NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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The Lacinia is a short peg-shaped projection arising from the inner edge of the
galea at its base. The two laciniæ project inwards opposite to one another at the
level of the articulation of the mandible, and assist in supporting the membrane at
the base of the skull.
The Stipes, which is directly continuous with the galea, is a much convoluted
piece, lying behind the pharynx and extending upwards to a point about midway
between the base of the mandible and the occipital foramen. It may be described
as consisting of a vertical rod, flattened transversely, and bent outwards in its
lower third, then inwards and again outwards at its upper end, where it articulates
with the cardo. The lower third of the inner edge is turned forwards, and there
is a blunt projecting spur on the middle of the outer edge. On the posterior
surface of this plate, at its lower end, there is a well-marked ridge, into which the
central strand of the galea runs, and to which the palp is attached.
The Cardo is a small wedge-shaped piece which connects the maxilla with the
cranial wall. Its base articulates with the stipes, while its apex, which is turned
outwards, rests in a notch in the thickened edge of the epicranium bounding the
central space. (Plate III, Fig. 13.) The cardo lies behind the base of the intracra
- nial tunnel, and is crossed by the attachment of this tube to a curved rod, to be
subsequently described.
The Labrum (Plate II, Fig. 7) is a thin spatulate slip, which is fused, except
for a short interval at its upper end, with the epipharynx. When the two are
separated by dissection, it is seen that the labium does not extend quite to the tip
of the epipharynx and that its margins are extremely thin and ill-defined. It is
attached to the clypeus by a narrow Y shaped slip of brown chitin.
The Epipharynx (Plate II, Fig. 7) is shaped like a broad two-edged sword. It
is directly continuous at its base with the anterior of the two chitinous plates which
form the pharynx, and has on its posterior surface a deep groove, the edges of
which are incurved so as to include about four-fifths of a circle. This groove, when
opposed to the hypopharynx, forms the canal up which blood is sucked. The tip
of the epipharynx is blunt, and is occupied by three small tubercles, two of which
are lateral and anterior, while the third is situated at the extreme tip. These
tubercles consist of a large number of small blunt teeth, arranged in rows in a
radiating manner. The edges of the epipharynx are fine and sharp, and show an
oblique striation, internal to the edges, and there is on each side a strong sup
- porting band of fibres running in a longitudinal direction, and internal to this,
between it and the canal, there is a small amount of cellular tissue, well seen in
cross section. (Plate II, Fig. 3.)
The Hypopharynx (Plate II, Fig. 8) resembles the epipharynx in shape, but is
more slender, and tapers more towards its point. It is pierced throughout its
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length by the salivary duct, which opens at its tip. The edges of the blade con-
tract sharply at the base, where the duct opens into the salivary reservoir. Th
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hypopharynx consists of two laminæ, which are readily separated in dissection.
The anterior and larger of these is continuous with the posterior plate of the
pharynx and with the salivary reservoir, while the posterior lamina is attached
to the cornua of the labium.
The Labium (Plate II, Figs. 4 and 6) has a somewhat complicated structure.
It is partly composed of thin chitinous plates, and partly of a flexible membrane,
which connects the chitinous parts so as to enclose a tubular space, in which there
are contained certain muscles and cellular structures. The distal half of the labium
is divided into two labellæ, on the opposing surfaces of which the arthrodial
membrane is replaced by a pseudo-tracheal membrane, similar to that of the house
fly. The loose connection of the chitinous plates, by means of the membrane,
permits of a considerable degree of movement of the labellæ on the labium and on
one another. The anterior surface of the labium is deeply grooved for the recep
- tion of the piercing mouth parts. The main support of the upper half of the labium
is a large shield-like plate, aptly termed by Meinert the scutum. This piece is
concave forwards, and forms the posterior and lateral walls of the upper half of
the organ. The narrow upper end is produced into a pair of cornua, which lie
behind the salivary receptacle and are attached to the non-chitinized membrane,
which, as will be seen subsequently, completes the base of the skull. The arched
upper border of the scutum has, on each side, a row of six long black macro chaetæ.
In the middle of this border a pair of short, pointed processes, called by Meinert
the cardines, project downwards.
In front of the scutum, and of course, separated from it by the muscles lying
between the membraneous folds, there is a somewhat ill-defined chitinous plate
which forms the floor of the groove in which the piercing parts lie. This plate is
also concave forwards, though its contour differs considerably at different levels.
The membrane which completes the wall of the labium in this region is attached
to the sides of this trough-like piece and to the corresponding sides of the scutum,
and is folded forwards, to a degree varying in different preparations, over the
groove in the anterior surface. In this way the membraneous wall of the labium
together with the maxillary palps, which lie in front of the upper part of the
labium, entirely conceal the rest of the mouth parts. Below the level of the
scutum, and external to the cardines, there is on each side an oval plate, called by
Meinert the Stylus Extensoers labellœ, and by Hansen the Lamina articuli secundi
labii. On the internal surface of this plate there is a thick dentate ridge, which
curves slightly inwards, so as to rest in the angle on the lower border of the scutum
external to the cardo. The pointed lower end of the plate fits into a notch in the
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chitinous framework of the labellæ, when the latter are brought parallel to the
surface on which the insect rests.
The Labellœ are oval lobes, attached to one another behind, but diverging in
front. They are set on the labium at an angle of about half a right angle, so that
their long axis points upwards and backwards from the labium. Their outer
surfaces are mainly membraneous, but are stiffened by the development, on each
side, of two thin plates of chitin along the border. The upper one of these plates
lies in the projecting posterior angles of the labella, and is square-shaped. Its
lower side is produced into a narrow spur, which runs forward across the outer
surface, and bears a row of six or seven short but stout black bristles. The anterior
side of the plate is incurved below this spur, so as to form a notch for the reception
of the " stylus." The lower plate is about twice the size of the upper, and lies in
the long axis of the labella. Its upper and anterior sides are thickened and ridged
for muscle attachment. Many short, stout, black hairs are scattered over the outer
surfaces and posterior borders of the labellæ ; they are especially numerous in the
larger chitinous plate. No special sense organs have been found in connection
with these setæ, although, from the manner in which the insect uses its labellæ,
one would expect to find something analogous to the tactile organs in the pro
- boscis of the house fly. The membrane attached to the scutum is produced down
- wards, and holds together the various chitinous plates described. It is continued
for a short distance round the anterior and posterior borders of the labellæ, but is
replaced, on their inner and opposing surfaces, by a pseudo-tracheal membrane.
This membrane is clear, transparent, and apparently structureless. Its surface is
traversed, on each side, by twenty-six deep grooves, which converge from the
posterior border inwards, so that, accommodating themselves to the oval shape
of the labellæ, the upper ones run downwards and inwards, the lower ones upwards
and inwards, while the middle ones are directly transverse. These grooves lead to,
but do not actually communicate with, a short but broader longitudinal groove
which runs along the inner border of each labella. When examined under a high
magnification, it is seen that these grooves are lined with, and presumably kept
open by, a series of minute incomplete chitinous rings, set side by side, transversely
to the axis of the groove. Each ring is about three-quarters of a circle, and each
end of it is thickened into a little knob, so that the edge of the groove has a beaded
appearance.
The labellar muscles arise from the scutum, and are inserted into the lower
chitinous plates. They consist of three pairs of muscles, not definitely separated
at their origin. The most posterior bundle runs directly downwards to the
" stylus " that is, the oval plate lying external to the cardines. The other fibres
are inserted into the upper and lower chitinous plates of the labellæ, some fibres
c
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being traceable almost to the tip. Contraction of all these bundles together will
retract the
labellæ, while contraction of the anterior fibres only will bring the
labellæ more parallel to the surface on which the fly is resting.
Two trachea enter the base of the labium. They expand at the region of th
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scutum into two comparatively large air sacs, from which the trachea can be traced
downwards for a short distance. There is also a pair of small nerves, lying internal
to the muscles.
Cellular Structures.—The membrane, and the chitinous plates developed in
it, are lined throughout by an indefinite layer of small round cells. The anterior
plate which forms the floor of the groove in which the piercing mouth parts lie is
lined by a single layer of small regular cubical cells, which extend forward to
the junction of the plate with the membrane. The pseudotracheal membrane
is lined throughout by a single layer of similar, but larger, cubical cells. There
are a few fat cells scattered about the interstices of the organ.
The space between the pseudotracheal membrane and the outer wall of the
labella is almost entirely filled up by a large mass of cells, which probably repre
- sent a labial salivary gland. (Plate II, Figs. 3 and 4.) These cells are remarkably
large, averaging about 40µ in their long axis. They are oval or pear-shaped and
are arranged with their narrow ends pointing towards the pseudotracheal mem
- brane, the innermost ones lying in contact with the cubical cells. The nucleus is
round or oval, and is situated at the broad end of the cell. The chromatin appears
to be separated into a large number of small rods, mainly aggregated about the
periphery. In many cells there is a second nucleus, usually very small, situated
at the narrow end of the cell. The protoplasm contains many fine granules, and
is, in a small proportion of the cells, vacuolated. In each cell there is a crescentic
or sickle-shaped area, near the nucleus, where the protoplasm is non-granular and
does not stain well, being only faintly tinged by cosin. In a few cells a distinct
vacuole can be made out inside this area.
It is evident from its structure that the labium exercises other functions as
well as the obvious one of protecting the other mouth parts. The pseudotracheæ
differ in no essential particular from those of the house fly, except that there is no
direct communication between them and the alimentary canal. This is not, how-
ever essential. If fluid is drawn inwards along the pseudotracheæ by capillary
attraction, it will come to lie in contact with the hypopharynx and epipharynx,
the distal ends of which lie between the two internal longitudinal pseudotracheæ.
It may then, either by capillary attraction or by contraction of the pharyngeal
muscles, be drawn into the pharynx. I have never myself seen Hamatopota drink,
but it appears to be generally agreed that Tabanidæ do so. Austen (13), quoting
Pontschinsky, states that in Russia horse flies are destroyed in large numbers by
pouring petroleum on the pools to which they resort.
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There are no ducts to be made out in connection with the labial gland cells
.Their secretion is probably exuded by means of the cubical cells lining the pseudo-
tracheal membrane.
(c) The Intracranial Tunnels.
These remarkable structures lie in the lower part of the head. Each consist
of a stout hollow chitinous tube, which opens on to the anterior and posterior sur
-faces in the positions already indicated in the description of the external char
-acters. The two tubes lie in a horizontal plane, and converge somewhat towards
one another from before backwards. The main portion of the tube is cylindrical
and is narrowest about its middle. The anterior end is funnel-shaped, and has a
wide attachment to the epicranium in the region of the clypeus and gena. The
posterior end, which lies in front of the cardo of the maxilla, is more irregular
and is produced into three tubercles. One of these projects inwards, while the
others are firmly attached by strands of chitin to the thickened lateral edge of the
epicranium, and to the curved rod which extends from the region of the orifice to
the occipital foramen.
From the upper surface of each of these tubes there arises a thin trabecular
membrane. This is triangular in shape, and is attached in front to the epicranium
at the junction of the clypeus with the gena, extending as high as the antennal
socket. The free upper margins of the two trabecular plates separate off a median
compartment in this region of the head, in which the pharynx and the pharyngeal
dilator muscles are contained.
These intracranial tunnels resemble those described in Chironomus (14) and
in Anopheles (7). They are of the nature of supporting buttresses, and the fact
that they are hollow has probably no more singificance than that, weight for weight,
a hollow cylinder is stronger than a solid rod. Their special functions will be
discussed in relation to the mechanism of feeding.
(d) The Structure of the Base of the Skull. (Plate III, Fig. 13.)
The chitin of the head capsule is deficient in a space extending from the oc
-cipital foramen to the bases of the mandibles, this area being filled in by a flexible
non-chitinized membrane. The space is bounded in front by the cornua of the
mandibles and the lacinæ of the maxillæ, and laterally by the thickened edge of
the epicranium, and by a specially developed curved rod. The central part of
the space is occupied by the pharynx, on each side of which lie the stipes of the
maxillæ.
The lateral curved rods, previously referred to, extend from the posterior
orifices of the intracranial tunnels to the occipital foramen. They are flattened
c 2
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and shaped like boomerangs. The lower end of each rod is produced into a tubercle
which projects inwards into the central space, while the upper ends are
produced inwards in front of the occipital ring, and are united by a few strands of
fibres, so as to form the anterior boundary of the foramen. About the middle of
each rod there is a constriction, and above this the rods are fused with the
epicranium.
The upper and posterior border of the occipital foramen is formed by a thick-
ened arched ridge, continuous with the posterior wall of the head capsule.
( e) The Homology of the Head.
It is of course somewhat difficult to assign the component parts of the head
to their respective segments with precision from a study of the adult insect, and
especially when, as in this case, few of the sutures marking the lines of fusion of
the segments can be made out. The parts in Hœmotopota, however, represent
a comparatively early stage in the adaptation to the suctorial habit, and by com-
parison with other insects of more elementary form it is possible to define the
extent of the various somites with some degree of certainty. The detailed
exposition of the present views on the segmentation of the insect head given in
Berlese's recent work (15) is adhered to in the following description. Berlese
regards the insect head as derived from six segments, the third of which is
wanting in the adult stage, except in Campodea. The first somite is the preoral
one, and bears the eyes. The labrum and clypeus belong to it. The second
segment bears the antennæ, and is well demarcated in this insect as the elevated
antennal plate. The sternite of this segment is represented by the anterior parts
of the intracranial tunnels and trabecular plates. The fourth segment bears the
mandibles. Its tergite lies behind the antennal plate and extends between the
eyes to the vertex. The sternite is represented by the genæ, the posterior
border, and an indefinite area of the posterior surface of the head; its extent may be
inferred from the area of attachment of the mandibular muscles. The mandible
itself shows the least possible differentiation from the mandibulate type which is
consistent with a blood-sucking habit. It is articulated to the gena, and, as will
be seen later, the arrangement of its muscles indicates that it is capable of a rela-
tively wide range of movement. Comparing it with Blatta and Anopheles, it
illustrates very well the process of adaptation to function. The fifth segment
bears the maxillæ. Its tergite occupies the lateral areas in the posterior surface
of the skull, external to the sutures ; the sternite is represented by the bases of the
intracranial tunnels. The primitive form of the maxilla has already been dis-
cussed. The tergite and sternite of the sixth segment are not separated. The
tergite occupies the median area of the posterior surface of the head, between the
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sutures, and encloses the occipital ring above. The sternite has undergone con-
siderable modification in the development of the membraneous area at the floor
of the skull. The palatal membrane, representing the gular region of other insects,
and the cornua and scutum of the labium, representing the submentum and
mentum, belong to the sixth sternite. The cardines of Meinert and the
stylus may represent the cardines and stipes of a primitive second maxilla.
II.—The Thorax. (Plate V, Fig. 19.)
The head is attached to the thorax by a short neck, which, on account of the
concavity of the posterior surface of the head, is not easily seen on the living insect.
There are no cervical sclerites.
The Thorax, as seen from above, is ovoid in shape, the pointed scutellum pro-
jecting backwards above the first abdominal segment. It is rather narrower than
the head, and is continuous with the abdomen without any " waist." It is dark
tawny brown in colour, with a dorsal median light stripe, continuous with that on
the abdomen, and two lateral stripes, the inner of which extends only through the
anterior half. It is covered throughout with short downy hairs, which are aggre-
gated into little tufts on certain of the sclerites, especially about the neck and
sides.
There is no differentiation of the thorax into three regions, and the separate
parts can only be satisfactorily studied by dissection.
(a) The Prothorax.
The sclerites of the prothorax close in the convex anterior end of the thorax,
and can only be satisfactorily seen after removal of the head. There are seven
paired and two unpaired sclerites.
On the ventral aspect, immediately behind the thoracic inlet, there are three
pairs of sclerites which belong properly to the neck. These " jugulares " lie in a
loose anthrodial membrane, and are not united directly to one another in the
middle line. The first, which is the largest, is dome-shaped, and bears a tuft of
dense hairs. It is free in the anthrodial membrane. The second piece is a trans-
verse oblong slip, attached externally to the epimeron ; it bears no hairs and is
not recognizable in the external surface of the thorax. The third jugular resembles
the second, except that it bears hairs, and is attached also to the episternum.
It lies in front of the articulation of the foreleg. The looseness of the thoracic
wall in this region permits of considerable distension during the act of feeding.
The remaining sclerites are readily homologized with those of a primitive
thoracic segment. The ventral sclerite is divided into a pair of Presterna a
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Sternum, and a Sternellum. The Presterna lie on the inner sides of the spaces in
which the forelegs are articulated, and behind the jugulares. They are quadri-
lateral in shape, and are firmly united by their inner borders, from which a thick
ridge projects upwards into the thoracic cavity. Their anterior borders form a
continuous thickened arch, to which the arthrodial membrane is attached.
The Sternum is a small semicircular plate, which is wedged in between and
behind the presterna, which project backwards on each side of it. The sternel-
lum is heart-shaped, and is inset between the diverging inner borders of the sterna
of the meso-thorax. The sternum and sternellum do not bear hairs, and are easily
recognizable on the external surface of the thorax.
The Episternum lies external to the articulation of the foreleg, and is firmly
attached to the meso-thorax. The Epimeron lies in front of the episternum, and is
closely adherent to it and to the præsternum. Both lateral sclerites are rounded
and convex, and are conspicuous on the external surface, having dense tufts of
hairs. The Pronotum is represented by two small convexities on the outer
edges of the prescutum ; they are continuous with the epimera.
Passing between the ventral and lateral sclerites there is a somewhat complex.
arrangement of fibrous bands, representing an antefurca, which serves both to
compensate for the laxity of the thoracic wall in the region of the jugulares, and
to provide for muscle attachments. (Plate IV , Fig. 14.) It consists of two parts,
arising from the sternum and sternellum respectively. The anterior fibres arise
from the arched border of the sternum, from the posterior fourth of the median
ridge between the presterna, and from a thick transverse strand which separates
the sternum from the sternellum. The fibres are collected into a broad-arched
transverse band, which, as it runs outwards, contracts to form a stout round cord.
The posterior fibres arise from the thickened and ridged lateral borders of the
sternellum, and run outwards and forwards. All these fibres run towards the
junction of the epimeron and episternum, and are mainly attached to the recurved
anterior margin by the latter sclerite. They form a strong supporting buttress
for this part of the thorax.
(b) The Mesothorax.
This constitutes the main part of the thorax. The tergite, which occupies.
the whole dorsal surface of the thorax, is separated into a Prescutum, Scutum,
and Scutellum. The two former pieces together make up the greater part of the
tergite, and the separation between them is only indicated by a short lateral
furrow anterior to the wing base. The Scutellum is a stout triangular piece,
with rounded margins, which overlaps the first abdominal segment, and conceals
the metathorax from above.