ARACHNIDA EMBOLOBRANCHIATA (_CONTINUED_)—PALPIGRADI—SOLIFUGAE =
SOLPUGAE—CHERNETIDEA = PSEUDOSCORPIONES
=Order IV. Palpigradi.=
_Minute Arachnids with three-jointed chelate chelicerae, and with the
last two joints of the cephalothorax free. The abdomen consists of
eleven segments with a fifteen-jointed flagellum._
In 1885 Grassi discovered, at Catania, a minute Arachnid which did not
fall into any of the established orders of Arachnida. He named it
_Koenenia mirabilis_. In 1893 Hansen collected several specimens in
Calabria, near Palmi and Scilla, and carefully redescribed the species
in conjunction with Sörensen.[322] It has been studied still more
minutely by Börner.[323]
There is a “head” portion, covered by a carapace, and bearing the
chelicerae, pedipalpi, and two pairs of legs. The two free thoracic
segments bear the third and fourth pairs of legs, recalling the
Schizonotidae (see p. 312), where the portion of the thorax bearing
these legs is separate, though covered by a single dorsal plate. There
are no eyes, but two hair-structures, believed to be sensory, are
present on the cephalothorax, and Börner has observed openings in the
second joint of the first pair of legs which have all the appearance of
“lyriform” organs, as found in Spiders (see p. 325).
The last three abdominal segments narrow rapidly, the last bearing the
anus. A fifteen-jointed caudal flagellum is carried, Scorpion-like,
above the animal’s back. The body and tail are each about a millimetre
in length, and the animal is of a translucent white colour.
The mouth is extremely simple, being merely a slit upon a slight
eminence. There are two sternal plates beneath the “head,” and one
beneath each free thoracic segment. The genital operculum is
complicated, and is situated beneath the second abdominal segment.
[Illustration:
FIG. 216.—_Koenenia mirabilis_, much enlarged. (After Hansen.)
]
Since 1885 several other species have been discovered in various parts
of the world. Two American forms possess three pairs of lung-sacs on
segments 4, 5, and 6 of the abdomen. Rucker[324] has suggested for them
the generic name of _Prokoenenia_, including _P. wheeleri_, Rucker, from
Texas, and _P. chilensis_, Hansen, from Chili. The others, styled by
that author _Eukoenenia_, have no lung-sacs. There are about ten
species, mostly from the Mediterranean region, but _E. augusta_, Hansen,
is found in Siam, _E. florenciae_, Rucker, in Texas, and _E. grassii_,
Hansen, in Paraguay.
=Order V. Solifugae (Solpugae).=
_Tracheate Arachnids, with the last three segments of the cephalothorax
free and the abdomen segmented. The chelicerae are largely developed and
chelate, and the pedipalpi are leg-like, possessing terminal
sense-organs._
The Solifugae are, in some respects, the most primitive of the tracheate
Arachnida. Their general appearance is very spider-like, and by the old
writers they are uniformly alluded to as spiders. The segmented body and
the absence of spinning organs, however, make them readily
distinguishable on careful inspection. They are for the most part
nocturnal creatures, though some seem to rove about by day, and are even
called “Sun-spiders” by the Spaniards. The night-loving species are
attracted by light. They are, as a rule, exceedingly hairy. Some are
extremely active, while the short-legged forms (e.g. _Rhagodes_, see p.
429) move slowly. They are capable of producing a hissing sound by the
rubbing together of their chelicerae. Only the last three pairs of legs
are true ambulatory organs, the first being carried aloft like the
pedipalps, and used for feeling and manipulating the prey.
There has been much controversy as to the poisonous properties with
which these creatures have been very widely credited by both ancient and
modern writers. The people of Baku on the Caspian consider them
especially poisonous after their winter sleep. The Russians of that
region much dread the “Falangas,” as they call them, and keep a Falanga
preserved in oil as an antidote to the bite. The Somalis, on the other
hand, have no fear of them, and, though familiar with these animals,
have not thought them worthy of the dignity of a name.
Several investigators have allowed themselves to be bitten without any
special result. Some zoologists have found and described what they have
taken to be poison-glands, but these appear to be the coxal glands,
which have an excretory function. Bernard[325] suggests that, if the
bite be poisonous, the virus may exude from the numerous setal pores
which are found on the extremities of the chelicerae. The cutting powers
of the immensely-developed chelicerae are usually sufficient to ensure
fatal results on small animals without the agency of poison.
Distant,[326] indeed, thinks they cannot be poisonous, for when birds
attack them they flee before their assailants.
The Solifugae require a tolerably warm climate. In Europe they are only
found in Spain, Greece, and Southern Russia. They abound throughout
Africa, and are found in South-Western Asia, the southern United States,
and the north of South America. They appear to be absent from Australia,
nor have any been found in Madagascar. Their usual food appears to be
insects, though they devour lizards with avidity. Some interesting
observations on their habits are recorded by Captain Hutton,[327] who
kept specimens in captivity in India. An imprisoned female made a burrow
in the earth with which her cage was provided, and laid fifty eggs,
which hatched in a fortnight, but the young remained motionless for
three weeks longer, when they underwent their first moult, and became
active.
A sparrow and musk rats were at different times placed in the cage, and
were speedily killed, but not eaten. Two specimens placed in the same
cage tried to avoid each other, but, on coming into contact, fought
desperately, the one ultimately devouring the other. It was noteworthy
that the one which was first fairly seized immediately resigned itself
to its fate without a struggle. As is the case with some spiders, the
female is said occasionally to kill and devour the male. A Mashonaland
species, _Solpuga sericea_, feeds on termites,[328] while a South
Californian _Galeodes_ kills bees,[329] entering the hives in search of
them. They are fairly good climbers. In Egypt _Galeodes arabs_ climbs on
to tables to catch flies, and some species have been observed to climb
trees.
[Illustration:
FIG. 217.—_Rhagodes_ sp., ventral view. Nat. size. _a_, Anus; _ch_,
chelicerae; _g.o_, genital operculum; _n_, racket organs; _p_,
pedipalp; 1, 2, 3, 4, ambulatory legs. (After Bernard.)
]
That their pedipalps, in addition to their sensory function (see p.
426), possess a sucking apparatus, is clear from an observation of
Lönnberg,[330] who kept specimens of _Galeodes araneoides_ imprisoned in
rectangular glass boxes, up the perpendicular sides of which they were
able to climb for some distance by their palps, but, being able to
obtain no hold by their legs, they soon tired.
=External Anatomy.=—The body of _Galeodes_ consists of a cephalothorax
and an abdomen, both portions being distinctly segmented. The
cephalothorax consists of six segments, the first thoracic segment being
fused with the two cephalic segments to form a sort of head, while the
last three thoracic segments are free, and there is almost as much
freedom of movement between the last two thoracic segments as between
the thorax and the abdomen. The “cephalic lobes,” which give the
appearance of a head, have been shown by Bernard[331] to be due to the
enormous development of the chelicerae, by the muscles of which they are
entirely occupied.
The floor of the cephalothorax is for the most part formed by the coxae
of the appendages, and the sternum is hardly recognisable in many
species. In _Solpuga_, however (see p. 429), it exists in the form of a
long narrow plate of three segments, ending anteriorly in a
lancet-shaped labium.
A pair of large simple eyes are borne on a prominence in the middle of
the anterior portion of the cephalothorax, and there are often one or
two pairs of vestigial lateral eyes.
The first pair of tracheal stigmata are to be found behind the coxae of
the second legs.
The mouth-parts take the form of a characteristic beak, consisting of a
labrum and a labium entirely fused along their sides. The mouth is at
the extremity of the beak, and is furnished with a straining apparatus
of complicated hairs.
The abdomen possesses ten free segments, marked off by dorsal and
ventral plates, with a wide membranous lateral interval. The ventral
plates are paired, the first pair forming the genital opercula, while
behind the second and third are two pairs of stigmata. Some species have
a single median stigma on the fourth segment, but this is in some cases
permanently closed, and in the genus _Rhagodes_ entirely absent, so that
it would seem to be a disappearing structure.
The appendages are the six pairs common to all Arachnids—chelicerae,
pedipalpi, and four pairs of legs. The chelicerae, which are enormously
developed, are two-jointed and chelate, the distal joint being
articulated beneath the produced basal joint. In the male there is
nearly always present, on the basal joint, a remarkable structure of
modified hairs called the “flagellum,” and believed to be sensory. It
differs in the different genera, and is only absent in the Eremobatinae
(see p. 429). The pedipalpi are strong, six-jointed, leg-like
appendages, without terminal claw. They end in a knob-like joint,
sometimes movable, sometimes fixed, which contains a very remarkable
eversible sense-organ, which is probably olfactory. It is concealed by a
lid-like structure, and when protruded is seen to be furnished, on its
under surface, with a pile of velvet-like sensory hairs.
The legs differ in the number of their joints, as the third and fourth
pairs have the femora divided, and the tarsus jointed. The first pair
has only a very small terminal claw, but two well-developed claws are
borne by the tarsi of the other legs. Each of the last legs bears, on
its under surface, five “racket organs,” believed to be sensory.
=Internal Structure.=—The alimentary canal possesses a sucking chamber
_within_ the beak, after which it narrows to pass through the
nerve-mass, and after an S-shaped fold, joins the mid-gut. This gives
off four pairs of thin diverticula towards the legs, the last two
entering the coxae of the third and fourth pairs.
At the constriction between the cephalothorax and the abdomen there is
no true pedicle, but there is a transverse septum or “diaphragm,”
through which the blood-vessel, tracheal nerves, and alimentary canal
pass. The gut narrows here, and, on entering the abdomen, proceeds
straight to a stercoral pocket at the hind end of the animal, but gives
off, at the commencement, two long lateral diverticula, which run
backwards parallel with the main trunk. These are furnished with
innumerable secondary tube-like diverticula, which proceed in all
directions and fill every available portion of the abdomen. The caeca,
which are so characteristic of the Arachnidan mid-gut, here reach their
extreme development. A pair of Malpighian tubules enter the main trunk
in the fourth abdominal segment.
Other excretory organs are the coxal glands, which form many coils
behind the nerve-mass, and open between the coxae of the third and
fourth legs. They have been taken for poison-glands.
There is a small endosternite in the hinder portion of the cephalothorax
under the alimentary canal.
The vascular system is not completely understood. The heart is a very
long, narrow, dorsal tube, extending almost the entire length of the
animal, and possessing eight pairs of ostia, two in the cephalothorax
and six in the abdomen. It gives off an anterior and a posterior vessel,
the latter apparently a vein, as it is guarded at its entrance by a
valve. The blood seems to be delivered by the anterior artery on to the
nerve-mass, and, after percolating the muscles and viscera, to divide
into two streams—the one returning to the heart by the thoracic ostia,
the other passing through the diaphragm and bathing the abdominal
organs, finally to reach the heart either by the abdominal ostia or by
the posterior vein.
[Illustration:
FIG. 218.—Nervous system of _Galeodes_. _abd.g_, Abdominal ganglion;
_ch_, cheliceral nerve; _ch.f_, chitinous fold; _ch.r_, chitinous
rod; _g.n_, generative nerve; _l_, labial nerve; _st_, position of
stigma. (After Bernard.)
]
The nervous system, notwithstanding the fact that the three last
thoracic segments are free, is chiefly concentrated into a mass
surrounding the oesophagus. Nerves are given off in front to the eyes,
the labrum, and the chelicerae, while double nerves radiate to the
pedipalps and to the legs. From behind the nerve-mass three nerves
emerge, and pass through the diaphragm to enter the abdomen. The median
nerve swells into an “abdominal ganglion” just behind the diaphragm, and
is then distributed to the diverticula of the alimentary canal. The
lateral nerves innervate the generative organs.
The respiratory system consists of a connected network of tracheae
communicating with the exterior by the stigmata, whose position has
already been described. There are two main lateral trunks extending
nearly the whole length of the body, and giving off numerous
ramifications, the most important of which are in the cephalothorax, and
supply the muscles of the chelicerae and of the other appendages.
The generative glands do not essentially differ from the usual Arachnid
type, though the paired ovaries do not fuse to form a ring. There are no
external organs, and the sexes can only be distinguished by secondary
characteristics, such as the “flagellum” already mentioned.
=Classification.=—There are about a hundred and seventy species of
Solifugae inhabiting the warm regions of the earth. No member of the
group is found in England, or in any except the most southern portions
of Europe.
Kraepelin[332] has divided the group into three families—Galeodidae,
Solpugidae, and Hexisopodidae.
=Fam. 1. Galeodidae.=—The Galeodidae have a lancet-shaped flagellum,
directed backwards. There is a characteristic five-toothed plate or comb
covering the abdominal stigmata. The tarsus of the fourth leg is
three-jointed, and the terminal claws are hairy.
[Illustration:
FIG. 219.—Chelicera and flagellum of _Galeodes_. (After Kraepelin.)
]
There are two genera, _Galeodes_, with about twelve species, and
_Paragaleodes_, with six species, scattered over the hot regions of the
Old World.
=Fam. 2. Solpugidae.=—The Solpugidae comprise twenty-four genera,
distributed under five sub-families. The toothed stigmatic plate is
absent, and the tarsal claws are smooth. The ocular eminence is
furnished with irregular hairs. The “flagellum” is very variable.
[Illustration:
FIG. 220.—Chelicerae and flagella of =A=, _Rhagodes_; =B=, _Solpuga_;
and =C=, _Daesia_. (After Kraepelin.)
]
(i.) The RHAGODINAE include the two genera, _Rhagodes_ (_Rhax_) and
_Dinorhax_. The first has twenty-two species, which inhabit Africa and
Asia. The single species of _Dinorhax_ belongs to East Asia. These
creatures are short-legged and sluggish.
(ii.) The SOLPUGINAE contain two genera—_Solpuga_ with about fifty
species, and _Zeriana_ with three. They are all inhabitants of Africa,
and some occur on the African shore of the Mediterranean.
(iii.) The DAESIINAE number about forty species, divided among several
genera, among which the principal are _Daesia_, _Gluvia_, and
_Gnosippus_. They are found in tropical regions of both the Old and the
New World.
(iv.) The EREMOBATINAE are North American forms, the single genus
_Eremobates_ numbering about twenty species. The flagellum is here
entirely absent.
[Illustration:
FIG. 221.—Chelicera and flagellum of _Hexisopus_. (After Kraepelin.)
]
(v.) The KARSHIINAE include the five genera _Ceroma_, _Gylippus_,
_Barrus_, _Eusimonia_, and _Karshia_. They are universally distributed.
=Fam. 3. Hexisopodidae.=—This family is formed for the reception of a
single aberrant African genus, _Hexisopus_, of which five species have
been described.
There are no claws on the tarsus of the fourth leg, which is beset with
short spine-like hairs, and in other respects the genus is peculiar.
=Order VI. Chernetidea.=
(CHERNETES, PSEUDOSCORPIONES.)
_Tracheate Arachnids, with the abdomen united to the cephalothorax by
its whole breadth. Eyeless, or with two or four simple eyes placed
laterally. Abdomen segmented, with four stigmata. Chelicerae chelate,
bearing the openings of the spinning organs. Pedipalpi large,
six-jointed, and chelate. Sternum absent or rudimentary._
The Chernetidea or “False-scorpions” constitute the most compact and
natural order of the Arachnida. There are no extreme variations within
the group as at present known, while all its members differ so markedly
from those of other Arachnidan orders that their true affinities are by
no means easy to determine.
The superficial resemblance to Scorpions which has won these animals
their popular name is almost entirely due to the comparative size and
shape of their pedipalpi, but it is probable that they are structurally
much more closely allied to the Solifugae.
Chernetidea are not creatures which obtrude themselves on the general
notice, and it is highly probable that many readers have never seen a
living specimen. This is largely due to their minute size. _Garypus
littoralis_, a Corsican species, nearly a quarter of an inch in length
of body, is a veritable giant of the tribe, while no British species
boasts a length of more than one-sixth of an inch.
Moreover, their habits are retiring. They are to be sought for under
stones, under the bark of trees, and among moss and débris. One species,
probably cosmopolitan, certainly lives habitually in houses, and is
occasionally noticed and recognised as the “book-scorpion,” and one or
two other species sometimes make themselves conspicuous by the
remarkable habit of seizing hold of the legs of flies and being carried
about with them in their flight. With these exceptions, the Chernetidea
are not likely to be seen unless specially sought for, or unless
casually met with in the search for small beetles or other creatures of
similar habitat. Nevertheless they are very widely distributed, and
though more numerous in hot countries, are yet to be found in quite cold
regions.
Though comparatively little attention has been paid to them in this
country, twenty British species have been recorded, and the known
European species number about seventy.
As might be expected from their small size and retiring habits, little
is known of their mode of life. They are carnivorous, feeding apparently
upon any young insects which are too feeble to withstand their attacks.
The writer has on two or three occasions observed them preying upon
Homopterous larvae. As a rule they are sober-coloured, their livery
consisting of various shades of yellow and brown. Some species walk
slowly, with their relatively enormous pedipalps extended in front and
gently waving, but all can run swiftly backwards and sideways, and in
some forms the motion is almost exclusively retrograde and very rapid. A
certain power of leaping is said to be practised by some of the more
active species. The Chernetidea possess spinning organs, opening on the
movable digit of the chelicera. They do not, however, spin snares like
the Spiders, nor do they anchor themselves by lines, the sole use of the
spinning apparatus being, apparently, to form a silken retreat at the
time of egg-laying or of hibernation.
=External Structure.=—The Chernetid body consists of a cephalothorax,
and an abdomen composed of twelve segments. The segmentation of the
abdomen is emphasised by the presence of chitinous plates dorsally and
ventrally, but the last two dorsal plates and the last four ventral
plates are fused, so that ordinarily only eleven segments can be counted
above and nine below.
The cephalothorax presents no trace of segmentation in the Obisiinae
(see p. 437), but in the other groups it is marked dorsally with one or
two transverse striae. The eyes, when present, are either two or four in
number, and are placed near the lateral borders of the carapace towards
its anterior end. They are whitish and only very slightly convex, and
are never situated on prominences. Except in _Garypus_ there is no trace
of a sternum, the coxae of the legs and pedipalps forming the ventral
floor of the cephalothorax.
In the Obisiinae a little triangular projection in front of the
cephalothorax is regarded by Simon[333] as an _epistome_. It is absent
in the other sub-orders.
The abdomen is armed, dorsally and ventrally, with a series of chitinous
plates with membranous intervals. The dorsal plates are eleven in number
(except in _Chiridium_, which has only ten), and are frequently bisected
by a median dorsal membranous line. There are nine ventral plates. There
is a membranous interval down each side between the dorsal and ventral
series of plates.
[Illustration:
FIG. 222.—=A=, _Chernes_ sp., diagrammatic ventral view, × about 12.
_a_, Anus; _ch_, chelicera; _g_, generative opening; _p_, pedipalp;
1, 2, 3, 4, legs. (The stigmata are at the postero-lateral margins
of the 1st and 2nd abdominal segments.) =B=, Tarsus, with claws and
sucking disc.
]
The chitinous membrane between the plates is very extensible, rendering
measurements of the body in these animals of little value. In a female
full of eggs the dorsal plates may be separated by a considerable
interval, while after egg-laying they may actually overlap. The four
stigmata are not situated on the plates, but ventro-laterally, at the
level of the hinder borders of the first and second abdominal plates.
The first ventral abdominal plate bears the genital orifice. In the same
plate there are two other orifices, an anterior and a posterior, which
belong to the “abdominal glands.” They were taken by some authors for
the spinning organs, but their function is probably to supply material
for the capsule by which the eggs are suspended from the body of the
mother (see p. 434).
The Chernetidea possess chelicerae, pedipalpi, and four pairs of
ambulatory legs, all articulated to the cephalothorax.
[Illustration:
FIG. 223.—Chelicera of _Garypus_. _f_, Flagellum; _g_, galea; _s_,
serrula. (After Simon.)
]
The chelicerae are two-jointed, the upper portion of the first joint
being produced forward into a claw, curving downward. The second joint
is articulated beneath the first, and curves upward to a point, the
appendage being thus chelate. This second joint, or movable digit,
bears, near its extremity, the opening of the spinning organ, and is
furnished, at all events in the Garypinae and Cheliferinae (see p. 437),
with a pectinate projection, the “galea,” arising at its base, and
extending beyond the joint in front. In the Obisiinae it is only
represented by a slight prominence.
Two other organs characterise the chelicerae of all the Chernetidea;
these are the “serrula” and the “flagellum.” Their minute size and
transparency make them very difficult of observation, and for a long
time they escaped notice. The serrula is a comb-like structure attached
to the inner side of the distal joint. The flagellum is attached to the
outer side of the basal joint, and recalls the antenna of a Lamellicorn
beetle, or the “pectines” of scorpions, a resemblance which gave rise to
the supposition that they are olfactory organs. It is more likely,
however, that they are of use in manipulating the silk.
The pedipalpi are six-jointed and are very large, giving these animals a
superficial resemblance to scorpions. According to Simon,[334] the
patella is absent, and the joints are _coxa_, _trochanter_, _femur_,
_tibia_, _tarsus_, with an apophysis forming the fixed digit of the
chela, while the sixth joint is the movable digit, and is articulated
behind the tarsus. These joints, especially the tarsus, are often much
thickened, but however strongly developed, they are always narrow and
pediculate at the base. The coxae of the pedipalps are closely
approximated, and are enlarged and flattened. They probably assist in
mastication, but there is no true maxillary plate articulated to the
coxa as in some Arachnid groups.
The legs are usually short and feeble, and the number of their
articulations varies from five to eight, so that it is not easy to be
certain of the homologies of the individual joints to those of other
Arachnids. The coxae are large, and form the floor of the cephalothorax.
They are succeeded by a short trochanter, which may be followed by
another short joint, the “trochantin.” Then come the femur and tibia,
elongated joints without any interposed patella, and finally the tarsus
of one or two joints, terminated by two smooth curved claws, beneath
which is situated a trumpet-shaped membranous sucker.
=Internal Structure.=[335]—The internal structure of the Chernetidea, as
far as their small size has permitted it to be made out, bears a
considerable resemblance to that of the Phalangidea. The alimentary
canal dilates into a small sucking pharynx before passing through the
nerve-mass into the large many-lobed stomach, but the narrow intestine
which terminates the canal is convoluted or looped, and possesses a
feebly-developed stercoral pocket.
Above the stomach are situated the spinning glands, the products of
which pass, by seven or more tubules, to the orifice already mentioned
on the distal joint of the chelicerae. The abdominal or cement-glands
are in the anterior ventral portion of the abdomen. No Malpighian tubes
have been found.
The tracheae from the anterior stigmata are directed forward; those from
the posterior stigmata backward. Bernard[336] has found rudimentary
stigmata on the remaining abdominal segments.
The heart is the usual dorsal tube, situated rather far forward, and
probably possessing only one pair of ostia. The nerve-cord is a double
series of ventral masses, united by transverse commissures. These
undergo great concentration in the last stages of development, but in
the newly-hatched Chernetid a cerebral mass and five pairs of
post-oesophageal ganglia can be distinguished.
There are two peculiar eversible “ram’s-horn organs,” opening near the
genital opening. They are said to be present only in the male, and have
been taken for the male organs, though other writers consider them to be
tracheal in function.
=Development.=—Some points of peculiar interest are presented by the
embryology of these animals, the most striking facts being, first, that
the whole of the egg is, in some cases at all events, involved in the
segmentation; and, secondly, that there is a true metamorphosis, though
the larva is not free-living, but remains enclosed with others in a sac
attached to the mother.
At the beginning of winter the female immures herself in a silken
retreat, her body distended with eggs and accumulated nourishment. About
February the egg-laying commences, thirty eggs, perhaps, being extruded.
They are not, however, separated from the mother, but remain enclosed in
a sac attached to the genital aperture, and able, therefore, to receive
the nutritive fluids which she continues to supply throughout the whole
period of development.
The eggs, which line the periphery of the sac, develop into embryos
which presently become _larvae_, that is to say, instead of further
development at the expense of yolk-cells contained within themselves,
they develop a temporary stomach and a large sucking organ, and become
for a time independent sucking animals, imbibing the fluids in the
common sac, and arranged around its circumference with their mouths
directed towards the centre. Subsequently a second embryonic stage is
entered upon, the sucking organ being discarded, and the albuminous
matter which the larva has imbibed being treated anew like the original
yolk of the egg.
It is an interesting fact that in this second embryonic stage a
well-marked “tail” or post-abdomen is formed, and the ganglionic
nerve-masses increase in number, a cerebral mass being followed by eight
pairs of ganglia in the body and eight in the tail. Subsequently a great
concentration takes place till, besides the cerebral mass, only five
closely-applied pairs of ganglia remain, corresponding to the pedipalpi
and the four pairs of legs. Moreover, the first pair advances, so as to
lie on the sides of, and not behind, the oesophagus.
[Illustration:
FIG. 224.—Three stages in the development of _Chelifer_. =A=,
Segmenting ovum; =B=, embryo, with post-abdomen, maximum number of
ganglia, and developing sucking apparatus; =C=, larva. (After
Barrois.)
]
There are two ecdyses or moults during development, a partial moult,
concerning only the ventral surface of the “pro-embryo” as it assumes
the larval form, and a complete moult at the final stage, before
emergence from the incubating sac.
At the end of winter the mother cuts a hole in the silken web, and the
young brood issues forth.[337]
=Classification.=—The order Chernetidea consists of a single family,
=Cheliferidae=. Nine genera are recognised by most authors, but their
grouping has been the subject of a good deal of difference of opinion,
largely dependent on the different systemic value allowed by various
arachnologists to the absence or presence of eyes, and to their number
when present. Simon takes the extreme view that the eyes are only of
specific value, and he is thus led to suppress two ordinarily accepted
genera, _Chernes_ and _Roncus_, which are separated chiefly by
eye-characters from _Chelifer_ and _Obisium_ respectively. He relies
rather on such characters as the presence or absence of _galea_,
_epistome_, and _trochantin_, and establishes three sub-families as
follows:—
(i.) CHELIFERINAE.—Galea. No epistome. Trochantin on all legs. Eyes two
or none. Sole genus, _Chelifer_ (_Chelifer_ + _Chernes_).
(ii.) GARYPINAE.—Galea. No epistome. Trochantin on legs 3 and 4 only.
Eyes four or none. Genera _Chiridium_, _Olpium_, and _Garypus_.
(iii.) OBISIINAE.—No galea. An epistome. No trochantin. Genera
_Chthonius_ and _Obisium_ (which includes _Roncus_).
Whatever be the value of the eyes in the classification of this group—
and Simon adduces strong arguments for his view—there can be no doubt of
their convenience in practical identification. Moreover, as
Pickard-Cambridge[338] points out, a grouping of the genera according to
the eyes results, as regards British species, in pretty much the same
linear arrangement as Simon’s classification, and it may therefore be
convenient to mention that, of the six genera represented in this
country, _Chthonius_ and _Obisium_ are four-eyed, _Roncus_ and
_Chelifer_ two-eyed, while _Chernes_ and _Chiridium_ are eyeless.
=Sub-Fam. 1. Cheliferinae.=—These Chernetidea have the cephalothorax
slightly narrowed in front, and generally marked dorsally with two
transverse striae, while the abdominal plates are bisected by a dorsal
longitudinal line. With the exception of _Chelifer cancroides_, which is
always found in houses, all the species are to be sought under bark,
though occasionally they are discovered under stones.
The two genera of this sub-family are _Chelifer_ and _Chernes_, the
species of _Chelifer_ being two-eyed, and those of _Chernes_ blind.
As already stated, Simon does not consider the possession of the two—
often very feebly developed—eyes of generic importance, and admits only
the genus _Chelifer_.
Five species of _Chelifer_ (including _Ch. cancroides_) and five of
_Chernes_ have been recorded in England.
[Illustration:
FIG. 225.—_Chelifer cyrneus_, enlarged. (After Simon.)
]
[Illustration:
FIG. 226.—_Chiridium museorum_, enlarged. (After Simon.)
]
=Sub-Fam. 2. Garypinae.=—The Garypinae have the cephalothorax greatly
contracted in front and often projecting considerably.
[Illustration:
FIG. 227.—_Olpium pallipes_, enlarged. (After Simon.)
]
There are three genera, _Chiridium_, _Olpium_, and _Garypus_.
_Chiridium_ is eyeless, and appears to have only ten segments in the
abdomen, the tergal plates of which are bisected. _C. museorum_ is found
in England, and is the only Chernetid except _Chelifer cancroides_ which
habitually lives in houses. _C. ferum_ is found under bark in the south
of France.
Neither _Olpium_ nor _Garypus_, which both possess four eyes and eleven
abdominal segments, have as yet been found in this country. _Garypus_,
like _Chiridium_, has the dorsal abdominal plates bisected. There is a
transverse stria on the cephalothorax, and the eyes are far from the
anterior border. In _Olpium_ the dorsal plates are undivided and the
eyes more anterior.
=Sub-Fam. 3. Obisiinae.=—The cephalothorax of the Obisiinae does not
narrow—and is, indeed, sometimes broadest—anteriorly. The chelicerae are
notably large, and the dorsal abdominal plates undivided. They are the
most active of the Chernetidea, ordinarily running backwards or sideways
with their pedipalpi closely folded up against the body. Four genera
usually admitted fall within this group:—_Obisium_, _Roncus_,
_Blothrus_, and _Chthonius_.
_Obisium_ has four eyes, parallel-sided cephalothorax, and curved chelae
on the palps. _Roncus_ is like _Obisium_ except in having only two eyes,
and is therefore disallowed by Simon, who also considers _Blothrus_ to
comprise merely eyeless species of _Obisium_. In _Chthonius_ the
cephalothorax is broadest in front, and the digits of the chelae are
straight.
The Obisiinae are found in moss and débris, and under stones. Three
species of _Obisium_, two of _Roncus_, and four of _Chthonius_ are
recorded in England.
The subjoined list of British species of Chernetidea is taken from the
monograph of the Rev. O. P. Cambridge, cited above:—
GROUP I.—Four eyes.
_Chthonius orthodactylus_, Leach.
„ _rayi_, L. Koch.
„ _tetrachelatus_, Preyssler.
„ _tenuis_, L. Koch.
_Obisium muscorum_, Leach.
„ _sylvaticum_, C. L. Koch.
„ _maritimum_, Leach.
GROUP II.—Two eyes.
_Roncus cambridgii_, L. Koch.
„ _lubricus_, L. Koch.
_Chelifer hermannii_, Leach.
„ _cancroides_, Linn.
„ _meridianus_, L. Koch.
„ _subruber_, Simon.
„ _latreillii_, Leach.
GROUP III.—No eyes.
_Chernes nodosus_, Schr.
„ _insuetus_, Camb.
„ _cimicoides_, Fabr.
„ _dubius_, Camb.
„ _phaleratus_, Simon.
_Chiridium museorum_, Leach.