NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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The Rostrum.
The rostrum has the shape of a truncated cone, broad end uppermost. Its
outer covering consists of a flexible membrane which is continuous at the
epistomal orifice with the chitin of the head capsule. At the distal end it is
attached to the wall of the haustellum. Within this membrane there is a
somewhat complex chitinous structure termed the fulcrum, part of which forms
the pharynx or sucking pump of the fly. This structure affords the necessary
rigidity to this part of the proboscis, and gives attachments to the various,
muscles which act on the joint between the rostrum and the haustellum.
The fulcrum resembles, to use Kraepelin's admirable simile, a Spanish
stirrup iron. It consists of a posterior plate, two lateral plates, and an anterior
arch. The posterior plate is oblong, slightly concave forwards anteriorly, and
narrower above than below. Its upper and lower borders are incurved, the
lateral angles of the former being produced upwards into a pair of stout cornua,
which are also intimately connected with the lateral plates. The lateral plates
are roughly triangular, and are continuous by their longest sides with the
posterior plate, projecting forward at right angles from it; their upper sides
are deeply incurved. The anterior side, which is much longer than the upper
one, is also incurved, but the contour is abruptly broken at the junction of
the middle and lower thirds by the projection forwards of a sharp spur, the
border being more concave below than above this point. The anterior arch is
situated at about the same level as the cornua on the posterior plate, and passes
between the upper anterior angles of the lateral plates; it is a thick ridged
bar, which gives attachment to part of the dilator muscles of the pharynx, and
is the part which is pressed against the clypeus when the rostrum is rotated in
retraction. It lies in, and is continuous with, the membraneous anterior wall
of the rostrum.
The pharynx, or sucking pump, of Musca resembles that of the Orthor-
raphic flies, consisting of two superimposed chitinous plates, which can be
separated by dilator muscles. The posterior of these plates is the one described
above as the posterior plate of the fulcrum; the anterior plate, which lies in
close contact with it when the organ is not in use, is of similar shape and
extent, but is much thinner, and is drawn away from the other by the action
of the dilator muscles. It has a thick ridge of chitin running down its middle
line, to which the muscle fibres are attached. The anterior plate, however,
is not simply pulled away from the other so as to leave a crecentic interval
between the two, but is bent forwards on each side as the muscles pull
the median ridge forward, so that the resulting cavity tends to be tri-radiate,
an arrangement which recalls that of the pharynx of the mosquito. The
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fulcrum lies in the anterior part of the rostrum; behind it there is a compara
-tively large space, which in the extended position is occupied by two large
cylindrical air sacs, which are of great importance in the mechanism of
extension.
In the middle line between these there is the salivary duct, and on either
side and behind there are numerous muscles, which act on the joints above and
below. These will be dealt with more particularly later.
The fulcrum does not extend quite to the lower end of the rostrum, and
communication between the food canal in the haustellum and the pharynx is
established by means of the intervention of a small chamber resembling a
miniature pharynx. This has been elaborately described by Kraepelin, who,
however, did not give it a definite name. Lowne termed the chitin forming
its wall the " hyoid sclerite." When describing the corresponding structure in
Lyperosia I suggested the name buccal cavity as having the double advantage
of indicating at once the position and the homology. The chamber consists, in
Musca, of a small triangular piece of thin chitin, arranged with its long axis
pointing towards the haustellum, and its broad base between the lower cornua
of the fulcrum, so that it occupies the interval between the two divisions of the
proboscis. The lateral angles are bent forward, and are connected with one
another by means of a thin membrane, which is also attached to the anterior
plate of the pharynx. The epipharynx and hypopharynx terminate in the
region of this cavity in a manner which will presently be described.
The Haustellum.
The main bulk of the haustellum consists of the labium, or fused second
maxillæ. The other parts are concealed in a groove on its anterior surface in
the normal conditions of the parts.
The labium is roughly cylindrical in shape, thickest in the middle, and
tapering a little at each end. Its wall consists of two pieces of chitin united
by a loose membrane, which is attached to the lateral edges of each. Of these
the posterior, termed the mentum (Plate II, figure 3) and homologous with the
mentum of other insects, is much the largest; it has the shape of a shallow
trough, with the concavity directed forwards, and is contracted at its lower
end. It forms the posterior and lateral walls of the labium, and has thickened
lateral borders. The arrangement of the parts at its lower end should be parti-
cularly noted, as they undergo important modifications in the blood-sucking
Muscids. The thickened lateral borders converge towards the distal end, and
become somewhat attenuated; the distal border, which is about one-third the
length of the lateral sides, has in its middle portion a deep square-shaped
incision, on the opposing sides of which there are two longitudinal rod-like
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thickenings of the chitin, extending the whole length of the sides of the incision.
Continuous with these there is on each side a second rod of the same length as
the sides of the incision; these two rods pass downwards distal to the lower
border of the labium, and diverge from one another. They have an import-
ant connection with the chitinous arch which supports the labella behind. If
they are examined in thoroughly cleared preparations, it will be seen that
although they are continuous with one another, there is a distinct thinning of
the chitin at the level of the distal border of the mentum, which suggests that
the proximal and distal portions of the rod are moveable on one another.
The other chitinous portion of the labium is very ill defined in Musca, and
can only be recognised in slightly cleared preparations and in sections. It is
a thin sheet of chitin, grooved to form a shallow trough, and situated imme-
diately behind the hypopharynx, the upper part of which is fused with it.
On each side of this groove there is a rod-like thickening, which extends from
a point a little distal to the base of the hypopharnyx to the labella, to the
anterior sclerite of which it is attached. The groove, with its lateral thicken-
ings, may be termed the labial gutter. It corresponds to the structure
described in Stomoxys under that name by Stephens and Newstead.
The membrane which unites the anterior and posterior plates of chitin, and
completes the wall of the labium, is in Musca very lax and of considerable
extent. The two chitinous portions are not continuous with one another at
any point.
The chief contents of the labium are the muscles which move the
labella; these will be briefly described in connection with the mechanism of
the proboscis. In addition there is the labial salivary gland, situated behind
the lower end of the labial gutter, and a nerve and trachea on each side.
There are no air sacs in the labium and the tracheæ are little larger than the
nerves.
The labrum-epipharynx (Plate II, figure 5) lies in the groove on the ante-
rior surface of the labium. It is shaped like a lance-head, its greatest breadth
being about one-third of its total length. There appears to be some difference
of opinion with regard to the composition of this organ, and the terms labrum
and labrum-epipharynx are used somewhat loosely in current literature. If
one bears in mind that the organ in Musca is homologous with the one in the
more primitive Orthorraphic flies its dual nature will be evident. The outer
part of the organ, consists of the labral element. It gives its shape to the
structure, and, arching over the anterior and lateral sides, terminates at about the
middle third of the posterior surface. The inner portion is the epipharynx, and
consists of a gutter having a depth of about half a circle. Between these two
chitinous parts there is an interval, which is partly occupied by a fan-shaped.
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muscle passing from the labrum to the epipharynx. The method of termina-
tion of the two constituent parts indicates clearly their different origin, for at
the proximal end they separate completely. The broad upper end of the
labrum becomes continuous with the membrane on the anterior surface of the
rostrum, and this of course is directly continuous with the clypeus of the head
capsule; the only real difference in the relations of the labrum in the Muscid
flies is that it has become separated from the head by the evagination of the
rostrum from the head cavity. At the sides of its upper border there are small
pits for the articulation of the labral apodemes, stout and heavily pigmented
rods which pass upwards and outwards, diverging from one another, into the
rostrum. The lower ends of the apodemes are pointed and barbed, the main
portion of the rod slightly sinuous, and the upper end expanded for muscle
attachment. Some of the muscle fibres attached to these apodemes are prob-
ably representatives of the labral muscles found in the Orthorraphic flies, and
the apodemes themselves may be derived from the portion of the labrum
which in the more primitive group is to be found inside the head capsule, and
to which the muscle of the labrum is attached. The upper end of the
epipharynx is a considerable distance proximal to that of the labrum, for it
projects into the rostrum as far as the upper part of the buccal cavity, to the
upper border of which it is connected by means of a soft membrane. In view
of the similarity in all essential points between the labrum-epipharynx in
Musca and that in the Orthorraphic flies, in which the epipharynx is in direct
chitinous continuity with the functional buccal cavity, I do not see any
grounds for accepting Kraepelin's view, that the organ is a mere evagination
from the head cavity.
The hypopharynx retains the structure usual in the Diptera, being a simple
spatulate slip perforated by the salivary duct. Its proximal third is fused with
the labial gutter. At its upper end it projects into the rostrum behind the
epipharynx, and is attached by a fine membrane to the pointed end of the
triangular plate which forms the main part of the buccal cavity. It will now
be evident that the buccal cavity is completed in its distal portion by the at-
tachment to it of the epipharynx and the hypopharynx, which converge to-
gether at this point, and are united to the borders of the chitinous plate by
membrane. This point, therefore, as correctly stated by Kraepelin, is the true
mouth of the fly. It will be shown later that as the blood-sucking proboscis
is evolved, the mouth is moved forwards, and the buccal cavity transformed
into a tube with complex walls.
I have already noted, in a previous paper, that the hypopharynx, like the
labrum-epipharynx, consists of two distinct laminæ, which enclose between
them the salivary duct. In Musca the labial lamina is only separate in theD
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distal third of the organ. Separation of whole of this lamina with the form-
ation of a free organ is found in the flies to be discussed later. The salivary
duct emerges from between the two laminæ just above the point of union of
the stomadael element with the walls of the buccal cavity.
The Labella.
The labella are oval lobes, separated from one another by a median
fissure which extends throughout their anterior surfaces, and is there continuous
with the groove on the anterior surface of the labium, but which is incomplete
behind. The two labella represent portions of the second maxillæ which have
remained distinct while the rest of the appendages have united to form the
labium. Each.lobe has an outer and an inner wall, the outer being composed
of thin plates of chitin with membraneous intervals, the inner of the highly
specialised pseudotracheal membrane, by means of which the fly absorbs its
food. The structure of this membrane is familiar from many descriptions, and
full details of its minute anatomy and mechanism will be found in Graham-
Smith's recent paper, so that it will be unnecessary to discuss it here, especially
since it is one of the first portions to disappear in the evolution of the blood-
sucking proboscis. The outer walls impart the necessary rigidity to the
labella, and have the chitin concentrated towards the periphery where the inner
and outer walls join one another ; at the anterior and posterior parts they bear
a large number of pigmented macrochætæ, which project forward as a fringe
distal to the organ.
There are two sclerites which support the labellar walls and at the same
time unite the labium and labella, forming joints which permit of free move-
ment between the two parts in an antero-posterior direction. The posterior of
these is known as the furca (Plate II., figure 3). It is shaped like a rather wide
horse-shoe, and embraces the posterior half of the labella. The posterior half
of the arch fits between the arms of the fork of the mentum, the short trans-
verse part lying opposite the incision between them. The furca is intimately
fused with the external labellar wall, and is also attached to the membraneous
portion of the wall of the haustellum in the region of the fork of the mentum,
so that when it is swung backwards and forwards between the arms of the fork
it carries the labellar wall with it ; there is in addition a small heart-shaped
piece of thin chitin attached to the transverse part of the arch, which passes
downwards in the middle line behind, and further assists the furca in its control
over the labella. The anterior or discal sclerite (Plate II, figure 4, Plate V,
figures 27 and 28) lies between the labella, and is attached to the labial gutter.
Its shape and position are of great importance from the comparative point of
view, as it undergoes important modifications in the blood-sucking forms.
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It is a racket-shaped loop of strong and heavily pigmented chitin, the broad
distal end of which forms the margin of the oral pit, while the proximal end,
corresponding to the handle, gives attachment to an important pair of muscles.
The distal end is shaped like an isosceles triangle, forwardly directed and with
rounded lateral sides and posterior angles; the base is incomplete in the middle
third, the remaining portions projecting inwards as thin flanges. Anticipating
subsequent descriptions, this portion of the discal sclerite may be called the
axial apophysis, a term used by Stephens and Newstead in their description of
the proboscis of Stomoxys. The proximal portion of the sclerite consists of
two strong rods, which arise from the thickened posterior angles, and pass
backwards, at first parallel to one another, but diverging slightly towards
their ends ; they are about equal in length to the axial apophysis. When
examined in profile it is seen that they are not quite in the same straight line
as the distal portion of the sclerite, but are curved a little upwards as they
diverge from one another. These portions of the sclerite may be termed the
labellar rods.
The position of the discal sclerite will be better understood by reverting
for a moment to the structure of the labium, as a whole. It is essentially a
hollow organ, the chitinous and membraneous walls of which are continuous
throughout the labium and labella, and enclose a space in which are contained
the muscles which move the labella, some tracheæ, and a labial salivary gland,
all of which lie in the hæmatocœle, a space continuous with the hæmatocœle
of the head and body of the fly. At the distal end, where the labium is
divided into the labella, it is invaginated, and so presents a double contour on
cross-section, the two opposing inner walls being the pseudotracheal mem-
branes. The discal sclerite is situated in the apex of the invagination, and the
converging lateral halves of the membrane are attached to the margins of
its expanded distal portion, all the channels terminating around this point. The
aperture enclosed by the sclerite is therefore the commencement of the food
canal and may be tomed the prestomum. The mouth, as already pointed out,
lies at the distal end of the buccal cavity.
The discal sclerite is attached to the two lateral rods which form the sides
of the labial gutter by short tendons, which pass from the attenuated ends of the
rods to the thickened posterior angles at the junction of the parallel with the
curved portions of the sclerite. Their attachment functions as a joint, and the
discal sclerite can be rotated on the fixed point through an angle of ninety
degrees or a little more ; in the closed position the labellar rods lie distal to and
in the same line as the labium ; when the labella are expanded for use they are
rotated so as to lie perpendicular to the labium, and therefore parallel to the
surface on which the fly is feeding.
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The prestomal teeth (Plate II, figures 6, 7 and 8), though of only secondary
importance in the economy of the house fly, have a special significance front
the comparative point of view as being the homologues of the powerful cut-
ting teeth of other Muscids. They are situated around the periphery of the
prestomum, their proximal ends being attached to the discal sclerite and their
distal ends directed outwards. The number and arrangement differ a good deal
in nearly allied species, but they always conform to the same general type.
Those of Musca domestica have been described by Kraepelin, and those of the
Blow-Fly by Graham-Smith. The arrangement in Musca nebulo resembles the
above. In Musca convexi frons it is somewhat simpler, as there is only one
row of teeth, and only four on each side. Their disposition in this species is
represented in figure 8, Plate II, where these are shown in side view, the
terminations of the pseudotracheal channels being also indicated. Each tooth
is a small petal-like slip of yellow chitin, contracted at its base to a rounded
cord and expanded and hollowed distally into a shallow trough. The inner
end is articulated to a thin raised flange of the discal sclerite, and the inner
half of each tooth is adherent to the membrane between the pseudotrachae ;
the distal margin is irregularly serrated, but on account of the extreme thinness
of this part of the tooth it is difficult to make out the precise outline.
The terminations of the pseudotracheal channels at the prestomum are
somewhat peculiar. As the channel approaches the orifice the rings develop
a median inward projection, so that when seen in optical section they appear
T-shaped instead of straight ; in the last complete ring this spur is produced
inwards almost to the margin of the orifice, though it does not appear to be
actually attached to it. On either side of the end of the channel, and em-
bracing the last ring, there are several curved filaments of chitin which unite to
form a thick strand, those of two sides passing inwards in contact with one
another, between the teeth, to be inserted into the flange on the discal
sclerite.
The Musculature and Mechanism.
It is not necessary to give here a detailed account of all the muscles and
other structures which are concerned in the mechanism of the proboscis, but
rather to indicate the main features of the movements and the musculature,
so that the modifications found in the blood-sucking members of the group
may be compared with the Musca type. The muscles are extremely difficult
to study, and the chances of making an error with regard to their exact points
of attachment are so great that it would not be worth while to attempt to
describe minor differences. A full account of the different muscles and their
actions will be found in Kraepelin's paper.
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The first movements to be considered are those which result in the exten-
sion and retraction of the proboscis ; as already stated, the whole organ is drawn
up underneath the head when not in use, and can be extended into a straight
line perpendicular to the long axis of the body when required. The labella
also can be folded and expanded. The movement has to be considered joint
by joint, though it must be remembered that in life all the joints are moved
at the same time, the extension and retraction taking place rapidly. The
extension of the rostrum on the head is brought about by a rotation of the
fulcrum on the anterior border of the epistomal orifice, in such a way that
its lower end passes forwards and downwards. The motive power for this is
to be found in the distension of the large air sacs which lie on either side of
the fulcrum, the air being expressed from the body of the fly into the head by
means of the respiratory movements of the fly, as described by Kraepelin.
The rostrum is extended, in fact, much as one would straighten the finger of
a glove by blowing into it. The extent of possible movement is approximately
rotation through a right angle, from the horizontal to the vertical position.
Extension of the haustellum on the rostrum is accomplished mainly by the
contraction of the muscles which are attached to the apodemes of the labrum,
passing between the expanded upper ends of the apodemes and the lower end
of the fulcrum ; they pull in a downward and inward direction, and so force
the l
abrum, and with it the labium, into line with the rostrum, by straighten-
ing the two at the loose joint in the region of the buccal cavity. The actual
point on which the rotation occurs appears to be the membraneous union
between the epipharynx and hypopharynx and the small chitinous plate which
forms the walls of the buccal cavity.
The movements in these two joints by which the proboscis is retracted
are both produced by muscular action. The fulcrum is rotated upwards and
backwards by means of two pairs of muscles. The anterior pair of these is
short, and passes between the internal surface of the head capsule, at a point
just below the antennae, and the prominent posterior horns of the fulcrum ;
its action causes the fulcrum to rotate on the fixed point provided by the
pressure of the anterior arch against the anterior edge of the epistomal orifice.
The posterior muscle is very long, and passes between the internal surface of
the head capsule in the region of the occipital foramen and the upper border
of the mentum. The two act together, and the fulcrum, and with it the rest of
the rostrum, is drawn upwards as it is rotated. The folding of the haustellum
on the rostrum is accomplished by means of certain groups of muscles lying
on the anterior surface of the rostrum ; these are somewhat ill-defined, and
many separate muscles which may fulfil this function have been described.
The most important ones appear to be a pair which pass between the distal