NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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The Sternum is divided into two quadrilateral plates, which are united along
the middle line at an angle, thus giving the thorax a keeled appearance. The
Episternum lies directly above the sternum, and resembles it in shape. Both these
sclerites are united to the adjacent plates, and to one another, the lines of junction
being marked on the internal surface by prominent ridges. The Epimeron is a
narrow and irregular vertical plate, extending from the wing base to the middle
line below. The upper third of its anterior margin is separated a short distance
from the episternum, the interval being filled in by arthrodial membrane, on
which the anterior series of wing sclerites lies. The middle third of the margin
is united with the mesosternum, and below this the two diverge, leaving an
interval in which there lies a conical coxal plate, with which the middle leg
articulates. The upper third of the epimeron is concealed, from within, by the
wing sclerites. Its posterior margin is ill-defined, and merges into the membrane
to which the squama is attached. The middle third is produced backwards,
forming a dome-shaped expansion, which lies below the squama, and which gives
origin to the mesophragma. The posterior thoracic spiracle lies in a membrane-
ous interval below this expansion. The lower third of the epimerion tapers to a
blunt point ; it is partially fused with the metathorax.
The Mesophragma is a funnel-shaped expansion, which projects backwards
and inwards into the thorax, materially diminishing the size of the thoracic outlet.
It arises from the upper and lower borders of the dome-shaped expansion on the
epimeron. From these borders thin sheets of chitin are reflected upwards and
downwards into the cavity of the thorax, those from the two sides meeting above
and below, so as to form a sort of inner chamber in the thorax. This funnel, which
is more extensive above than below, extends backwards as far as the first
abdominal segment, and contains all the organs passing from the thorax to the
abdomen. In cross sections of the posterior end of the thorax it appears as an
isolated ring of chitin, giving attachment to the longitudinal muscles of the thorax.
Between the scutum and prescutum above, and the episternum below, there is a
narrow longitudinal interval, filled in by arthrodial membrane. This is prolonged
downwards to separate the episternum from the epimeron. The anterior thoracic
spiracle lies at the anterior end of this space.
(c) The Selerites of the Wing Base.
These are arranged in two sets. The anterior set consists of three wedge-
shaped rods, which are attached to one another and to the adjacent sclerites by
their expanded upper ends. their pointed apices hanging down internal to the
episternum and the arthrodial membrane. The largest rod is situated between
and internal to the other two. The pointed anterior angle of its base is attached
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to the prescutum, near the lateral furrow separating it from the Scutum, by a
strong band of fibres. The anterior rod is attached at its base to the largest rod.
The posterior wedge is shorter and stouter than the others, and is attached by the
anterior angle of its base to the first rod, and by its apex to the anterior margin of
the epimeron, by stout cords.
This set of sclerites, by virtue of the elasticity of the fibrous cords connecting
them with the dorsal and lateral walls of the thorax, and by means of the muscles
attached to them, will antagonise the longitudinal muscles, by reducing the vertical
diameter of the thorax.
The posterior sclerites are directly associated with the wing, and form a series
of levers by which the wing can be folded and unfolded. They are five in number,
and are of too irregular a shape to be accurately described. Their arrangement is
indicated somewhat diagramatically in the figure. (Plate V, Fig. 19.) The costal
vein terminates in a round knob, produced downwards into a fibrous cord. The
radial vein terminates in a very complex sclerite, which lies upon the upper end
of the epimeron, and receives, in its anterior border, the fibres from the costal
vein. Co-relation between the two important veins of the wing is thus secured.
The posterior margin of the wing terminates in a conical sclerite, which lies behind
the radial sclerite but is not connected with it. The squama is closely connected
with a curved rod lying in the membrane at its base, and this rod is attached
above to the base of the sclerite which terminates the posterior border of the
wing. Below these four rods there lies a Y shaped rod, which is the governing
part of the mechanism. This rod is horizontal, the fork of the Y being anterior.
The lower limb of the fork is received into a little pit in the epimeron; the upper
limb is bifid, and interlocks with the bifid lower end of the radial sclerite. The
posterior long limb of the Y is attached to the curved rod at the base of the
squama.
If one bears in mind that when the wings are folded on the abdomen the
squamæ are folded on the wings, the mode of action of these levers becomes obvious.
When the long arm of the Y is pushed up, the radius, and with it the costa, are
pressed backwards, while the squama is pushed in the reverse direction.
The Medi-furca is composed of two large lateral expansions, set on a Y shaped
stalk. It lies in the angle between the mesosterna, and is attached, somewhat
loosely, to the posterior part of the ridge between them. These lateral expansions
are formed from two plates of chitin, lying one above the other. The plates are
roughly oblong in shape, but with their external anterior angles produced forwards;
they are united along their inner and anterior borders, but diverge from
one another behind, and are separated posteriorly by a small vertical plate, so that
they enclose a boat-shaped cavity. Behind this expansion there is another about
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one-fourth the size, and formed in the same way from two triangular plates.
These are attached in front to the plate representing the stern of the boat, and
converge to meet one another behind. The fork of the stalk is much thickened
and ridged to resist the strain which falls on it during the contraction of the trans-
versely acting muscles attached to the apodeme.
(d) The Metathorax.
The metathorax is much reduced in size, and the separate segments are diffi-
cult to recognise. The mesophragma passes backwards through the metathorax.
The Meta-notum is a narrow thickened arched ridge, which lies beneath the
scutellum, where it articulates with the tergite of the first abdominal segment.
The pleural sclerite is undivided, and forms a narrow plate extending from the
insertion of the halter to the middle line. It is attached to the meta-notum above
and behind, and to the epimerion of the mesothorax in front. The meta-sternum
is a small oval plate, fused with the pleural sclerites. It bears the post-furca, and
its posterior borders are deeply excavated for the reception of the coxæ of the hind
legs.
The Post-furca lies on the metasternum, anterior to the coxæ. It consists of
a pair of pointed wing-like expansions, united with one another and supported by
stout ribs. It is attached to the median ridge between the two lateral halves of
the metasternum. Each expansion is supported by a Y shaped rib, the inner
limb of which runs to the extreme tip of the apodeme. These ribs converge
inwards and backwards, and those of the two sides are connected by a sheet of
chitin. They do not meet posteriorly, but diverge again, and terminate in a pair
of stout-hooked processes, which lie between the coxæ, and are closely connected
with the median ridge on the metasternum.
(e) The Wings.
The wings, when in the resting position, extend a little beyond the tip of the
abdomen, and are held in a tectile manner. Their general shape is indicated in
the figure. (Plate I.) They are light brown in colour, and are mottled with patches
of a lighter tint. The inner half of the posterior border of the wing is indented,
so that a large anal lobe, a smaller alula, and a still smaller antisquama, can be
distinguished. The squama is specially differentiated, and is relatively rather
large and conspicuous. It lies in a plane almost at right angles to the wing when
the latter is moderately extended, and, as the wing is folded backwards, the squama
is folded on it so that it lies in contact with the antisquama. The special mechanism
by which this is accomplished has already been described. The squama is some-
D
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what thicker than the rest of the wing, and has a thickened margin, which bears a
fringe of fine hairs.
The distribution of the wing veins is sufficiently indicated in the text figure.
The terminology employed is that of Conistock and Needham (16). The Costa
extends to the tip of the wing and is continuous with the thickened posterior border.
It bears four rows of small spines, which extend all round the wing. The Radial
vein divides into four branches, of which the second represents the fused second and
Venation of Wing.
co., costa; sc., subcosta. r. 1, 2, 3, 4, the four divisions of the radial vein; m. 1, 2, 3, the three
divisions of the median vein; cu. 1, 2, the two divisions of the cubital-vein; an., the anal
vein; h., the humeral cross-vein; ay., the axillary crossvein ; rm., the radial median cross-vein;
mcu., the median cubital crossvein; R, R1, etc., the radial cells; M. M1, the median cells;
Cu, 1 and 2, the cubital cells; A., the anal cell; an., the anal lobe; al., the alula; as., the
antisquama; sq., the squama.
third radial veins. The first radial vein, the stoutest in the wing, bears a single
row of small spines. At its termination there is a small oval area of dark pigmen-
tation, the " stigma." The division between the fourth and fifth radial veins is
peculiar. The fourth vein turns forwards and then outwards from its origin, and
from the angle so produced a stout spur projects inwards in line with the vein.
At first sight it appears as if an adventitious cross vein had been developed,
but a comparison of other Tabanidœ indicates that an obtuse angle is by no
means uncommon at this junction. The peculiarity is the projecting spur.
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(f ) The Legs.
The legs are composed of five joints, as in other Diptera. The joints differ-
somewhat in the three legs. The coxa of the foreleg is long and cylindrical, and
articulates with the thorax at a comparatively wide membraneous interval external
to the presternum. The femur resembles the coxa, but is longer, and is covered
with stout black setæ. The tibia is shorter and stouter than those of the other
legs, and is covered with setæ. The coxa of the middle legs is much reduced in
size ; it articulates with the coxal plate which lies behind the mesosternum. The
femur resembles that of the foreleg, except that the setæ are restricted to a small
area just above the lower end. The tibia of this leg has two stout conical spines
at its lower end. The coxæ of the hind legs are broad and oval, and lie almost in
contact with one another, on the metasternum. The femur and tibia resemble
that of the middle leg, except that the hind tibia does not bear spines. The meta-
tarsus is the same in the three legs. There are five joints, the first of which is much
larger than the rest. The next three joints are short and conical. The terminal
joint bears two large symmetrical curved claws, and three pulvilli with glandular
hairs.
The fore tibia is coloured yellow in its proximal half ; the mid and hind
tibia are yellow throughout, except at the lower end, and in small areas about the middle
and the upper end.
(g) The Halters.
These are situated on the anterior edge of the metathorax, just behind the
posterior spiracles. Each consist of a somewhat flattened ovoid knob, borne on a
slender stalk, which terminates in an expanded base. The upper and lower sur-
faces of the halters are pigmented, and are covered by extremely fine hairs. The
proximal expanded end of the stalk, and a small elevated cushion lying behind it,
have on them a large number of minute shallow pits, on the margins of which there
are a few small hairs.
III.—The Abdomen.
The abdomen is the same breadth as the thorax, and is about twice as long.
It consists of seven visible segments, which decrease in width from before back-
wards, so that the last is about two-thirds the breadth of the first. The posterior
segments are flattened from above downwards. The coloration resembles that of
the thorax. The median light stripe is continued down the abdomen, and in addi
-tion there is on. each segment a round spot on each side, and a narrow area on the
posterior margin, of the same fawn colour. The whole abdomen has a scanty
covering of downy hairs, and there is a small fringe of short stout setæ in the
posterior border of the last segment. A pair of small oval anal lobes projects
from under cover of the last segment.
D 2
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The pleural membrane which unites the tergites and sternites, and which bear
the spiracles, is capable of considerable distension, and is conspicuous in a recently
fed insect.
Under cover of the seventh segment there are several small sclerites, which,
with the anal lobes, may be taken to represent four reduced segments. Dorsally
there is a plate resembling the seventh tergite, but about one-fourth the size; be-
hind this there are two pairs of small pieces, and the two anal lobes. On the ven-
tral aspect the several pieces are fused into one, which is roughly square-shaped,
with the posterior half of its lateral borders expanded. These sclerites are situated
in an arthrodial membrane, and when they are drawn out, it is seen that the ante-
rior margin of the first piece is attached to the posterior margin of the seventh
segment, so that they form a continuous but retracted part of the abdominal wall.
The arrangement probably represents, and functions as, an ovipositor. (Plate
VII, Figs. 29 and 30.)
INTERNAL STRUCTURE.
I.—The Alimentary Canal.
This consists of the following parts:—
Pharynx.*
Œ s o p h a g u s , w i t h i t s d i v e r t i c u l u m .
Proventriculus.
Mid-gut.
Hind-gut, divisible into—
I l e u m, wi t h t h e Ma l p i g h a n t u b e s .
Colon.
Rectum, with its papillæ.
The Pharynx is the most important part of the sucking apparatus of this insect.
It consists of two oblong vertical chitinous plates, the anterior of which is continuous
with the epipharynx, the posterior with the hypopharynx. These plates are con-
cave forwards, and are closely apposed to one another in a state of rest; they are
united to one another by their lateral borders, and the pouch so formed opens
below into the canal between the epipharynx and hypopharynx, and above into the
first part of the œsophagus. The upper lateral angles of the pharynx are pro-
duced into two long cornua, which project upwards, and slightly forwards, into
the cranial cavity. The pharynx lies on the palatal membrane, in front of and in-
ternal to the stipes of the maxilla. (Plate III, Fig. 13.)
* The nomenclature of Meinert is followed here. The first part of the œsophagus corresponds to the
pharynx of the mosquito.
F i g u r e 2 . — T h e C h i t i n o u s p l a t e o f t h e œ s o p h a g u s . A . W h e n t h e p o u c h i s d i l a t e d . B . W h e n e m p t y .
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The Œsophagus consists of two parts, differing in structure and in function.
The first part is accessory to the pharynx, and might perhaps be more correctly
described as part of it, since it plays an important part in the mechanism of feeding
(17).
It is a dilatable chamber, situated between the cornua of the pharynx, and
lying in a horizontal plane, that is, at right angles to the pharynx and proboscis.
The superior and lateral walls of this pouch are formed by a quadrilateral plate of
chitin. This plate is concave downwards, and its four angles are turned down-
wards towards the pharynx, and are connected with it by a thin membrane which
completes the chamber. This membrane is attached to the margins of the chitin-
ous plate above, and to the cornua of the pharynx and to the upper margins of the
two pharyngeal plates below. The second part of the œsophagus turns directly
backwards from the posterior angle of the first part, through the brain, occipital
foramen, and neck, into the thorax. It terminates just behind the thoracic inlet
in a small dilatation, in the dorsal aspect of which there is an aperture, by which
the œsophagus communicates with the proventriculus. The intracranial portion
of the œsophagus has a thin semi-membraneous wall, but as it passes backwards
the chitin thins, and is replaced by a layer of small round cells, set on a basement
membrane.
The œsophageal diverticulum is a small bilobed sac, which lies in the second
abdominal segment, to the left of the middle line. It is connected with the œso-
phagus by a fine duct, which arises from the posterior end of the terminal posterior
dilatation of the œsophagus, and runs backwards beneath the proventriculus and
between the salivary glands. The walls of the diverticulum are composed of deli-
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cate interlacing fibrils, in the meshes of which there are many round cells. The
duct has the same structure as the thoracic part of the œsophagus, with which it is
directly continuous, the distinction between them being only marked by a narrow-
ing of the lumen.
The œsophageal diverticulum is usually distended with gas. Sometimes it
is found to contain blood, but it apparently does not function in the same way as
the corresponding organs of the mosquito, since it is frequently found to be empty
in specimens dissected immediately after feeding.
The Proventriculus extends throughout the thorax, lying between the lateral
expansions of the medifurca and on the mesophragma, and terminates in the second
segment of the abdomen by joining the mid-gut. Its anterior portion is broad, flat,.
and has a mammillated surface ; the posterior portion contracts to form a simple
narrow tube. At its anterior end there is on each side a wide wing-like expansion,
the prominent anterior angle of which is produced forwards and outwards into a
small pointed tubercle. It should be noted that the proventriculus lies throughout
on a higher level than the œsophagus and its direct continuation, the œsophageal
diverticulum ; the opening into the œsophagus is on the ventral surface of the pro-
ventriculus, a little behind its anterior end. The mammillated appearance of the
surface is produced by folds in the wall; these are arranged somewhat irregularly,
but there are usually two rows of small elevations to each of the surfaces, and one
row in each lateral margin. The lumen of the proventriculus is a transverse slit,
which becomes rounded in the posterior portion. The wall of the proventriculus
is composed of a layer of round or oval cells, attached to a stout basement mem-
brane. These cells, however, have the power of forming a large amount of secre-
tion, which is collected in the cells towards their free margins, and eventually
extended into the lumen. The appearances seen on section consequently differ
widely at the various stages of this process. When the secretion is fully formed but
not yet thrown off, the cells are elongate, and are arranged in rather indefinite rows.
The nuclei are round or oval, and stain deeply. The borders of the cells adjoining
the lumen are converted into a structureless hyaline cosinophil mass, in which the
distinction between adjacent cells is lost. This hyaline layer is carried into all
the folds of the wall, and renders the mammillation of the surface more marked.
At a later stage this secretion is thrown off, and the lumen is found to be filled with
a coarse granular mass, which takes up the cosin stain only faintly. The
cells are flattened, ,and their nuclei lie close to the basement membrane, with their
long axes parallel to it. The surface of the proventriculus is then not so mammil-
lated, the lumen is less flattened. At a still later stage the lumen is found to be
empty, and the wall closely resembles that of the œsophagus, being composed of a
simple layer of round cells, the inner ends of which project into the lumen. These
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remarkable changes are related to the feeding of the fly, and one of the same nature
as those described by Schaudinn (18) in the mosquito.
The Mid-gut is the chief digestive part of the alimentary canal and is the only
part in which blood is found in a recently fed insect. It lies in the second and
third segments of the abdomen ,and is embedded in a mass of fat body, to which,
and to the adjacent coils of the Malpighian tubes, it is closely bound down by a
rich supply of tracheæ. It is oval or pear-shaped, the narrow anterior end, which
is generally a little dilated when the gut is full of blood, merging indefinitely with
the proventriculus, while the posterior end is sharply demarcated from the rest of
the gut by the insertion of the Malpighian tubes. The wall of the gut consists of
a single layer of long cylindrical cells, arising from a thin basement membrane,
and two thin layers of muscle fibres, the inner layer being circular, the outer layer
longitudinal. The cells, with their basement membrane, are collected throughout
the cavity into small villi, so that the mid-gut has a stellate appearance on cross
section. The cells lying between the villi are typically columnar and are regularly
arranged, while those on the sides of the villus are more elongate, their attached
ends being compressed. The appearance of the protoplasm between the nucleus
and the free border of the cell varies in different preparations. In some it is clear,
hyaline, and eosinophil, while in others it is vacuolated, and the border of the cell
is frayed out, the cells having discharged their secretion after the manner of goblet
cells. The lumen of the gut often contains large numbers of degenerated free
nuclei, evidence of the degeneration of the epithelium. There are many small
round cells, by means of which the columnar layer is regenerated, lying between the
attached ends of the cells; these are most conspicuous when the large cells
have discharged their secretion.
The Hind-gut commences at the origin of the Malpighian tubes and is conti-
nued backwards as a simple narrow tube. The first part, immediately behind
the mid-gut, is somewhat dilated; the succeeding narrow portion is bent upon
itself, passing first from left to right, then to the left again, bending upwards. The
gut then dilates to twice its previous diameter, and runs directly backwards to
join the rectum. The wall consists of a single layer of short cubical cells, on a
basement membrane, and a few scattered bands of circular and longitudinal muscle
fibres. Although there is no difference in structure, the anterior narrow portion
may for convenience be termed the Ileum, the dilated posterior portion the Colon.
The dilated anterior portion of the Ileum, in a considerable proportion of cases,
has been found distended by two or three large gregarine cysts. The Colon
frequently contains many large reddish granules, the remains of blood.
The Rectum is separated from the colon by a slight constriction of the gut.
It is pear-shaped, the narrow end opening beneath the anal plates. The structure