra_, it
appeared to be _de rigueur_ to eat the vanquished, and on such occasions
only was any considerable amount of food consumed.
The scorpions were not, however, anxious to fight, greatly preferring to
retire if possible; but when incited to combat, their sting was quickly
fatal to any mature insect, to spiders and to centipedes. Curiously
enough, however, insect larvae, though badly wounded, did not succumb to
the poison. Newly-hatched scorpions mounted on the mother’s back, where
they remained motionless for a week, entirely unfed. They then underwent
a moult, after which they were able to forage for themselves.
=External Structure.=
The chitinous plates of the prosoma are fused to form a carapace. Six
segments are clearly indicated by the six pairs of appendages, but,
though the development of _Scorpio_ affords little direct evidence of
the fact, there is reason to believe that there once existed a
pre-cheliceral segment,[241] as has been clearly proved in the case of
the spiders. An embryonic pregenital segment has also been recognised.
The six prosomatic appendages are those proper to the Arachnida, being
the chelicerae, pedipalpi, and four pairs of ambulatory legs. The
mesosoma, which is broadly attached to the prosoma, comprises six
segments, indicated ventrally by the genital operculum, the pectines,
and the four pairs of pulmonary stigmata. The last of the broad
abdominal segments, which tapers abruptly, belongs to the metasoma,
which also comprises six segments, and is succeeded by the post-anal
spine or sting.
=Prosoma.=—Near the middle of the carapace are two median eyes, and on
its antero-lateral borders are usually to be found groups of smaller
eyes, numbering from two to five. All the eyes are simple. There is a
difference, however, in their development, the median eyes being
_diplostichous_, or involving two layers of hypoderm, while the lateral
eyes are _monostichous_, and pass through a stage strikingly like the
permanent condition of the eyes of _Limulus_. The arrangement of various
slight longitudinal ridges on the dorsal surface of the carapace is of
systematic importance. On the ventral surface, just in front of the
genital operculum, is a sternum, never large, and sometimes barely
visible. Its shape and size constitute one of the principal family
characteristics.
[Illustration:
FIG. 169.—_Buthus occitanus_. =A=, Dorsal view; =B=, ventral view.
(After Kraepelin.)
]
=Mesosoma.=—The dorsal plates or terga are distinct, and are connected
by soft chitin with their corresponding sterna.
Beneath the second abdominal segments are borne the “pectines” or
comb-like organs. In their structure four portions are distinguishable,
an anterior lamella or shaft attaching them to the body, a middle
lamella, the teeth, and the fulcra, a series of small chitinous pieces
intercalated between the bases of the movable teeth.
Beneath the third, fourth, fifth, and sixth segments are the paired
openings of the lung-sacs.
=Metasoma.=—The first segment is usually and the remainder are
invariably enclosed in complete chitinous rings and show considerable
variations in their comparative size and shape, and in the arrangement
of the ridges and keels with which they are usually furnished. The
post-anal segment is more or less globular at its base, constituting a
“vesicle,” and terminates in a fine curved point, the “aculeus,”
perforated for the passage of the delicate poison-duct. With the abdomen
fully extended the point is directed downward, but in the attitude of
attack or defence, when the “tail” is carried horizontally over the
back, the sting points forward in the neighbourhood of the animal’s
head.
[Illustration:
FIG. 170.—=A=, Diagram of a Scorpion’s leg; 1, coxa; 2, trochanter; 3,
femur; 4, patella; 5, tibia; 6, protarsus; 7, tarsus; _p.s_, pedal
spur; _t.s_, tibial spur. =B=, Fourth tarsus of _Palamnaeus
swammerdami_; _l_, lateral lobe. (After Pocock.)
]
=Appendages.=—The three-jointed chelicerae are powerful and chelate. The
first joint is small, but the second is strongly developed and bears at
its anterior end on the inner side a projection which forms the
immovable finger of the chela. The third joint, or movable finger, is
articulated on the outer side, and both fingers are armed with teeth
whose arrangement is useful in distinguishing the species. The pedipalpi
consist of six joints. The coxa is small and has an inwardly directed
lamella which assists in feeding. The trochanter is also a small joint,
bearing, normally at right angles to the longitudinal axis, the powerful
humerus or femur. Then follows the brachium or tibia, again directed
forward, and the last two joints form the chela or “hand,” the terminal
joint or movable finger being on the outer side as in the chelicerae. In
systematic determination special attention is given to the “hand.” In
some forms the upper surface is uniformly rounded, while in others a
“finger-keel” divides it into two flattish surfaces almost at right
angles. The biting edges of the fingers are usually furnished with rows
of minute teeth arranged characteristically in the different genera. The
ambulatory legs are seven-jointed, though, unfortunately, authors are
not agreed upon the nomenclature of the joints. Kraepelin[242] names
them coxa, trochanter, femur, tibia, and three-jointed tarsus, and
Simon[243] agrees with him. Pocock’s names[244] are coxa, trochanter,
femur, patella, tibia, protarsus, and tarsus, and it is certainly
convenient that each joint should have a separate name, but it must be
borne in mind that the tibia of different authors is not always the same
joint. Special attention must be directed to the three terminal joints,
which furnish highly important characteristics. The tibia (in Pocock’s
sense) is sometimes provided with a “tibial spur” at its lower distal
extremity. From the soft arthrodial membrane between the protarsus and
tarsus may proceed one or more dark-tipped claw-like spurs, the “pedal
spurs.” The terminal joint (tarsus of Pocock) is variously furnished
with hairs and teeth, and always ends in a pair of well-developed
movable claws beneath which a much reduced and sometimes almost obsolete
third claw is distinguishable. The tarsus generally projects in a
“claw-lobe” over the base of the superior claws, and sometimes lateral
lobes are present. The first and second coxae have triangular maxillary
lobes directed towards the mouth. The third and fourth coxae are fused
together on each side, and those on one side are separated from those on
the other by the sternum. In other respects the four pairs of legs are
usually similar.
=Internal Anatomy.=
The =alimentary canal= is a fairly uniform tube, nowhere greatly
dilated. The very small mouth leads into a small suctorial chamber, and
this is connected by a narrow oesophagus, which pierces the cerebral
nerve-mass, with a slightly dilated portion which receives the ducts of
the first pair of gastric glands, often called salivary glands. The
succeeding portion in the prosoma receives four or five more pairs of
ducts from the well-developed gastric glands. In the rapidly narrowing
first metasomatic segment the intestine receives one or two pairs of
Malpighian tubes, and thence proceeds to the anus, situated ventrally in
the last segment.
The =vascular system= is of the usual Arachnid type, the heart being a
seven-chambered dorsal longitudinal vessel lying in a pericardium, with
which it communicates by seven pairs of valvular ostia. Lankester[245]
has demonstrated several pairs of superficial lateral veins connecting
two deep-seated ventral venous trunks with the pericardium. The
lung-books are, so to speak, pushed in to dilatations of these trunks,
so that some of the lateral veins carry blood newly aerated by the
lung-books directly to the pericardium.
The =nervous system= is not greatly concentrated except in the prosoma,
where there is a single ganglionic mass which innervates not only the
whole prosoma but the mesosoma as far as the first and sometimes the
second pair of lung-books. There are two mesosomatic ganglia, variously
situated in different genera, and each metasomatic segment has its
ganglion.
The =generative organs= are more or less embedded in the gastric glands.
There are two testes, each composed of a pair of intercommunicating
tubules, and connected by a common vas deferens with the generative
aperture, which is furnished with a double protrusible intromittent
organ. A pair of vesiculae seminales and a pair of accessory glands are
also present. The female possesses a single ovary, consisting of a
median and two lateral tubules, all connected by cross branches.
In addition to the external sclerites a free =internal skeletal plate=,
situated in the prosoma between the alimentary canal and the nerve-cord,
furnishes convenient fulcra for muscular attachment. It is known as the
“endosternite.”
Brauer[246] has made the most complete study of the development of
_Scorpio_, and two of the most interesting of his conclusions may be
mentioned here. He has shown the lung-books to be derived from gills
borne on mesosomatic appendages. Moreover he found in the embryo five
pairs of segmental ducts—in segments 3–6 and 8—and demonstrated that
those of segment 5 persisted, though without external aperture, as coxal
glands, and those of segment 8 as the genital ducts.
=Classification.=
More than 350 species of scorpions have been described, but many of
these are “doubtful,” and probably the number of known forms may be put
at about 300. These are divided by Kraepelin[247] into six families and
fifty-six genera. The best indications of the family of a scorpion are
to be found in the shape of the sternum, the armature of the tarsi, and
the number of the lateral eyes, while assistance is also to be derived
from the shape of the stigmata and of the pectines, and from the absence
or presence of a spine beneath the aculeus.
The six families are: Buthidae, Scorpionidae, Chaerilidae, Chactidae,
Vejovidae, and Bothriuridae.
=Fam. 1. Buthidae.=—_Sternum small and generally triangular. Tibial
spurs in the third and fourth legs. Generally a spur beneath the
aculeus. Lateral eyes three to five in number._
There are two sub-families: BUTHINAE and CENTRURINAE.
The BUTHINAE, which possess a tibial spur, comprise fourteen genera,
most of them Old World forms. The principal genera are _Buthus_, which
contains about 25 species, and _Archisometrus_ with 20 species. One
genus only, _Ananteris_, is South American, and it includes only a
single species. The genus _Uroplectes_, with 16 species, is almost
entirely African.
The CENTRURINAE, without tibial spur, are New World scorpions, though
_Isometrus europaeus_ (_maculatus_) is cosmopolitan. The principal
genera are _Tityus_ with 30 species, _Centrurus_ with 13, and
_Isometrus_ with 6.
=Fam. 2. Scorpionidae.=—_Sternum broad and pentagonal, with sides
approximately parallel. No tibial spur, but a single pedal spur.
Generally three lateral eyes._
Nearly a hundred species of Scorpionidae have been described,
distributed among fifteen genera. The following sub-families are
recognised: Diplocentrinae, Urodacinae, Scorpioninae, Hemiscorpioninae,
and Ischnurinae.
The DIPLOCENTRINAE have a spur under the aculeus. They form a small
group of only eight species. The principal genus, _Diplocentrus_, is
entirely Neotropical, but _Nebo_ has a single Old World representative
in Syria.
The URODACINAE, with the single genus _Urodacus_, are Australian
scorpions. As in the next sub-family, there are rounded lobes on the
tarsi, but there is only a single keel on the “tail,” and the lateral
eyes are two in number. Six good and three doubtful species are
recognised.
The SCORPIONINAE are Asiatic and African forms, and are recognised by
the tarsi having a large lobe on each side, by the convex upper surface
of the “hand,” by the presence of two median keels on the “tail,” and by
the possession of three lateral eyes. _Palamnaeus_ (_Heterometrus_) has
sixteen species in the Indian region. There are about thirty species of
_Opisthophthalmus_, all natives of South Africa. _Pandinus_ includes
about ten species, but there are only two species of the type genus
_Scorpio_, _S. maurus_ and _S. boehmei_.
The sub-family HEMISCORPIONINAE was formed for the reception of the
single Arabian species _Hemiscorpion lepturus_. Its most striking
characteristic is the cylindrical vesicle of the tail in the male.
The ISCHNURINAE differ from the Scorpioninae chiefly in the absence of
the tarsal lobes, the presence of a well-marked finger-keel, and the
generally more depressed form of the body and hand. In the opinion of
some authors they should be separated from the Scorpionidae as a
distinct family, the Ischnuridae. There are more than twenty species,
divided among six genera. The type genus _Ischnurus_ has only the single
species _I. ochropus_. There are eight species of _Opisthacanthus_,
which has representatives in Africa and America.
=Fam. 3. Chaerilidae.=—_Sternum pentagonal with median depression or
“sulcus” rounded posteriorly. Two pedal spurs. Stigmata circular. Two
lateral eyes with a yellow spot behind the second. Pectines very short._
This small family has the single genus _Chaerilus_ with but seven
species, natives of the Oriental region.
=Fam. 4. Chactidae.=—_Two pedal spurs. Two lateral eyes (or, rarely, no
eyes) but without yellow spot. Characteristic dentition on movable
finger of “hand.”_
There are three sub-families, Megacorminae, Euscorpiinae, and Chactinae.
The MEGACORMINAE include but a single Mexican form, _Megacormus
granosus_. There is a single toothed keel under the “tail,” and all the
under surface is spiny. There is a row of long bristles under the
tarsus.
In the EUSCORPIINAE the upper surface of the hand is divided into two
surfaces almost at right angles by a strong finger-keel. This is a small
group of about six species found in the Mediterranean region. The two
genera are _Euscorpius_ and _Belisarius_.
The CHACTINAE are without any marked keel on the hand. The scorpions of
this sub-family are found in equatorial South America and the West
Indies, where there are more than twenty species divided about equally
between the four genera _Chactas_, _Broteas_, _Broteochactas_, and
_Teuthraustes_.
=Fam. 5. Vejovidae.=—_No tibial, but two pedal spurs. A single row of
hairs or papillae under the tarsus. Sternum generally broader than long.
Elongate stigmata, and three lateral eyes._
Seven of the eight genera of this family include only American forms,
the principal genus being _Vejovis_, with about ten species. The genus
_Scorpiops_, however, belongs to the Indian region and numbers more than
ten species.[248]
=Fam. 6. Bothriuridae.=—_Sternum much reduced and sometimes hardly
visible, consisting of two slight, nearly transverse bars._
Of the seven genera of this family one, _Cercophonius_, is Australian.
The other six genera include some dozen South American forms,
_Bothriurus_ having four species.
=Order II. Pedipalpi.=
_Arachnids with non-chelate, two-jointed chelicerae, powerful pedipalpi,
and four pairs of legs, of which only the last three are ambulatory, the
first being used as tactile organs. The cephalothorax is usually covered
by an undivided carapace, but the pedunculated abdomen is segmented.
Respiration is by lung-books._
The Pedipalpi are a little-known group of animals of nocturnal habits.
Though rarely seen they are widely distributed, being found in India,
Arabia, the greater part of Africa, and Central and South America. They
are of ancient origin, a fossil genus, _Graeophonus_, of the
Tarantulidae (Phrynidae, see p. 312), occurring in the Carboniferous
strata in North America. They live under stones and bark, and in caves,
where, when disturbed, they seek safety in crannies in the rock.
Little is known of their habits, but they are believed to feed chiefly
upon insects. The female _Tarantula_ carries the developing eggs,
somewhat after the manner of the Chernetidea (see p. 434), in a bag
beneath the abdomen, the under surface of which becomes concave and
dome-like during the period of gestation.[249]
[Illustration:
FIG. 171.—_Thelyphonus_, diagrammatic ventral view; about natural
size. _c_, Coxal joint of pedipalp; _g_, generative opening; _p_,
pedipalp; _sp_, spiracles; _st_, sternal plates; 1, 2, 3, 4,
ambulatory legs. (After Pickard-Cambridge.)
]
=External Structure.=—The external features which the members of this
Order have in common are the segmented pediculate abdomen (9 to 12
segments), the two-jointed non-chelate chelicerae, the antenniform first
pair of legs, and the presence of two pairs of lung-book stigmata
beneath the abdomen. The constituent families differ so much in outward
form that they must be dealt with separately.
The Thelyphonidae or “Whip Scorpions” (see p. 312) have a long-oval
carapace bearing well-developed eyes, two in front, and a group of three
or five on either side some distance behind. The pedipalpi are chelate,
and have their basal joints fused beneath the mouth, being thus
incapable of any masticating motion.
The first legs are six-jointed, and have multi-articulate tarsi; the
others are seven-jointed, and their tarsi, in some species at least, are
tri-articulate. The abdomen consists of two portions, a wide
nine-jointed pre-abdomen and a short narrow three-jointed post-abdomen,
to which a filiform tail is articulated. Beneath the cephalothorax,
between the coxae of the legs, is a distinct sternal plate in two
portions (Fig. 171). The first abdominal ventral plate is largely
developed, and covers two segments. Behind it are the median genital
opening and two pulmonary stigmata, while the other stigmata are behind
the second ventral plate, which corresponds to the third abdominal
segment. On the last abdominal segment there are often two or four
light-coloured spots called “ommatoids,” and considered by some authors
to be organs of sight. Laurie, however (_vide infra_), thinks it more
probable that they are olfactory in function.
The Schizonotidae (see p. 312) have a two-jointed carapace, and do not
possess more than two eyes. There is a short unjointed tail-piece.
In the Tarantulidae (Phrynidae) the whole body is much flattened and
extended laterally, the undivided carapace being reniform, and broader
than long. The long non-chelate pedipalps have their basal joints free
and movable, and there are several sternal plates. There are nine
abdominal tergal plates, the last three diminishing rapidly in size, and
the last plate covering a button-like terminal portion of the abdomen.
The first abdominal ventral plate is largely developed, as in the
Thelyphonidae, and the genital orifice and pulmonary stigmata are in the
same situation as in that group. The Tarantulidae have glutinous glands
in the first abdominal segment which are capable of spinning a few
irregular threads.
In the whole group paired circular depressions are conspicuous dorsally
on all the abdominal segments. These indicate the points of attachment
of the dorso-ventral muscles.
=Internal Structure.=—The anatomy of the Pedipalpi has been very
inadequately studied. Disconnected notes on various points of structure
have been published by various morphologists, but no complete
investigation has yet been made of the internal organs. This is largely
due to the difficulty of obtaining material, and the bad state of
preservation of the internal parts of such specimens as have been
available for dissection.
The following points have been made out in the anatomy of
_Thelyphonus_.[250]
The alimentary canal commences after the mouth with a pharynx which,
though not dilated, is furnished with sucking muscles. It then narrows
into an oesophagus which passes through the nerve-mass, and afterwards
dilates to form the mid-gut, which immediately gives off two large
lateral diverticula which extend backwards, each having five lobes.
There are also two median diverticula which proceed from the ventral
surface and pass through the endosternite. The abdominal portion of the
canal is entirely concealed by the great “liver” mass which communicates
with it by four paired ducts in the anterior part of the abdomen. Behind
the fourth abdominal segment the gut is narrow till it expands in the
seventh segment into an hour-glass-shaped stercoral pocket which,
according to Laurie, is a portion of the mesenteron.
The excretory organs are the Malpighian tubes and the coxal glands. The
former are generally described as entering the anterior portion of the
stercoral pocket, but according to Laurie they pass along its ventral
surface, attached to it by connective tissue, and really enter at the
posterior end. The coxal glands are well developed, and lie beneath the
endosternite, opening near the first coxae.
The nervous system is much concentrated and of the usual Arachnid type.
The median abdominal nerve has a ganglion towards its extremity,
supplying, according to Bernard,[251] the muscles which move the tail.
The heart is extremely long, and varies little in width. It has nine
pairs of ostia[252]—two in the thorax and seven in the abdomen. The
generative glands are paired, and in the male there are large seminal
vesicles. In the most ventral portion of the abdominal cavity lies a
remarkable asymmetrically-situated gland, the “stink-gland.” It consists
of a number of secretory tubules communicating with two elongated sacs,
one of which lies beneath the nerve-cord, and therefore medially, while
the other lies far to the left. Their ducts proceed to the anus or its
vicinity.
The caudal organs, or white spots which, as already mentioned, are
usually found on the last of the three post-abdominal segments of
_Thelyphonus_, are of doubtful function. They have been variously
explained as the stink-gland orifices, and as organs sensitive to light
(“ommatoids”). Laurie[253] was unable to find any pore in this region,
nor was there any of the pigment so characteristic of organs of sight.
The histological structure indicated a sense-organ rather than a gland,
but the use of these organs is entirely conjectural.
=Classification.=—The order Pedipalpi is divided into three families—
Thelyphonidae, Schizonotidae and Tarantulidae. The first two are
considered by some authors to form a sub-order, UROPYGI, or tailed
Pedipalpi, while the Tarantulidae constitute the remaining sub-order
AMBLYPYGI, the members of which are tailless.
=Fam. 1. Thelyphonidae.=[254]—This family comprises nine or more genera,
differing chiefly in the position of the eyes, the structure of the
genital operculum, the armature of the pedipalps, and the presence or
absence of “ommatoids” in the anal segment.
The three following genera are among those most likely to be met with.
Two ommatoids are present in each.
_Thelyphonus_ has a spine on the second ventral plate, and a deep median
impression on the male genital operculum, which is, however, absent from
that of the female. There are about fifteen known species of this genus,
inhabiting Southern Asia and the East Indies.
_Typopeltis_ has ridges running forward from the lateral eyes. The
middle third of the female operculum is raised and deeply impressed in
the middle. This genus is represented in China and Japan.
_Mastigoproctus_ has a short and stout coxal apophysis of the pedipalp,
without a tooth on its inner side. It is found in Mexico, Brazil, and
the West Indies. Other genera are _Thelyphonellus_ (Demerara),
_Labochirus_ (Ceylon), _Hypoctonus_ (Burma), _Mimoscorpius_
(Philippines), _Uroproctus_ (Assam), _Abalius_ (New Guinea), without
ommatoids, and _Tetrabalius_ (Borneo), with two pairs of ommatoids.
=Fam. 2. Schizonotidae (= Tartaridae).=—This family contains only two
genera, _Schizonotus_ (= _Nyctalops_, Pickard-Cambridge, nom. preocc.
Aves) and Trithyreus[255] (= _Tripeltis_, Thorell, nom. preocc.
Reptilia). They are very small, pale-coloured forms (about 5 mm. in
length), found in Burma and Ceylon.
=Fam. 3. Tarantulidae=, better known as =Phrynidae=. Pocock has shown
that Fabricius established the genus _Tarantula_ from the species _T.
reniformis_ in 1793, while there is no earlier record of Olivier’s
_Phrynus_, established for the same species, than Lamarck’s citation of
it in 1801. The family is divided into three sub-families, Tarantulinae,
Phrynichinae, and Charontinae.
(i.) The TARANTULINAE are new-world forms, represented by three genera,
_Tarantula_, _Acanthophrynus_ (_Phrynopsis_), and _Admetus_
(_Heterophrynus_), in Central and South America and the West Indies.
(ii.) The PHRYNICHINAE belong to the Old World, being found in Africa,
India, and Ceylon. _Phrynichus_, _Titanodamon_ and _Nanodamon_ are
genera of this sub-family.
(iii.) The CHARONTINAE are natives of South-East Asia and the Pacific
Islands. There are five genera and eight species.