ARACHNIDA EMBOLOBRANCHIATA (_CONTINUED_)—ACARINA—HARVEST-BUGS—PARASITIC
MITES—TICKS—SPINNING MITES—STRUCTURE—METAMORPHOSIS—CLASSIFICATION
=Order IX. Acarina (Acari, Acaridea).=
_Arachnids with unsegmented,[349] non-pediculated abdomen. Respiration
by tracheae, or by the general surface of the body. Month parts
suctorial, but frequently capable of biting or piercing. Metamorphosis
always observable._
The Acarina or Mites are remarkable not so much for the number of their
species, which is very considerable, as for the vast multitude of
individuals of the Order, which is far in excess of that of any other
Arachnid group. This fact is correlated with their minute size. Very few
Mites exceed half an inch in length, while very many are microscopic
creatures, often measuring less than the hundredth of an inch. Taken all
round, a millimetre may be considered a large size for a Mite.
There is much variety of habit within the Order. All Mites live
principally on fluid nutriment, but it may be obtained from living
animals or plants or from decaying organic matter. Some are entirely
parasitic upon plants or animals; others attach themselves to animals in
their larval stage, but are free when adults; while others, again, live
an entirely independent and predaceous life.
The greater number of the Mites are too small to strike the eye. Some of
them have, however, contrived to attract attention, in no very agreeable
manner. Every one knows the Mite popularly called the “Harvest-bug,” but
to this day there is some uncertainty as to its identity. It was
described as a separate species under the name of _Leptus autumnalis_,
and Mégnin was the first to show that it was the larval form of one of
the Trombidiidae (see p. 472). Most authors have considered it the larva
of _Trombidium holosericeum_, but Murray referred it to the genus
_Tetranychus_. The difficulty is that the minute creature cannot be
removed from its victim without such injury as to prevent it from being
bred out and the mature form determined. Brucker[350] has recently
compared a large number of “Harvest-bugs” taken from human beings with
the figures and descriptions of the larvae of certain Trombidiidae given
by Henking and Berlese, and he determined them as the larvae of _T.
gynopterorum_. Quite possibly, however, more than one genus is concerned
in the production of this pest.
That certain skin-diseases are due to Mites (Demodicidae, Sarcoptidae)
is a fact which is widely known. The fruit-grower, too, has to take
cognisance of the Order, for his trees may suffer from “Red-spider”
(_Tetranychus telarius_), and his black-currant bushes fail under the
attack of the “Gall-mite” (_Eriophyes_ or _Phytoptus ribis_). The
curious swellings or galls which disfigure the leaves of many trees are
sometimes of insect origin, but they are often due to Mites.
Domestic pets suffer greatly from Acarine parasites. A large number of
species confine their attention exclusively to the feathers of birds
(_Analges_, etc., see p. 466). One curious genus, _Syringophilus_, is
parasitic _within_ the feathers, feeding upon the pith of the quill.
Heller of Kiel discovered them in 1879, but the researches of Trouessart
first showed their frequent presence and very wide distribution. He
found that they entered by the superior umbilicus of the feather, and
disappeared by the inferior umbilicus when the feathers moulted or the
infested bird died.
It is probable that the comparatively large Mites of the group
_IXODOIDEA_ (see p. 468), commonly called “Ticks,” are the most widely
known of the order. They attack wild and domestic animals and man, and
are nearly always acquired from vegetables, such as brush or herbage. It
would seem likely that many of these creatures can never have the chance
of attaching themselves to animals, and it has been suggested that
animal juices are a luxury but not a necessity to them, and that they
can live, if need be, on vegetable sap, but further investigations have
quite dispelled this view.
The suspected connection between the North American Tick, _Boophilus
annulatus_, and the cattle disease known as Texas fever or “red water,”
since clearly proved by the researches of Smith and Kilborne, led to the
careful investigation of the life-history of that creature, and this was
undertaken by Curtice.[351]
The female Ticks laid eggs a few days after dropping off the cattle,
egg-laying lasting a week or more. The eggs took from three to five
weeks to hatch, and the larvae attached themselves to cattle, on which
they remained a fortnight, becoming mature and fertilised before they
again sought the ground. The whole cycle occupied a time varying from
six to ten weeks, a period apparently much exceeded by some members of
the family.
Lounsbury[352] has recently made out the life-history of the South
African “Bont” tick, _Amblyomma hebraeum_.
The eggs are deposited in the soil, ten to twenty thousand eggs in all
being laid by one female. The larvae climb neighbouring plants and seize
passing animals. After the third day of attachment they begin to
distend, and they generally fall off, fully distended, on the sixth day,
immediately seeking a place of concealment, where they become torpid.
Under natural conditions the nymph does not emerge for at least eleven
weeks, and then it behaves in the same way as the larva, again attaching
itself to an animal for six days. A new time of torpidity and
concealment ensues, again of at least eleven weeks’ duration, when the
final moult takes place and the mature tick emerges. The males at once
attach themselves to animals, but the females hesitate to fix
themselves, except close by a male. For four days after fixation the
male appears to exercise no attraction for the female, but after that
period he shows great excitement at her approach. She, however, does all
the courting, the male remaining fixed in the skin of the host. After
pairing, the female distends greatly, attaining her maximum size (nearly
one inch in length) in about a week, when she lets go and descends to
the earth to lay eggs. If unmated, she detaches herself within a week,
and seeks another host. Oviposition lasts from three to nine weeks, and
the development of the egg from eleven weeks to six months. At least a
year is occupied in the whole cycle. These ticks, and many others,
communicate disease[353] by inoculation, conveying it from one animal to
another.
No poison-glands have been demonstrated in the Acari, the function of
the salivary glands of the Ticks being probably to prevent the
coagulation of the blood of their victims.
It is an important point in the mode of life of the Ticks that they can
live for a long time without food. Mégnin[354] states that he kept an
_Argas_ alive for four years, entirely without nutriment.
In the Tetranychinae (see p. 472), glands apparently homologous with the
salivary glands of the Ticks have taken on the function of spinning
organs. According to Donnadieu,[355] these glands, which resemble
bunches of grapes, and are possessed by both sexes, open into the buccal
cavity at the base of the chelicerae. The gummy fluid exudes from the
mouth, and is combed into threads by the pedipalps. The legs of these
mites are furnished terminally with curious hairs ending in a round
knob, which are supposed to have some relation to their spinning habits.
The males are the busiest spinners, the time of the females being
largely occupied in laying eggs among the excessively fine threads of
silk with which the Mites cover the under surface of leaves. In the
Eriophyidae (see p. 464) corresponding glands are thought to furnish an
irritant fluid which causes abnormal growths or galls upon vegetable
tissues.
=External Structure.=—It is often stated, but erroneously, that there is
no distinction between cephalothorax and abdomen in the Mites. Certainly
no such division can be made out in the Hydrachnidae (see p. 472) or in
some other forms, but in the majority of Acari the cephalothorax is
clearly marked off by a transverse groove or suture. In some cases the
anterior portion of the cephalothorax is movably articulated with the
rest, and forms a sort of false head called a “capitulum.” In most Mites
the chitinous integument is soft and non-resistant, but it is otherwise
with the Oribatidae or “Beetle-mites” (see p. 467), which are nearly all
covered by an extremely hard and coriaceous armature.
Eyes are sometimes absent, sometimes present in varying numbers. They
seem here to be of remarkably little systematic importance, as otherwise
closely allied species may be either eyed or eyeless.
Normally Mites possess the usual Arachnid appendages, chelicerae,
pedipalpi, and four pairs of ambulatory legs. The anterior appendages
are, however, subject to a very great degree of modification, while in
one Family, the Eriophyidae (Phytoptidae), the legs are apparently
reduced to two pairs.
The chelicerae are sometimes chelate, in which case they are
two-jointed, the distal joint or movable finger being always articulated
_below_ the immovable finger. Sometimes they terminate in a single claw
or blade, the movable joint being obsolete. In the Ticks they exist as
two long styles or piercing weapons, serrate on the outer edge.
The pedipalpi vary very much in structure, according to the habits of
the particular form to which they belong. In the Sarcoptidae (see p.
466) they are hardly recognisable owing to the extent to which they have
coalesced with the maxillary plate. In many of the free-living forms
they are leg-like feeling organs, but in others they are raptorial,
being not precisely chelate, but terminating in a “finger-and-thumb”
arrangement which is of use in holding prey. The extreme development of
the raptorial palp is found in _Cheyletus_ (see p. 473), in which the
whole appendage is remarkably thick and strong, and the “finger” is a
powerful chitinous claw, while the “thumb” is replaced by movable
pectinated spines of chitin. The Water-mites have a palpus adapted for
anchoring themselves to water-weeds, the last joint being articulated
terminally with the penultimate joint, and bending down upon it.
Finally, in the “Snouted mites” (Bdellidae, see p. 471) the palpi are
tactile or antenniform, often strongly recalling the antennae of
weevils.
The maxillary plates which arise from the basal joints of the pedipalps
are always more or less fused, in the Mites, to form a single median
transverse plate, constituting the lower lip or “labium” of some
authors. In some of the Oribatidae the fusion of the maxillae is only
complete at the base, and the free points are still of some use as
masticating organs. In those free living Mites which have undergone no
great modification of the mouth-parts two other portions can be
distinguished, the upper lip or “epipharynx,” and the “lingua,” which
forms the floor of the mouth, and is for the most part concealed by the
maxillary plate.
The legs are usually six- or seven-jointed, and are subject to great
variation, especially as regards the tarsus or terminal joint. This may
bear claws (1–3) or sucking disks, or a combination of the two, or may
simply take the form of a long bristle or hair.
The Cheese-mite has a claw surrounded by a sucker—like Captain Cuttle’s
hook within his sleeve. The claws of those species which are parasitic
on the hairs of animals are sometimes most remarkably modified.
[Illustration:
FIG. 239.—Diagram of the viscera of an Oribatid Mite, greatly
enlarged. C, C, Lateral caeca of stomach; _g_, cerebral ganglion;
_od_, _od_, oviducts; _oe_, oesophagus; _pr.g_, pro-ventricular
gland; _ps_, pseudo-stigmatic organ; _st_, stomach; _tr_, _tr_,
tracheae. (Partly after Michael.)
]
=Internal Structure.=—The minute size of most Mites has rendered
research upon their internal structure a matter of great difficulty, and
there are still many obscurities to be removed. Those forms which have
been subjected to examination present a tolerable uniformity in the
structure of the principal organs, but the brief description here given
will not, of course, apply to aberrant groups like the Vermiformia. A
marked concentration is noticeable throughout the Order, and is best
exemplified by the nervous system.
The mouth leads into a sucking pharynx, which narrows to form the
oesophagus. This passes through the nerve-mass in the usual Arachnid
fashion, and widens to form the ventriculus or stomach. The oesophagus
varies considerably in width in the various groups, being very narrow in
those Mites which merely suck blood, but wider in vegetable-feeders like
the Oribatidae.
The stomach is always provided with caeca, but these are not nearly so
numerous as in some other Orders of Arachnida. There are always two
large caeca directed backwards, and there may be others. They are most
numerous in the Gamasidae (see p. 470), which sometimes possess eight,
some being prolonged into the coxae of the legs, as in Spiders. At the
sides of the anterior part of the stomach there are usually two
glandular bodies, the pro-ventricular glands. In those Mites in which
the alimentary canal is most differentiated (_e.g._ Oribatidae) three
parts are distinguishable behind the stomach, a small intestine, a
colon, and a rectum, but in most groups the small intestine is
practically absent. The Malpighian tubes, very variable in length, enter
at the constriction between colon and rectum.
In some of the Trombidiidae there appears to be a doubt as to the
existence of a hind-gut at all. A body having the appearance of the
hind-gut, and occupying its usual position, is found to contain, not
faecal matter, but a white excretory substance, and all efforts to
discover any passage into it from the stomach have been unsuccessful.
Both Croneberg[356] and Henking[357] came to the conclusion that the
stomach ended blindly, and that the apparent hind-gut was an excretory
organ. Michael,[358] in his research upon a Water-mite, _Thyas
petrophilus_, met with precisely the same difficulty, and was led to the
belief that what was originally hind-gut had become principally or
entirely an excretory organ.
The nervous system chiefly consists of a central ganglionic mass,
usually transversely oval, and presenting little or no indication of the
parts which have coalesced in its formation. Nerves proceed from it in a
radiate manner, but no double nerve-cord passes towards the posterior
end of the body. As above stated, it is perforated by the oesophagus.
The vascular system is little understood. In 1876 Kramer[359] wrote that
he was able to perceive an actively pulsating heart in the posterior
third of the abdomen in specimens of _Gamasus_ which had recently
moulted, and were therefore moderately transparent. No other
investigator has been equally fortunate, though many capable workers
have sought diligently for any trace of a dorsal vessel in various
Acarine groups.
It would appear that the blood-flow in most Mites is lacunar and
indefinite, aided incidentally by the movements of the muscles, and
perhaps by a certain rhythmic motion of the alimentary canal, which has
been observed to be most marked during the more quiescent stages of the
life-history.
The internal reproductive organs have the ringed arrangement generally
observed in the Arachnida. The two testes, which are sometimes bi-lobed,
are connected by a median structure which may serve as a vesicula
seminalis, and there are two vasa deferentia which proceed to the
intromittent organ, which is sometimes situated quite in the anterior
part of the ventral surface, but at others towards its centre. The male
Mite is often provided with a pair of suckers towards the posterior end
of the abdomen, and sometimes accessory clasping organs are present.
In some Mites there is no intromittent organ, and Michael[360] has
described some remarkable cases in which the chelicerae are used in the
fertilisation of the female, a spermatophore, or bag containing
spermatozoa, being removed by them from the male opening and deposited
in that of the female. The most remarkable instance is that of _Gamasus
terribilis_, the movable joint of whose chelicera is perforated by a
foramen through which the spermatophore is, so to speak, blown and
carried as a bi-lobed bag, united by the narrow stalk which passes
through the foramen, to the female aperture.
The ovaries are fused in the middle line, and connected by oviducts with
the tube (vagina or uterus) which passes to the exterior. There is often
an ovipositor.
Professor Gené of Turin[361] described, in 1844, some remarkable
phenomena in connection with the reproduction of Ticks. The male
_Ixodes_ introduced his rostrum into the female aperture, two small
white fusiform bodies emerging right and left from the labium at the
moment of introduction. On retraction they had disappeared. When the
female laid eggs, a bi-lobed vesicle was protruded from beneath the
anterior border of the scutum and grasped the egg delivered to it by the
ovipositor, appearing to manipulate it for some minutes. Then the
vesicle was withdrawn, and the egg was left on the rostrum, and
deposited by it in front of the animal. When the vesicle was punctured,
and so rendered useless, the unmanipulated eggs quickly shrivelled and
dried up.
Lounsbury[362] has recently confirmed Professor Gené’s observation as to
oviposition in the case of a South African Tick, _Amblyomma hebraeum_.
The respiratory organs, if present, are always in the form of tracheae.
These are usually long and convoluted, but not branching. The spiral
structure is difficult to make out in these animals, and in the
Oribatidae at least, instead of the external sheath being fortified with
a spiral filament of chitin, there is a very delicate enveloping
membrane with an apparently unbroken chitinous lining, which can,
however, by suitable treatment, be resolved into a ribbon-like spiral
band.[363] The position of the stigmata is very variable, and is
utilised to indicate the main groups into which the Mites have been
divided.
The Oribatidae possess two curious cephalothoracic organs which were for
a long time considered respiratory. These are in the form of two bodies,
like modified hairs, which protrude from sockets on the dorsal surface
of the cephalothoracic shield. Michael[364] has shown that these have no
connection with the tracheae, and he regards them as sensory organs—
possibly olfactory. They are generally referred to as the
“pseudo-stigmatic” organs.
In the Oribatidae, at all events, well-developed coxal glands are
present. In many Mites, especially the Ixodoidea or Ticks, the salivary
glands are large and conspicuous.
=Metamorphosis.=—All Mites undergo a metamorphosis, varying in
completeness in the different groups. Altogether six stages can be
recognised, though they are seldom or never all exhibited in the
development of a single species. These are ovum, deutovum, larva, nymph,
hypopial stage, and imago.
THE OVUM.—All Mites lay eggs. It is frequently stated that the
Oribatidae are viviparous exceptions, but though some of them are
perhaps ovoviviparous, most deposit eggs like the rest of the Order. A
phenomenon which has probably helped to foster this erroneous view is
the occasional emergence from the dead body of the mother of
fully-formed larvae. Towards winter it is not unusual for the mother to
die at a time when her abdomen contains a few ripe eggs, and these are
able to complete their development internally.
THE DEUTOVUM.[365]—In a few cases (_Atax_, _Damaeus_) a stage has been
observed in which the outer envelope of the egg becomes brown and hard,
and splits longitudinally, so as to allow the thin inner membrane to
become visible through the fissure. More room is thus obtained for the
developing larva, which is, moreover, protected, over most of its
surface, by a hard shell. The deutovum stage may occur either within the
body of the mother, or after the egg has been laid.
THE LARVA.—Omitting, for the moment, the very aberrant Vermiformia (see
p. 464), it is the almost universal rule for the egg to hatch out as a
hexapod larva. The larvae of the genus _Pteroptus_ are said to be
eight-legged. Winkler has stated that the early embryo of _Gamasus_
possesses eight legs, of which the last pair subsequently atrophy, but
this observation requires confirmation.
THE NYMPH.—The nymph-stage commences on the acquisition of eight legs,
and lasts until the final ecdysis which produces the imago. This is the
most important period of Acarine life, and is divided into a prolonged
active period, during which the animal feeds and grows, and an inert
period, sometimes prolonged, but at others very short, and differing
little from the quiescence observable at an ordinary moult, during which
the imago is elaborated. In many species the nymph is strikingly
different from the imago; in others there is a close resemblance between
them. It would appear, from the cases which have been most thoroughly
investigated, that the imago is not developed, part for part, from the
nymph, but that there is an “histolysis” and “histogenesis” similar to
that which occurs among certain insects (see vol. v. p. 165). There may
be more than one nymphal stage.
THE HYPOPIAL STAGE occurs in the Tyroglyphinae, the “Cheese-mite”
sub-family. Here some of the young nymphs assume an entirely different
form, so different that it was for a long time considered to constitute
a separate genus, and was named _Hypopus_. The animal acquires a hard
dorsal covering. The mouth-parts are in the form of a flat blade with
two terminal bristles, but with no discernible orifice. The legs are
single-clawed, and all more or less directed forward, and they are
articulated near the middle line of the ventral surface. Suckers are
always present under the hind part of the abdomen.
It appears that these remarkably modified nymphs are entrusted with the
wider distribution of the species, and that they are analogous to the
winged individuals which occur in the parthenogenetic generations of the
Aphidae. The ordinary _Tyroglyphus_ is soft-bodied, and requires a moist
environment, and exposure to the sun or prolonged passage through the
air would be fatal to it. The hypopial form is much more independent of
external conditions, and its habit is to attach itself by its suckers to
various insects, and by this means to seek a new locality, when it
resumes the ordinary nymph-form and proceeds with its development.
=Classification.=—There is no generally accepted classification of the
Acarina, though several eminent Arachnologists have attempted of late
years to reduce the group to order. Widely different views are held
concerning the affinities of certain groups, and there is no agreement
as to the value to be accorded to the characters which all recognise.
Thus Canestrini[366] allows thirty-four families, while according to
Trouessart[367] there are only ten.
Trouessart’s scheme of classification is in the main followed in the
present chapter.
=Sub-Order 1. Vermiformia.=
This Sub-order includes the lowest and most aberrant forms of the Mites.
They are entirely parasitic, and of very small size. The abdomen is much
elongated, and is transversely striated. There are two families,
Eriophyidae[368] (Phytoptidae) and Demodicidae.
=Fam. 1. Eriophyidae (Phytoptidae).=—These are the so-called Gall-mites.
The curious excrescences and abnormal growths which occur on the leaves
and buds of plants are familiar to every one. Various creatures are
responsible for these deformities, many being the work of insects,
especially the Cynipidae among the Hymenoptera, and the Cecidomyiidae
among the Diptera. Others, again, are due to Eriophyid Mites.
Though the galls originated by Mites are often outwardly extremely
similar to those of insect origin, they can be at once distinguished on
close examination. Mite-galls contain a single chamber, communicating
with the exterior by a pore, usually guarded with hairs, and the Mites
live gregariously within it, apparently feeding upon the hairs which
grow abundantly on its inner surface. In Insect-galls each insect larva
lives in a separate closed chamber.
[Illustration:
FIG. 240.—Vermiform Mites, highly magnified. =A=, _Demodex
folliculorum_; =B=, _Eriophyes_ (_Phyptoptus_) _ribis_.
]
The Eriophyidae are unique among Mites in possessing only two pairs of
legs, situated quite at the anterior part of the body. The mouth-parts
are very simple.
There are three genera, _Eriophyes_ (_Phyptoptus_) with about one
hundred and fifty known species, _Monochetus_ with a single species, and
_Phyllocoptes_ with about fifty species.
Among the best known examples are _Eriophyes tiliae_, which produces the
“nail-galls” on lime-leaves, and _E. ribis_, the “black-currant
Gall-mite,” which feeds between the folded leaves of the leaf-buds, and
gives rise to swelling and distortion.
=Fam. 2. Demodicidae.=—The single genus _Demodex_ which constitutes this
family consists of a few species of microscopic Mites which inhabit the
hair-follicles of mammals, and are the cause of what is known as
“follicular mange,” some other forms of mange being due to members of
the succeeding family. _Demodex_ possesses eight short, three-jointed
legs, each terminated by two claws. The abdomen is much produced, and is
transversely striated. About ten species have been described, but of
these five are probably varieties of _D. folliculorum_ (Fig. 240, A),
which infests Man.
=Sub-Order 2. Astigmata.=
The Astigmata are Mites of more or less globular form, with chelate
chelicerae and five-jointed legs. All members of the group are eyeless.
Their habits are very various, some feeding on vegetable matter and
others on carrion, while a large number are parasitic on animals.
Tracheae are absent. There is only one family.
[Illustration:
FIG. 241.—=A=, Leg of a fowl infested with “leg-scab”; =B=, female of
_Sarcoptes mutans_, greatly magnified. (After Neumann.)
]
=Fam. 1. Sarcoptidae.=—No tracheae or stigmata. Apical rostrum.
Oviparous or ovoviviparous. The seventy genera and 530 odd species of
this family are divided into a number of sub-families, of which the
principal are the Sarcoptinae, the Analgesinae, and the Tyroglyphinae.
(i.) The Sarcoptinae are the so-called “Itch-mites.” They are minute
animals, with bodies transversely wrinkled and legs terminating in
suckers or bristles. The genus _Sarcoptes_, which includes about fifteen
species, lives in tunnels which it burrows in the skin of mammals.
(ii.) The Analgesinae are the “Birds’-feather Mites.” The principal
genera are _Pterolichus_ (120 species), _Pteronyssus_ (33 species),
_Analges_ (23 species), _Megninia_ (42 species), and _Alloptes_ (33
species).
(iii.) The Tyroglyphinae[369] have received the popular name of
“Cheese-mites,” from the best known example of the group. They are
smooth-bodied, soft-skinned white Mites, with legs usually terminating
in a single claw, sometimes accompanied by a sucker. They are for the
most part carrion-feeders, living upon decaying animal or vegetable
matter, but a few are parasitic on mammals, insects, and worms.
There are sixteen genera, including about fifty species. _Tyroglyphus
siro_ and _T. longior_ are common Cheese-mites. Other species live in
decaying vegetables and food-stuffs. Some of the genus _Glycyphagus_
(_G. palmifer_, _G. plumiger_) are very remarkable for the palmate or
plumose hairs which decorate their bodies. The remarkable hypopial stage
in the development of _Tyroglyphus_ has been mentioned on p. 463. The
Tyroglyphinae are the lowest of the free-living Acarine forms.
=Sub-Order 3. Metastigmata.=
The four families which constitute this sub-order comprise a large
number of Mites in which the tracheae open near the articulation of the
legs, and consequently in a somewhat posterior situation. The families
are Oribatidae, Argasidae, Ixodidae, and Gamasidae.
=Fam. 1. Oribatidae.=—The Oribatidae or “Beetle-mites” are free-living
Acari, with tracheae of which the stigmata are concealed by the
articulation of the legs. The cephalothorax is distinctly marked off
from the abdomen, and bears dorsally two “pseudo-stigmatic” organs. The
rostrum is inserted below the cephalothorax. These Mites gain their
popular name from the beetle-like hardness of their integuments. They
are oviparous or ovoviviparous. Eyes are always absent.
[Illustration:
FIG. 242.—Oribatid Mites. =A=, _Cepheus ocellatus_, × 24; =B=, ventral
view of _Hoploderma magnum_, closed, × 20. (After Michael.)
]
These are small creatures, seldom attaining the twentieth of an inch in
length. They are vegetable-feeders (except, perhaps, _Pelops_), and are
to be found in dead wood or vegetable débris, under bark, or among moss
and lichen. In winter they often take refuge under stones. It is
impossible at present to estimate the number of existing species, for
only a few localities have been systematically worked for them, and
their small size has prevented their inclusion, in any numbers, in the
collections of scientific expeditions. Our knowledge of the group is
likely, however, to be largely extended, for it has been found that they
reach England alive and in good condition from the most remote regions
if moss or other material in which they live is collected when not too
dry, and hermetically sealed up in tin cases.
About twenty genera and more than 220 species are at present known.
_Pelops_ has much elongated chelicerae, with very small chelae at the
end. There are ten species, found in moss and on bushes. _Oribata_
numbers about fifty species, found in moss and on trees. _Notaspis_, in
which the last three legs are inserted at the margin of the body, has
about thirty species, found among moss and dead leaves. _Nothrus_ is a
short-legged genus with flat or concave dorsal plate, often produced
into very remarkable spiny processes. There are twenty-two species found
under bark and among moss and lichen. _Hoploderma_ (_Hoplophora_) is
remarkable for its power of shutting down its rostrum and withdrawing
its legs in a manner which leaves it as unassailable as a tortoise or an
armadillo.
Though the Oribatidae are all eyeless, they are distinctly sensitive to
light, not wandering aimlessly till they reach a shadow, but apparently
making straight for a dark spot when subjected to strong illumination.
Some species have a curious habit of collecting dirt and débris on their
backs, so as entirely to obscure the often very remarkable disposition
of the spines and processes with which they are furnished.
[Illustration:
FIG. 243.—Capitulum of _Boophilus australis_; ventral view. _p^1_,
_p^2_, _p^3_, _p^4_, The four articles of the palp; _m_, the
mandible or chelicera; _d_, its digit; _n_, the hypostome.
]
The next two families include the animals commonly known as Ticks, the
largest and most familiar of the Mite tribe. Of recent years they have
attracted much attention as the conveyers, to man and domestic animals,
of certain diseases due to blood-parasites (see p. 457, _n._), and our
knowledge of their structure and habits has greatly increased in
consequence. Hitherto they have generally been considered to constitute
a single family, the Ixodidae, but a section of them so differ from the
rest as to require their removal to another family, the Argasidae, so
that it is necessary to employ a super-family name—IXODOIDEA—to embrace
the whole group.
Ticks are parasitic on mammals, birds, and reptiles, some shewing a
marked partiality for a particular host, others being much more catholic
in their tastes. Both sexes in the Argasidae, but the females only of
the Ixodidae, are capable of great distension, but when unfed they are
all somewhat flat animals with laterally extended legs and rather
crab-like movement.
All Ticks possess a small, movable “false head” or _capitulum_ bearing
mouth-parts which are exceedingly characteristic of the group. The
chelicerae are cutting instruments with their distal ends serrated
outwardly, and there is always present a hypostome beset with recurved
teeth which serve to maintain a firm hold on the tissues into which it
is thrust. On either side of the chelicerae are the four-jointed palps,
leg-like in the Argasidae, but more rigid and rod-like in the Ixodidae,
where their inner margin is often hollowed so as to enclose the
chelicerae and hypostome when the palps are apposed. There is a
conspicuous pair of spiracles near the coxae of the fourth pair of legs.
[Illustration:
FIG. 244.—_Ornithodoros talaje_, under surface, × 5. (After
Canestrini.)
]
=Fam. 2. Argasidae.=—The Argasidae are leathery Ticks without a shield
or _scutum_, and with free, leg-like palps. The capitulum is never more
than partially visible when the adult animal is viewed dorsally. Their
hosts are always warm-blooded animals. Two genera are usually
recognised, _Argas_ and _Ornithodoros_, though recent discoveries of new
forms have tended towards their fusion. _Argas reflexus_ and _A.
persicus_ have been proved to convey a Spirochaete disease to fowls, and
the latter, under the name of the “Mianeh Bug” has long possessed an
evil reputation for the “poisonous” effect of its bite on human beings.
In Mexico the “Turicata” (_Ornithodoros turicata_) and the “Garapata”
(_O. megnini_) are greatly dreaded, while human “tick fever” on the
Congo has been traced to the instrumentality of _O. moubata_.
[Illustration:
FIG. 245.—Female Sheep-tick, _Ixodes ricinus_.
]
=Fam. 3. Ixodidae.=—These are the more familiar Ticks, possessing a
_scutum_ or shield, which covers the whole back of the male, which is
capable, therefore, of little distension, whereas it forms only a small
patch on the front part of the body of the distended female. There are
ten genera, _Ixodes_, _Haemaphysalis_, _Dermacentor_, _Rhipicentor_,
_Rhipicephalus_, _Boophilus_, _Margaropus_, _Hyalomma_, _Amblyomma_, and
_Aponomma_.
_Ixodes ricinus_ is the common English sheep-tick. Species of
_Boophilus_ are parasitic on cattle the world over, and _B. annulatus_
is the transmitter of Texas fever. _Rhipicephalus_ and _Amblyomma_ are
large genera which include several species of economic importance. For
example, _R. sanguineus_ conveys canine piroplasmosis, and _A. hebraeum_
causes “heart-water” in South African cattle. The genus _Aponomma_
confines its attention to reptiles, and some of its species are
exceedingly ornate.
Neglecting _Margaropus_ and _Rhipicentor_, which include only a very few
aberrant forms, the following entirely artificial key will serve to
differentiate the genera of the Ixodidae:—
1. A pair of eyes on the lateral borders of the scutum 2
No eyes 6
2. Capitulum long, much longer than broad 3
Capitulum short 4
3. Unicolorous, ♂ with chitinous plates near anus _Hyalomma_
Generally ornate, ♂ without anal plates _Amblyomma_
4. Generally ornate, ♂ without anal plates, but with
enlarged 4th coxae _Dermacentor_
Unicolorous, ♂ with anal plates and normal coxae 5
5. Palpi very short, spiracle circular _Boophilus_
Palpi medium, spiracle comma-shaped _Rhipicephalus_
6. Capitulum short; 2nd article of palp projecting
laterally _Haemaphysalis_
Capitulum long 7
7. Unicolorous, elongate, on birds or mammals _Ixodes_
Generally ornate, broad-oval, on reptiles _Aponomma_
Neumann has recently revised the Ixodoidea in a series of papers
published in the _Mémoires de la Société zoologique de France_,[370] but
the work is not obtainable as a whole. A monograph, by Nuttall,
Warburton, Cooper, and Robinson, is now in course of publication at the
Cambridge University Press.[371]
=Fam. 4. Gamasidae.=—The Gamasidae are carnivorous Mites, either
free-living or parasitic on animals. The chelicerae are chelate, and the
palps are free. The tarsi have two claws, accompanied by a “caruncle” or
sucking disc. They are mostly pale-coloured Mites, with a smooth, more
or less scutate covering. The three principal sub-families are
Gamasinae, Uropodinae, and Dermanyssinae.
Of the GAMASINAE, _Gamasus coleoptratorum_ is the well-known
Beetle-parasite so frequently seen on _Geotrupes_. It is often
confounded with another species of similar habits, _G. crassipes_.
The curious Beetle-parasites attached to their victim by a thread belong
to the genus _Uropoda_ of the UROPODINAE. The connecting filament, which
the Mite can sever at will, for a long time puzzled observers. It was
variously construed as a silken cord of attachment, and as a sort of
umbilical cord, through which the Mite drew nourishment from the Beetle.
On more careful investigation it proved to be connected with the anus of
the Mite, and to consist of its consolidated excrement.
The DERMANYSSINAE are all parasitic on warm-blooded animals, principally
birds and bats. _Dermanyssus avium_ is the common parasite infesting
fowls and cage-birds.
=Sub-Order 4. Heterostigmata.=
=Fam. Tarsonemidae.=—This is the sole family of the sub-order. It
comprises a number of minute vegetable-feeding Mites which have been
little studied, though they are probably the cause of considerable
injury to the leaves and buds of plants.
=Sub-Order 5. Prostigmata.=
In these Mites the stigmata are situated anteriorly, in the rostrum or
the thorax. In the Water-mites the tracheae have atrophied, but these
creatures are clearly Trombidiidae which have taken to an aquatic life.
[Illustration:
FIG. 246.—_Bdella lignicola_, x about 50. (After Canestrini.)
]
=Fam. 1. Bdellidae.=—The Bdellidae are sometimes known as the “Snouted
Mites” on account of the very prominent forwardly-directed “capitulum”
or false head. They have chelate chelicerae and tactile palps, which are
often “elbowed,” like the antennae of weevils. Eyes may be present or
absent. They are usually of a bright red colour, and are free-living and
predaceous, though in their larval stages they may often be found
attached to the limbs of insects and spiders.
The minute active scarlet Mites of the genus _Eupodes_ and its allies
perhaps come within this family. Their legs are six-jointed.
The remaining families of the Prostigmata (Halacaridae, Hydrachnidae,
and Trombidiidae) all have raptorial palps, and clawed or piercing
chelicerae.
=Fam. 2. Halacaridae.=—This is a small group of marine Mites. In their
usually prominent capitulum they resemble the Bdellidae. In some
respects they recall the Oribatidae, having hard integuments, and their
legs being articulated near the margin of the body. They do not swim,
but crawl upon weeds and zoophytes, or burrow in the mud.
[Illustration:
FIG. 247.—_Atax alticola_, x 16. (After Canestrini.)
]
=Fam. 3. Hydrachnidae.=—The Hydrachnidae are the Fresh-water Mites.
Their legs are provided with long close-set hairs, and thus adapted for
swimming. They are predaceous, and in their young stages are often
parasitic on water insects. A familiar example is _Atax bonzi_, which
lives within the shell of the fresh-water mussel.
=Fam. 4. Trombidiidae.=—The predaceous palps of the Trombidiidae are
generally of the “finger-and-thumb” type. The tarsi are two-clawed,
without caruncle. This group may be divided into six sub-families.
[Illustration:
FIG. 248.—_Tetranychus gibbos_us, x 50. (After Canestrini.)
]
(i.) The LIMNOCHARINAE or “Mud-mites” connect the Hydrachnidae with the
typical Trombidiidae. They are usually velvety and of a red colour. They
do not swim, but creep. The larva of _Limnocharis aquaticus_ is
parasitic on _Gerris lacustris_.
(ii.) The CAECULINAE bear a strong general resemblance to the Harvestmen
or Phalangidae. _Caeculus_ is so similar to the Phalangid genus
_Trogulus_ that it was considered by Dufour to belong to the same order.
(iii.) The TETRANYCHINAE or “Spinning mites” are phytophagous, and do
much injury to plants, sucking the sap from the leaves and giving them a
blistered appearance. _Tetranychus telarius_ is the “Red-spider” of
popular nomenclature.
(iv.) The CHEYLETINAE are remarkable Mites with fleshy, semi-transparent
body, and enormously developed raptorial pedipalpi, which are extremely
formidable weapons of attack. They do not creep or run like most Mites
but proceed by a series of short leaps. _Cheyletus_ is the principal
genus.
The curious genus _Syringophilus_, which is parasitic in the interior of
birds’ feathers, appears to be a degenerate Cheyletine.
(v.) The ERYTHRAEINAE are minute, active Mites, usually red in colour,
free-living and predaceous.
(vi.) The TROMBIDIINAE include most of the moderate-sized, velvety red
Mites which are commonly known as “Harvest-mites,” and their larvae, the
so-called Harvest-bugs, frequently attack man. _Trombidium holosericeum_
is a well-known example.
=Sub-Order 6. Notostigmata.=[372]
This sub-order has been established for the reception of the curious
genus _Opilioacarus_.
=Fam. Opilioacaridae.=—Mites with segmented abdomen, leg-like palps,
chelate chelicerae, and two pairs of eyes. There are four dorsal
abdominal stigmata. Four species of the sole genus _Opilioacarus_ have
been recorded, _O. segmentatus_ from Algeria, _O. italicus_ from Italy,
_O. arabicus_ from Arabia, and _O. platensis_[373] from South America.
APPENDICES TO ARACHNIDA
I. AND II.
TARDIGRADA AND PENTASTOMIDA
BY
ARTHUR E. SHIPLEY, M.A., F.R.S.
Fellow and Tutor of Christ’s College, Cambridge, and Reader in Zoology
in the University