NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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and to the anterior borders of the sides of the fulcrum, converting it into a closed
cavity, which is occupied by the pharyngeal muscles. Posteriorly it runs down
-wards to the lower end of the rostrum, enclosing the muscles, tracheæ and sali
-vary duct, which lie posterior to the fulcrum. The membrane is thrown into
numerous folds when the proboscis is withdrawn.
The Palps are attached to the membrane on the anterior surface of the ros
-trum, in its lower third. Each arises from a small raised and partially chitinized
area, on the outer projecting angle of which there are two large macrochetæ. The
palps are clavate, moderately long, and clothed with numerous very fine recum
-bent hairs and many large pigmented chetæ. The palps are folded back against
the anterior surface of the rostrum when it is withdrawn, and are erected when
it is extended.
The Pharynx (Plate V, Fig. 22) is composed of two chitinous plates, normally
superimposed and in contact with one another. The posterior of these is the
posterior plate of the fulcrum. The anterior plate is of the same size and shape,
but is somewhat thinner. It has a stout median ridge running down its anterior
surface. The sucking action is produced, as in other diptera, by the separation of
these two plates.
The sucking portion of the pharynx does not extend quite to the lower end of
the rostrum. At the extreme end the food canal consists (Plate IV, Fig. 20) of a
concave posterior plate, only partially chitinized, and a membraneous anterior
wall. A little higher up (Plate V, Fig. 20) two lateral plates are developed, and
the attenuated end of the anterior plate is seen appearing in the anterior wall.
This arrangement permits of flexion at the junction of the rostrum and haustel
-lum when the proboscis is withdrawn.
The Salivary Duct in the upper portion of the rostrum is a simple tube of thin
chitin, lying in the middle line on the posterior surface of the pharynx. At the
lower end the walls are membraneous, and the duct lies embedded in a dense
mass of cellular tissue. A short distance above the point at which the duct
emerges from the haustellum there is a small valve resembling that of Musca.
The Muscles of the Rostrum (Plate V, Fig. 22)) resemble those in Musca, though
some of those described by Kraepelin have not been traced. Their function is to
retract the proboscis and to assist in extending it.
The Dilators of the Pharynx (D. ph) arise from the sides of the fulcrum through
-out its upper two-thirds, and from the adjacent membrane covering in the anterior
surface. They run inwards and backwards to be inserted into the anterior plate
of the pharynx, most of the fibres being attached to the median ridge on that
plate.
5
The Flexors of the Haustellum (F. H.) arise from the sides of the occipital
foramen, and are inserted into the distal ends of the labral apodemes; they
lie external to the tracheal sacs in the rostrum.
The Retractors of the Haustellum (R. H) are long round bundles, situated
behind the pharynx and on either side of the salivary duct. They arise from
the inner surface of the posterior wall of the head cavity, and, passing downwards
behind the brain and external to the occipital foramen, are inserted into the
thickened mass of chitin at the base of the theca, by a membraneous expansion.
The Extensors of the Haustellum (Ex. H.) arise from the cornua at the distal
end of the fulcrum, and run obliquely upwards and outwards to the proximal end
of the labral apodemes.
The Retractors of the Fulcrum (R. F.) arise on each side from the internal edges
of the genæ, and are inserted into posterior cornua at the proximal end of the
fulcrum. They lie in a fold of membrane at the anterior lateral angle of the
rostrum.
The Tracheæ resemble those of Musca. There are two large tracheal sacs,
lying on either side of the salivary duct. In dissection these sacs can frequently
be separated out while still distended with air, appearing as large sausage-shaped
white bodies. In sections they are usually constricted, and so come to have a
corrugated outline and a relatively thick wall.
The Haustellum.
This comprises the true mouth parts, viz. , the labium, epipharynx, hypo-
pharynx and the labium. Of these only the labium can be distinguished without
dissection, the other parts being concealed in the labial gutter. The labium con
-sists of two pieces of chitin, the theca and the labial gutter, and a connecting
membrane.
The Theca, which forms the main part of the labium, is a hollow scaphoid
trough, much deeper in its upper part than below, and having its open side directed
forwards. The margins of the space so formed are thickened into strong ridges,
which run parallel to one another throughout the length of the labium. The
upper end of the theca is somewhat truncated and narrowed, and contains at the
extreme end a Y shaped median ridge for muscle attachment. The upper end of
the labial gutter is fused with the converging margins at this end of the theca.
At the lower end there is another Y shaped median ridge, the lateral arms of which
are reinforced by fibrous bands from the lateral ridges of the theca, and which pass
outwards, downwards and slightly forwards, to articulate, as will be seen later,
with the furca of the labella. Near the tip of each of these rods there is a small
.
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barb-like projection. In preparations treated with potash for a prolonged time,
it is seen that these projecting rods are not of equal thickness throughout, but that
there is, about the middle of their length, a weaker portion which probably func
tions as a joint.
The Labial Gutter (L. G.) is a thick deeply pigmented rod, grooved on its
anterior surface in such a way that it appears U shaped in transverse section. It
lies in front of the theca, and is connected with it by a rather tough membrane,
which stretches between the lateral ridges of the theca and the borders of the
groove. At its upper end it is intimately fused with the theca so that no move
- m e n t i s p o s s i b l e b e t w e e n t h e m . O n t h e u p p e r h a l f o f i t s p o s t e r i o r s u r f a c e t h e r e
is a thin chitinous expansion, projecting backwards into the cavity of the theca,
and reaching almost to the posterior wall, which gives origin to an important pair
of muscles. This expansion, when seen in profile, has the shape of the keel of a
racing yacht, and corresponds exactly to the similar structure in Stomoxys. At
the lower end of the labial gutter the sides are produced downwards a little beyond
the bottom of the trough and terminate in gently rounded knobs which converge
slightly towards one another. In this way a simple socket is formed, into which, as
will appear later, a pair of labellar rods is articulated. (Plate II, Fig. 7.)
The Hypopharynx (Plate II, Fig. 2) is a slender slip of chitin arising from the
base of the labial gutter and terminating in a fine attenuated point. It lies at the
bottom of the gutter throughout and terminates just above its lower end, being
slightly shorter than the labrum-epipharynx. It is pierced throughout by the
salivary duct. When seen in cross section it is found to consist of two distinct
laminæ, separated by a small amount of cellular tissue. The inner lamina is cir
- c u l a r , a n d c o n s t i t u t e s t h e d u c t , b e i n g d i r e c t l y c o n t i n u o u s w i t h t h e s a l i v a r y d u c t
in the rostrum and head. The outer lamina is flattened at the tip, but becomes
rounded as it passes upwards in such a way as to accommodate itself to the bottom
of the trough in which it lies. The base of the hypopharynx is enlarged into a
thick knob of densely pigmented chitin, which projects upwards a little above the
upper end of the theca, to which and to the superimposed labrum-epipharynx it is
closely attached.
The Labrum-Epipharynx (Plate II, Fig. 3) is a lanceolate slip of chitin, flattened
at its apex, but expanding gradually in an antero-posterior direction towards its
upper end, so that at its base its depth is almost as great as its width. It is
attached, so loosely that it is easily bent forward in dissection, to the thick mass
of chitin at the upper end of the hypopharynx and labial gutter. As it passes
downwards it closes in the open side of the labial gutter, its bluntly rounded tip
lying in the hollow formed by the two projecting lateral tubercles. On its under
surface there is a deep groove, the overlapping sides of which gradually converge
7
backwards and inwards towards one another from the apex to the upper fourth,
where they meet and so form a closed canal. On cross section the labrum-epi
-pharynx, like the hypopharynx, is seen to consist of two distinct laminæ. The
outer of these composes the anterior surface and the posterior surface as far as the
edges of the groove, and in the upper fourth, where the groove has become a canal,
presents a continuous contour, convex in front and behind. The internal lamina
forms the wall of the groove and canal. At its tip it is semi-circular and concave
backwards, but as the edges of the groove close in it becomes more circular, until,
about the upper fourth, it forms a complete canal, the lumen. of which is continuous
with that of the pharynx in the rostrum. The inner and outer laminæ are united
with one another at the edges of the groove throughout, and are also firmly
attached to one another by a strong band of fibres lying in the middle line behind
the groove. In this way two lateral spaces are formed, one on each side of the
central canal; these are roughly triangular in shape, and contain a series of
muscles and some cellular tissue. The upper end of the inner tube projects a
little above the outer lamina and terminates in a thickened flange.
On each side of the groove of the labrum-epipharynx there is a row of minute
oval flattened tubercles, projecting into the lumen; three similar ones are situated
on the posterior surface of the flattened tip of the organ. These tubercles recall
the similar structures found in Stomoxys, but they do not bear spines, and no
suggestion as to their function can be offered.
The interval between the inner and outer laminæ of the labrum-epipharynx
is occupied by a series of muscle bundles passing, in a somewhat fan-shaped manner,
from the outer to the inner wall. They probably function as an accessory pump,
by dilating the lumen of the groove and canal.
The labral apodemes pass from the two upper angles of the labrum-epipharynx
upwards, and slightly forwards, into the rostrum. Each is a deeply pigmented
slightly sinuous rod, about two-thirds the length of the labrum-epipharynx. The
lower end is shaped like an arrow-head with a single barb, and is fitted into a small
round socket on the lateral edge of the labrum-epipharynx, where it contracts
sharply at the base. The upper end is flattened and expanded for muscle attach
-ment.
The relations of the labial gutter, hypopharynx and the labrum-epipharynx.
to one another are best studied in serial transverse sections. At its lower end
(Plate IV, Fig. 14) the labial gutter encloses a horseshoe-shaped space, at the
bottom of which the hypopharynx, which here is very small and circular in
outline, is seen. The front of the space is closed in by the flat tip of the labrum
-epipharynx. A little higher up (Plate IV, Fig. 15) both the labrum-epipharynx
and the hypopharynx are much enlarged, and the groove on the posterior surface
9
The Labella.
The Labella are joined to form a single organ, the distinction between the two
being indicated by a deep median groove on the anterior surface. The external
wall, forming the aboral surface, consists of membrane, strengthened in certain
areas by thin plates of chitin. It is supported by a horse-shoe shaped rod, the
furca, which also functions as a joint between the labium and labella. The
internal or oral surface, which when the labella are in use is expanded to form a
globular end to the proboscis, is formed by a simple pseudo-tracheal membrane.
The sides of the groove on the anterior surface are specially modified to form the
cutting organ of the fly.
The Furca (Plate II, Fig. 6) lies immediately below the Y fork at the lower end
of the theca, and encircles the posterior and lateral walls of the organ. Its distal
ends are thickened into small tubercles, just behind which there is on each side a
small spur projecting upwards and backwards, so as to form a small notch. The
projecting arms of the fork at the end of the theca rest in this notch, which form
the joint on which the labella are swung backwards and forwards.
The External wall of the labella is attached to the furca; in addition there are
three special plates, one posterior and two lateral, developed to give the furca, in
its movements, more control over the loose membrane. The posterior plate is
heart-shaped, and arises from the posterior part of the arch, running downwards
almost to the edge of the pseudo-tracheal membrane and ending in a blunt point.
The lateral plates are triangular in shape, the base arising indefinitely from the-
membrane near its edge about the middle of the external surface on each side.
The apex terminates in a fine hook, which is attached to the tubercle at the ex
-tremity of the lateral arm of the furca.
Besides these plates, which are definitely chitinized and do not bear hairs,
the external wall is stiffened in other less definitely outlined areas. There are two
heart-shaped ones as the extreme anterior end of the labella, the apices of which
are attached to the ends of the labellar rods, and two on the posterior surface, on
either side of and below the posterior plate of the furca. The lateral area between
these plates is thickened,along the extreme edge, where it joins the pseudo-tracheal
membrane. Except on the posterior and lateral plates, the external surface of the
labella is loosely covered with large and small pigmented hairs, arranged in an
irregular manner, some forming a loose fringe on the lower border. At the extreme
edge of the external surface the ordinary epidermal membrane is replaced by a
narrow band of scales. (Plate II, Fig. 8.) These are of three kinds. Immediately
below the fringe of hairs there are rows of small irregularly rounded scales, loosely
and irregularly set on the membrane, and about seven deep. These are succeeded
by a definite row of larger semi-circular scales, the straight edge of which projects.
C
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downwards. Below these there is ,another row of larger scales, resembling in shape
axe-heads with the basal part somewhat expanded. The broad ends of these scales
overlap the pseudo-tracheal membrane; from the narrower upper end of each
there arises a small hair, which projects downwards towards the pseudo-tracheae.
The Pseudo-tracheal membrane constitutes the oral surface of the labella.
hen the organ is extended for use the membrane presents a globular convex
surface, traversed by the pseudo-tracheal channels, which converge from the peri
-phery to the region of the teeth in the anterior median fissure. When at rest the
oral surfaces of the two sides are in contact with one another, the lower margin of
the organ being formed by the narrow scaly membrane of the external surface.
The membrane resembles that of the non-blood-sucking muscids. It is homogene
-ous, transparent, and apparently structureless. The channels are minute folds,
horse-shoe shaped in cross section, the edges being contracted so as to enclose
about three-quarters of a circle. The lumen is maintained by a series of fine chitin
- ous rings, closely set side by side, the extremities of each ring being turned out
-wards and backwards, so that when seen flat the edges of the channel have a
beaded appearance. There are twelve channels on each side arranged as follows.
Each channel arises from the edge of the pseudo-tracheal membrane, being over
-lapped at its origin by one of the hair-bearing scales of the external surface. The
anterior set of four converge towards the middle line to form one large trunk, which
dips inwards anterior to the teeth, at the lower end of the groove on the anterior
surface of the labella. The next five channels remain separate and run to the
middle of the oral pit. The posterior set of three unite to form a single larger
channel, which dips inwards posterior to the teeth. Finally all those from one
side unite to form a single channel, which, as it passes upwards, gradually looses
its chitinous rings and becomes replaced by a short duct with cellular walls.
The Biting Apparatns.
The remarkable biting apparatus of this fly lies in the groove on the anterior
surface of the labella, and is developed from their internal walls. It consists of
two sets of strong teeth, four on each side, certain serrated scales, two lateral
rods, which, for convenience in description, will be termed the labellar rods, and a
median piece of chitin which connects the rods and also functions as a tooth. These
structures are most conveniently described from above downwards.
The Labellar rods are of strong and deeply pigmented chitin. Each rod is
about one-third the length of the labial gutter, somewhat flattenel from side to
side, and has its upper end turned slightly outwards and forwards. At the junc
- t i o n o f t h e u p p e r
a n d m i d d l e t h i r d .
k b e r P i s o n t h e p o s t e r i o r s u r f a c e a s m a l l t u b e r c l e ,
11
Posterior to which the rod is considerably reduced in diameter and tapers to a
blunt point. The lower end of each rod is expanded and hollowed out so as to
form a shallow cup for the reception of the bases of the teeth. These rods lie on
either side of the bottom of the groove on the anterior surface of the labella, and
are attached throughout to the membrane forming the labellar wall. The tuber
-cles on their posterior surfaces fit into the end of the labial gutter, and, when the
labella are erected, press against the projecting tubercles of the lateral walls of the
gutter, the arrangement forming, in fact, a simple ball and socket joint. The
posterior thirds of the rods project upwards above the lower end of the gutter.
The Axial apophysis (Plate II, Fig. 4) is situated behind the lower ends of the
labellar rods, and binds them firmly together. It is shaped like a shield, the lower
end being produced to a sharp point, which lies a little below the bases of the
teeth. It thus forms the floor of the labellar groove at the lower end; above it,
the labellar rod.s are united by a tough membrane.
The Teeth (Plate III, Figs. 9 and 10) resemble rose thorns in shape except that
the apex is blunt, and the borders serrated near the apex. They are arranged
side by side in the antero-posterior axis of the labellum, arising from the internal
wall by expanded bases which lie immediately below the lower end of the labellar
rod. Their pointed ends project downwards in the lower end of the anterior
groove. The two middle teeth are larger than the other two, and are inserted a
little higher up, so that the upper limit of the bases of the four teeth shows a curved
contour, corresponding to the cup-shaped end of the labellar rod. The short
interval between the bases of the teeth and the labellar rod is occupied by a band
of strong fibrous tissue, which serves to retain them in position.
The Serrated blades (Plate III, Fig. 9) are minute leaf-like organs with deeply
serrated margins, which lie below the teeth. Each blade is a narrow lanceolate
slip of chitin, about 20 μ long by 5 broad, and with about seven serrations on each.
margin, each incision going nearly to the middle lino. They are arranged as
follows. There are ten tufts consisting of five or six blades each, the blades arising
at different levels and being superimposed. Each two tufts are carried on a
common bifurcated stalk, which arises between the teeth at the level of the upper
limit of their bases, and bifurcates at the lower limit. The tufts arising from each
common stalk diverge from one another below the teeth, and turn upwards super
- f i c i a l t o t h e p s e u d o - t r a c h e æ . T h e w h o l e o f t h e b l a d e s a r e t h u s c o l l e c t e d i n t o
twenty bunches, ten on each side, all of which turn outwards at the orifice of the
labella, forming a sort of brush at the tip of the proboscis.
Other Structures .—In addition to the teeth and serrated blades there are, certain
minor structures to be noted. On the stalk of each of the serrated blades there
are three pairs of short broad spines (Plate III, Fig.11)' which project downwards
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and slightly forwards. At the point of bifurcation each stalk bears a pair of
leaf-like scales, resembling the serrated scales, but having an entire margin and
a granular surface.
The Muscles of the Haustellum (Figs. 17, 18 and 19) are well developed, and fill
up the cavity of the labium. Their main function is to bring the biting appa
-ratus into action.
The Extensors of the Labellar Rods (M') arise from the sides of the keel of
the labial gutter, and run directly downwards, occupying the middle area of the
theca. They are inserted into the posterior third of the labellar rods.
The Retractors of the Furca (M") arise from the sides of the theca through-
out its extent, and from the thickened chitin at its upper end. The upper
fibres run downwards, the lower ones obliquely downwards and inwards to the
furca, and are inserted along its upper border and into the tuberosities at the end
of its lateral arms.
In addition to these muscles there is a small bundle of fibres arising in the
upper part of the theca which pass downwards on either side of the labial gutter,
in contact with the membraneous anterior surface of the haustellum. These are
inserted in the membrane, some distance above the theca, and probably act as
accessory retractors.
Cellular Structures .—The membraneous and chitinous walls of the proboscis
are lined throughout by an indefinite layer of the small round cells, the hypoder
-mis. In certain regions, chiefly on the posterior surface of the labial gutter and
at the bases of the teeth, this layer is replaced by a definite row of cubical chitino-
genous cells, usually separated from the pigmented chitin by a layer of fibrous
non-chitinized tissue. This fibrous layer is very distinct at the lower end of
the labial gutter.
In sections of the labella there is, if the organ was in the extended position,
a considerable interval between the cellular walls on each side. This space repre
- s e n t s t h e h æ m a t o c œ l . I t h a s n o d e f i n i t e b o u n d a r i e s , b e i n g , s t r i c t l y s p e a k i n g , a n
intercellular space. It is in this space that the blood collects when the labella are
distended.
The Labial Salivary Gland resembles that of Musca. It lies behind the lower
end of the labial gutter, and is enclosed in a thin membraneous capsule. It con
-sists of large granular uninucleated cells, round or oval in shape, each containing
a large vacuole with a chitinous lining. The ducts leading out of these cells have
not been traced.
The Tracheœ in the haustellum are continuations of the air sacs in the rostrum.
'There are two small lateral tracheæ of the ordinary type, lying among the muscles