NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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either side is a malpighian tubule. The rectum has some diverticula and
terminates in the anus.
The mouth. —The thick and leathery integument of the head passes, almost
without a break, into the basal joint of the pedipalps, the hypostome, and a
conical prolongation, which ensheaths the cheliceres as far forwards as the level
of the proximal end of the second palpal segment (Pl. II fig. 1c).From
this point the bare cheliceres lie upon the slightly grooved upper surface of
the hypostome. Between the hypostome and the cheliceres, and, further back,
between the hypostome and the conical prolongation of the head already men-
tioned, is a horizontal fissure, the mouth (Pl. II, fig. 9 m). Observed from
the ventral surface this fissure is not apparent. Dorsally it may be seen
opening at the side of the cheliceres, which together are of smaller extent
transversely than the hypostome upon which they lie. During life the chelicerest
are constantly being protruded and retracted, either simultaneously or indivi-
dually. Their digits may be projected beyond the end of the hypostome, but
are, as a rule, retracted, so as to lie in the hollow of this organ. With the
aid of a needle, the cheliceres may readily be separated from one another, and
the mouth exposed. The basal joints of the palps, as they join the integument
of the head, lie over the edges of the mouth and finally completely close it in
at the sides. The mouth, therefore, which anteriorly is a horizontal fissure
(Pl. II, fig. 5 m), open at the sides, becomes posteriorly a pocket (Pl. II,
fig. 6 m). This pocket, though of small extent, is of considerable significance.
Its roof is formed by the under surface of the dorsal conical projection and
consists of unbroken chitin of medium thickness. Above this, lying in loose
sheaths in the substance of the projection, are the cheliceres (Pl. II, figs. 5 and
6 at .) The cheliceres therefore do not come into connection with this portion
of the mouth at all. The floor of the pocket is continuous with the floor of
the rest of the mouth, that is with the upper surface of the hypostome. Upon the
floor of the pocket is the opening of the pharynx. This is small and is protected
by a projecting tongue, which springs from its posterior margin. The actual
opening is therefore in the form of a V, the limbs of which are directed back-
wards (P1. II, fig. 9 op.). Posteriorly the pocket ends abruptly, and into its
two posterior lateral angles open the ducts of the salivary gland of either side.
The buccal pocket is bounded laterally by the junction of the integument of
the pedipalps with that of the swollen base of the hypostome and the dorsal
conical prolongation of the head. The chitin at this place is thickened and
not only surrounds the buccal pocket, but gives rise also to important
prolongations which pass inwards (Endoschlerites).
Though the tick and the mosquito belong to an entirely different class of
anthropod, and the structures are probably in no way homologous, the buccal
30
cavity of the tick bears a close resemblance to the salivary receptacle described
and figured by Dutton25in the mosquito. In both the tick and the mosquito
the pharynx opens at an angle into the buccal cavity; in the tick on the ventral,
and in the mosquito on the dorsal surface. In both there is a short cul-de-sac,
lying behind the point of entry of the pharynx, and in both this direct conti-
nuation backwards of the mouth receives the salivary ducts or duct.
The Cheliceres. —(Pl. II, fig. 3 a.)—The cheliceres, as seen externally, are
but a portion of the whole organ. In their entirety they are fang-like structures
consisting of a bulbous posterior portion and a narrow anterior extension. The
anterior extensions are narrow, strongly chitinised bars which have already
been described as lying over the mouth. In their passage through the dorsal
conical prolongation of the head they lie surrounded by a loose sheath of a
synovial nature. They each contain two tendons passing from the muscles in
the bulbous portion to the digit. The posterior halves of the cheliceres are
swollen to form large elongated chitinous bulbs which lie closely approximated
in the head and have already been mentioned as projecting into the general
cavity of the body. The inner ends are devoid of chitin and admit muscles and
tracheæ. They also give attachment to numerous muscular bundles, which
pass in a radiating manner from an extended origin on the dorsum, to be inserted
into the cheliceres. The cavity of the bulbous portion is occupied by numerous
muscular bundles, which converge to, and end in, the tendons already mentioned.
The Endoschlerites of the head. —(Pl. II, figs. 3 and 9.)—Where the basal
joint of the pedipalps in Ornithodoras blends with the base of the hypostome and
the dorsal conical prolongation there is a marked thickening of chitin. From
this situation an extension passes inwards, blending with the posterior pocket
of the mouth, and forming an almost complete transverse bar. From the same
situation there extends backwards an extremely strong chitinous bar, which
becomes expanded to form, with its fellow of the opposite side, a large horizon-
tal plate. This plate extends almost completely across the cavity of the head,
and projects freely into the body cavity. Its upper surface forms, on either
side, a broad shallow groove, upon which rest the bulbous portions of the
cheliceres. The under surface of the plate gives origin to many muscle bundles,
which pass to the pharyngeal pump. By means of the anterior rod-like portions
of the endoschlerite, the salivary ducts are supported and carried forwards to
the mouth.
The pharynx.—The pharynx forms a typical pumping organ such as
occurs in the majority of the blood-sucking Insecta (Pl. II, figs. 7 and 9). It
consists of chitinous plates arranged so as to give in section a triradiate lumen
having a ventral and two lateral limbs. The chief plates are arranged so that
one lies beneath, and at a short distance from, the chitinous expansion of the
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endoschlerite. The remaining two have a lateral position. Each edge is formed
by a double fold, extending the whole length of the structure, and thus allowing
a concertina-like expansion to take place, when traction is made by the
surrounding muscles. The whole organ is broadly fusiform in shape, narrow at
its insertion into the mouth and at its junction with the œsophagus. The
anterior portion is bent sharply upwards, and enters the floor of the mouth
nearly at right angles. The pharyngeal pump and its muscles occupy the
space between the boldly convex lower surface of the head and the endoschleral
plate. In dissections it is so surrounded by dense muscle that it is somewhat
difficult to display without treatment with caustic. Posteriorly the pharynx ends
suddenly in the soft and narrow œsophagus.
The salivary ducts.—(Pl. II, figs. 3 and 9 s.)—The short salivary ducts,
enter the head on either side by passing close to the ventral surface over the
fold of chitin which surrounds the rostrum. They pass forwards, lying at first
by the side of, and later just beneath, the edge of the endoschleral plate.
Passing to the anterior portion of the endoschlerite they lie in a groove within
this. Eventually they pierce the chitin and reach the mouth. The fact that
these ducts enter the alimentary tract in front of the pharynx is very much
against the view that they represent the ducts of the so-called salivary glands
of Scorpio or of the spiders. The situation in which they o pen strongly suggests
their homology with the glands serving as poison glands in spiders. These
glands open by a duct, which passes through the poison fang, i.e., the pedipalp.
Were they to open at the base of this organ, they would exactly represent the
condition in the tick. In the tick it seems likely that the dense endoschlerites
are in some way connected with the remnants of the conjoined basal segments
of the pedipalps, and it is noteworthy that the salivary ducts are intimately
connected with these structures.
The æsophagus and proventricular fold. —The œsophagus is a short
straight tube, which perforates the central ganglion in its course from the
pharynx to the alimentary sac (Pl. III, fig. 1). It is lined with a layer of clear
columnar cells with small nuclei. The outlines of these cells have irregularities,
which are mutually adapted to one another, and thus give rise to a dove-tailing
arrangement. Externally there are muscular fibres. Shortly before entering
the alimentary sac, the œsophagus receives a number of muscular bundles
which pass in, in a radiating manner, to be inserted into its walls (Pl. III, figs.
3 to 5). At the entrance of the œsophagus into the large blood sac there is a
small solid organ. In section this is seen to consist of a thick fold of epithelium,
of the same general character as that in the œsophagus, but composed of more
columnar and less irregular cells. In the fold there are some thick circular
bands of muscular tissue. Outside this are longitudinal fibres passing from the
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œsophagus to the gut. The epithelium of the fold passes imperceptibly into
that of the œsophagus, but ends abruptly on reaching the wall of the alimentary
sac. The organ is very similar to, though still more rudimentary than, the
proventricular fold in the mosquito and probably has a similar function. The
importance of this fold in the trypanosome infection of Culex makes its presence
in ticks of moment in connection with infection by spirochetes.
The alimentary sac and its diverticula. —(Pl. III, fig. 7.)—These organs
when freshly distended with blood form smooth, dark-red lobulated masses. In
Ornithodoros the lobulations are very marked, and the diverticula are extremely
irregular in outline. In Hyalomma they are long and sausage-shaped. In Rhipi-
cephalus they usually show constrictions and ballooned portions. As the amount
of blood diminishes, the diverticula become almost black in colour, and exhibit
innumerable small lobulations.
In the young ungorged tick the diverticula are long and narrow. The walls
show active pulsatile movements. This movement, although it can have no
effect in drawing blood from the host, undoubtedly must serve to distribute the
fluid to the different parts of the sac.
From the entrance of the œsophagus and extending backwards to the
neighbourhood of the rectum, there is a large central reservoir which occupies
the dorsal prominence already noted in Neumann's description of the species.
This structure extends forward a little beyond the entrance of the œsophagus,
so that this latter is situated upon the ventral surface of the sac. From the
ventral surface posteriorly a conical tag passes downwards behind the sperma-
theca towards the rectum. This at first contains blood, but as it narrows
becomes a clear tube of capillary character. This portion of the canal appears
functionally inactive, and it can play no part in the passage of matter from the
sac to the rectum. In Ornithodoros savignyi, therefore, the alimentary system
is practically a closed one.
From the central reservoir a number of blind diverticula are given off. It is
stated in some text-books that the arrangement of the diverticula of ticks varies,
but, as a matter of fact, the disposition and extent of the diverticula is very con-
stant and according to a very definite plan in all ticks. In Ornithodoros the system
of diverticula is rather more complex than in the Ixodidæ. There is an anterior,
lateral, and posterior series. The anterior series is found only in Ornithodoros,
and is absent in Rhipicephalus and Hyalomma. It consists of a single medium
diverticulum of small size extending forward so as to lie over the cephalic gland
(Pl. III, fig. 7 d. ant. ). The lateral diverticula arise together about the level of
the entrance of the œsophagus. They are three in number on either side. The
anterior branch is short, the median one is somewhat longer, whilst the posterior
lateral diverticulum is of large size and great length. The anterior and middle
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branches divide terminally into two or more, usually three, cul-de-sacs. The
posterior branch divides into two branches, which, after passing down in the
neighbourhood of the stigma, curve in on to the ventral surface. The more
anterior of the branches ends by the side of the common genital duct. The
most posterior surrounds, on either side, the anus and ends a short distance in
front of this structure. In Ornithodoros irregularities in the arrangement of the
diverticula are sometimes seen. Thus, the median lateral diverticulum may be of
large size and give rise to the anterior of the two ventral branches (Pl. III, fig.
7). The posterior series consist of four lateral and one median, rather short,
diverticula. They all arise close together immediately beyond the atrophied
connection with the rectum. The median diverticulum is unrepresented in the
Ixodidæ which I have dissected.
The various sulci and prominences seen on the surface of Ornithodoros have
relation to these alimentary diverticula. On the dorsum, the transverse sulcus
limits posteriorly the central alimentary sac. Ventrally, the region lying between
the coxæ of the four pairs of legs supports upon its inner surface, with which the
viscus is in actual contact, the cæcal ends of the posterior lateral diverticula.
The lesser prominences correspond in nearly every case with a particular diver-
ticulum and the sulci with the intervals between two diverticula.
The structure of the alimentary sac.—The structure of the sac and of its
diverticula is identical. The cavity is lined by a single layer of large cells
resting upon a thin basement membrane (Pl. III, fig. 6). Externally there are
very large single muscular fibres of a peculiar nature, arranged circularly and
longitudinally. These form an open meshwork with square meshes as in the
mosquito. The lining epithelial cells are large cells with reticular protoplasm
and large vescicular nuclei. Some of them are seen projecting freely into the
lumen. Such cells are of especially large size, and have their inner portions
much swollen and vacuolated, and they may contain globules of a dense black
nature as well as red cells in various stages of intra-cellular digestion. In
addition to these large projecting cells there are smaller cells, whose nuclei
are situated nearer to the basement membrane. Practically all the cells of the
sac contain small densely black granules, evidently derived from the digestion
of the blood in the lumen. In undistended diverticula the epithelium may form
a more or less continuous lining to the tube, but in the distended tube the cells
become very unevenly distributed, being almost absent in some places, whilst in
others they form very striking projecting masses.
The process of digestion. —Twenty-four hours after a meal, the greatly
distended diverticula are found to contain a soft and imperfect coagulum, from
which a considerable amount of fluid blood may drain. On examining the
blood at this time the blood corpuscles are seen in an apparently unchanged
F
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condition. Scattered through the fluid, however, are numbers of intensely
black granules of a globular shape and measuring from 5 µ to .5µ or
less in diameter. In sections these granules are seen collected especially at
the periphery of the newly taken in blood, but they are also present in large
numbers scattered throughout the mass. The black granules are derived
from a previous meal, and there is therefore a considerable degree of mixture
between the new blood and the contents of the diverticula prior to the
meal.
If the diverticula be examined at some considerable time after digestion
has been in progress, some further changes will be observed. On cutting open
a diverticulum under normal saline a number of reddish granules, not unlike
grains of cayenne pepper, will be seen lying in the still partially fluid blood.
These appear quite loose and free from attachments and when washed out fall
to the bottom of the dish or among the viscera. On examination they are seen
to be globular masses composed of an aggregation of a large number of yellow,
red or black granules of large size. Such bodies are especially large, and contain
very coarse granules ; in Rhipicephalus and Hyalomma and in ticks of these
genera the contents of the tube which appear at first sight to be soft coagulum
may be found to be composed almost entirely of these bodies. It was at first
conjectured that the bodies were the inner swollen portions of the cells of the
lining epithelium, which had taken up material and become detached. Further
investigation, however, has shown that each is an entire cell containing a well-
marked nucleus. If films are made of the contents of the sac twenty-four hours
after a meal of blood, there will be seen, in addition to the leucocytes of the host,
cells derived from the epithelium of the sac. Many of these are evidently the
smaller undistended cells, previously noted as lying near to the basement
membrane, now detached in the preparation of the specimen. They contain a
large circular or oval nucleus, and finely reticular or partially vacuolated
protoplasm. Other cells are seen of a similar nature, but larger and with portions
of the protoplasm marked by vacuolated and stored with black granules. In
addition to such cells, which are those of the epithelium of the sac, there are
others which appear to be of a different nature. They are large circular cells
with a darkly staining and not very large nucleus. Their substance is markedly
vacuolated and crowded with matter which they have evidently engulfed, blood
corpuscles, black granules, fragments of chromatin, etc. In sections they are
seen, even in specimens made so soon as six hours after the ingestion of
blood, apparently lying in isolated positions far removed from the walls of the
sac. There appears reason to believe that these cells have a function to perform
as wandering digestive cells. Their relation to the epithelium of the sac is not
clear. As digestion advances, they become more and more replete with material
35
and increase in size until they are readily visible to the naked eye as the red
granules already noted. In the early stages of digestion, cells packed with
chromatin bodies, and resembling superficially macrophages containing the
Leishman-Donovan parasite are seen. The nature of these cells which are seen
both in Ornithodoros and in Rhipicephalus is not clear.
Although a prominent part in digestion is taken by the free cells just
alluded to, there can be no doubt that the epithelium lining the diverticula also
takes an active part in the process. The swollen and vacuolated portion of the
large projecting cells is crowded with products of digestion very much as is that
of the free cells. The smaller cells lying nearer the basement membrane are also,
as a rule, packed with fine black granules, though they rarely contain the large
granules seen in the other cells.
The intensely black and opaque globules which have been described, are
highly characteristic of digestion in the tick, and undoubtedly represent the
ultimate condition to which blood remaining in the gut is reduced by the digestive
process. It probably represents only the portion of food not assimilable, for in
Ornithodoros, ticks of which genus may be kept alive for long periods without
food, the diverticula, after some weeks, contain an inky black material consisting
entirely of the granules under discussion. In ticks of the Ixodidæ family the
digestion is not, as a rule, carried to such an extent as to allow of the whole
contents being so reduced, and the female usually dies whilst the diverticula
still contain much red blood.
As the contents of the diverticula are digested, the muscle fibres, which in
the fully distended organ slightly indent the surface, sink more and more into the
body of the viscus. The result is that the wall between the fibres becomes
ballooned and forms, eventually, flask-like pockets with only a narrow opening
connecting with the lumen. This tendency to the formation of pockets is seen in
Rhipicephalus, but it proceeds to an extraordinary degree in Ornithodoros, in
which genus the most puzzling appearances may arise both in dissection of the fresh
tick, where the whole gut is seen studded with cyst-like bodies, and in sections
where without a clue to the condition it is almost impossible to make out the
relations of the parts. The epithelium is, as a rule, present in the pockets, though
it is generally more noticeable on the ridges formed by the contracted muscular
fibres. The remains of ingested blood, in the form of black granules, is present
both in the pockets and in the lumen. Ticks ( Ornithodoros) examined months
after a meal still have the diverticula loaded with the black material. As has been
previously noted, waste matter is not passed into the rectum and any remnant of
food not absorbed must remain in the diverticula until the death of the tick. The
method by which absorption takes place has not been ascertained. In no case
have I ever detected the black pigment in cells in the tissues or body cavity.
F 2
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In the case of parasites, these could (by passing through the thin-walled
diverticula) at once gain access to the body cavity, and so reach at once many
important tissues, including the naked ova, with great ease. It is interesting to
note that the system of diverticula is directly derived from the yelk of the ovum.
In the nymph the remains of this substance can still be detected (see
Section IV), lying in the canal.
The salivary glands and ducts. —(Pl. IV, figs. 1 to 3.)—The salivary fluid
in Ornithodoros is secreted by two large glands of racimose type, which lie in
an oblique position over the bases of the first two pairs of legs on either side
(Pl. III, fig. 8). The posterior ends of the glands are situated upon the
anterior ventral trachea and the coxal gland. Their anterior ends almost touch
the base of the cheliceres.
The gland consists of a number of lobules, formed of closely aggregated glo-
bular acini, the ducts from which unite in a tree-like manner and open at intervals
around a central duct. This duct arises near the free extremity of the gland and,
passing through its whole length, ends as the short salivary duct of one side.
The acini are globular in shape, and consist each of a few large cells, which
nearly fill the acinus. Externally there is a limiting membrane. A short
spirally striated duct passes from the acinus and joins similar ducts to form the
lobular duct (Pl. IV, fig. 2).
Examined in the fresh condition, the cells of the acini are seen to be loaded
with large refractile granules. On rupturing an acinus these are set free in the
surrounding fluid. In sections of the gland the acini show a somewhat complex
structure. One half of the acinus nearly always shows one condition, whilst the
opposite half shows another. The individual cells may show one or other of the
following conditions, which are probably different stages in the formation of the
secretion.
The most conspicuous cells are those in which the protoplasm is crowded
with large round or oval granules, which stain deeply with hæmatein, and have a
well defined outline and a central darkly stained mass. The granules vary some-
what in size, but average 3 to 5 µ. in diameter. As a rule the cell in which they
occur is closely packed with them and the nucleus is not distinguishable
(Pl. IV, fig. 3 b).
On the opposite side of the acinus a cell is seen containing large granules
with indistinct outlines, which stain much less intensely with hæmatein, and
possess no central nuclear mass (Pl. IV, fig. 3 c).
In many cases areas of vacuolated and coarsely reticular protoplasm
are present (Pl. IV, fig, 3 d).
The nuclei of these cells are small and appear to have undergone degener-
ative changes.