NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
28
and the length of the organ is reduced. In other words, the distal end of the
proboscis is evaginated. The two labella now present a rounded appearance,
with no trace of the fissures on the anterior and posterior surface, and all the
structures which in the position of rest lie on the internal surface and point
distally now lie on the external surface and are directed backwards. The distal
margin of the organ is formed by the bases of the teeth, which, it should be
noted, form a concave border directed downwards and backwards, not in the
direct transverse axis of the proboscis. At each end of the tooth plate there is
a short forward projection, and if one focusses carefully one sees that these are
the two ends of the anterior border of the discal sclerite, which lies immediately
behind the united bases of the teeth. The teeth themselves arch upward and
backward from their bases, so as to point in the direction of the long axis of
the proboscis, and the petiolated blades and rod-like hairs lie in line with them.
Behind these there lies the fringe of hairs, which in the resting state form the
distal fringe. They are now, however, directed backwards well out of the way
of the teeth. The external plate of chitin is displaced backwards so as to reach
the furca, and it, and the smaller plates of chitin which lie in the triangular
space between the end of the labial gutter and the furca, are very loosely
arranged, and ruckled up. The position of the furca is important. Instead of
being arranged as a transverse arch, it is rotated backwards so that it makes an
angle of half a right angle with the long axis of the labium, pointing upwards
and towards the head. The lateral arms of the furca are thus displaced
backwards for a distance equal to the length of the external plate. With regard
to the labium, the transverse ridges and furrows are much more conspicuous
than they are in the resting condition. As I have been unable to obtain clear
-ed preparations fixed. in this position the position of the labrum-epipharynx
and hypopharynx cannot be defined, but it is evident that their distal ends
must now be much nearer the distal end of the proboscis than they are in the
resting position.
We have now to discuss the possibilities of the proboscis as a biting and
piercing organ, and in doing so it will be convenient to clear the ground by
noting certain limitations under which the organ is placed by reason of its
anatomical structure. In the first place, the labrum and the hypopharynx
are placed at once out of court ; they do not reach to the end of the proboscis,
and even supposing that they could reach it in the evaginated condition, they
could not project sufficiently far to be effective unless the discal sclerite were
already inserted into the wound. Moreover, they are not provided with cutting
ends, but are more or less flaccid.
The arrangement of the joints of the proboscis is such that it is not adapted
for the propagation of force from the body of the fly, as any pressure from
29
behind would tend to fold up the organ. This point has already been dealt with.
The cutting weapons must therefore be the teeth, and. the teeth only, and
the question to decide is as to the particular motion by which they act.
Stephens and Newstead, in their account of Stomoxys, have suggested that they
cut by a semirotatory motion, after the manner of a carpenter's auger, the
muscles involved being those which pass between the gutter and the wall of the
labium. This hypothesis cannot stand, for it would necessitate the presence of
a joint adapted for a rotatory motion between the teeth and the rest of the
proboscis, and such a joint does not exist and is not described by them ; indeed,
the only joint which would be of use for such a method of action would be one
between the base of the labial gutter and the mentum, assuming the teeth to be
firmly fixed on the end of the discal sclerite and that the sclerite was firmly
fixed on to the end of the labial gutter. Rotatory motion of any sort would
be a very extraordinary deviation from the type of motion possible in Musca,
and in the case of Lyperosia, as in Stomoxys, it is put entirely out of the
question on account of the nature of the articulations.
The one motion about which there is no doubt is that of the eversion of the
labella and the upward displacement of their outer walls. One can see this
take place under ether, and can produce it by pulling on the muscles of the
bulb in dissections. These muscles are by far the largest in the proboscis, and
one would naturally expect them to play an important part in the mechanism
of the making of the wound. By their contraction they cause the furca to be
rotated in an upward direction, on the fixed point provided by its articulation
with the ventral sclerites, in such a way that its lateral arms are displaced from
their transverse position to one in which they make with the long axis of the
labium an angle of about half a right angle, the ends of the furca pointing
towards the head, and as the rotation takes place the chitinous external wall of the
labellum is also displaced, both on account of its intimate connections with the
furca and because it receives directly some of the fibres of insertion of the ten
-don of the muscle of the bulb. Now the internal walls of the labella are firmly
attached to the distal end of the discal sclerite, and the sclerite itself is practi
-cally a fixture on the end of the labial gutter, so that what happens when. the
full amount of traction is exercised on the outer walls is that the invagination
is undone, and the end of the discal sclerite becomes the most distal point of the
proboscis, as is shown in figure 13. As this unfolding takes place the teeth,.
which are firmly attached to the end of the discal sclerite, are rotated through
about half a circle, so that their serrated points come to point upwards instead
of downwards.
This movement of the teeth constitutes, I believe, the essential act in the
mechanism of the proboscis, and it is by a repitition of this movement that the
E2
30
wound is made. I have referred in a previous paper to the rapidity of action of
which insect muscle is capable, and the action of the muscles of the bulb in
undoing the invagination at the end of the labella, and so causing the teeth to
press upwards, appears to me to be another instance of it. The muscles which
are inserted into the furca are not only much larger in this fly than they are in
.Musca , but they are proportionately very much larger than those of the anterior
set, and are undoubtedly the dominant motive force in the proboscis.
Retraction of the furca is not the only result of the action of these muscles.
It has already been pointed out that the labrum-epipharynx and the hypo-
pharynx do not reach to the distal end of the proboscis in the resting position.
In fact, they are so much shorter than the combined labium and labella that
their distal ends only appear in sections about the level of the furca, and distal
to them there is the whole length of the discal sclerite. Now the discrepancy
in length is partly compensated for by the retraction of the labellar walls, by
means of which the distal end of the discal sclerite becomes the terminal point
of the proboscis, but even this is not sufficient, as is clearly shown by actual
measurements of cleared proboscides, to bring their distal ends into such a position
that they can perform their functions, and further shortening is required before
they can reach the wound. This is accomplished by the shortening of the whole
of the mentum. In the early part of this paper it was stated that the whole of
the chitinous wall of the mentum is traversed by a closely set series of ridges
and furrows. Now these do not present the same appearance in all prepara
-tions, and a careful comparison of those in proboscides which are in the position
of rest and those in the position of action shows that the more the lateral arms
of the furca are displaced backwards the more marked the ridges and furrows
become, and it follows that in each individual specimen the more marked the
furrows are the shorter the mentum will be, for the crinkling of the wall must
reduce its length. I have been unable to obtain a cleared preparation in which
complete evagination has occurred, but I have no doubt that if one could obtain
one it would. show that the evagination of the labella and the crinkling of the
mentum taken together had reduced the total length of the labium just suffi
-ciently to bring the labium-epipharynx and. the hypopharynx to the distal end
of the proboscis. The muscles of the bulb, the retractors of the furca, bring
this about by pressing the furca on the ventral sclerites, so that the movement
of the furca is an actual displacement as well as a rotation on its fixed point.
The labrum-epipharynx and the hypopharynx are of course quite incapable
of being moved on the labium, owing to the close attachments at their proximal
ends. The labial gutter cannot move on the mentum, for, although there is no
firm chitinous union, they are closely held together by means of the membrane
which unites their lateral borders throughout their length, and at the proximal
31
end the attachment is still more close. When the mentum is shortened
the labial gutter must therefore remain of the same length, and it is a little
difficult at first to see how this difficulty is to be overcome, for if the two
remain in the same plane retraction of the mentum alone will not affect the
relations of the discal sclerite and the terminations of the labrum-epipharynx
and the hypopharynx. Two points which have already been mentioned
help to solve this difficulty. In the first place, the proboscis when in use is not
held perpendicularly, but points a little forward, and the distal margins of the
discal sclerite form a line parallel to the surface of the skin of the host when
the proboscis is held at this angle, as will be seen from figures 1 and 5, Plate
11. If the angle at which the labella make with the labium in the position
of rest be compared with that in the position of action, it will be seen that in
the latter case the angle is considerably greater, that is to say, the retraction
of the mentum is accompanied by a flexion of the joint between the labial
gutter and the discal sclerite. The amount of flexion is small, but it is enough
to materially shorten the distance between the base of the labium and the
opening at the distal end of the discal sclerite. This flexion is produced by the
action of the retractors of the furca, acting through traction on the inner
walls of the lab ella.
By means, therefore, of the contraction of the retractor muscles of the
furca, the most powerful muscles in the proboscis, the furca is rotated so that its
arms pass in the direction of the head, and so carries with it the external
wall of the labella, which are turned inside out, the teeth being drawn through
the wound and exercising their obvious cutting function as they go ; it is also
displaced bodily upwards, room for it being made by the actual shortening of
the mentum. The discal sclerite is at the same time flexed on the end of the
labial gutter, and in this way the distance between the proximal and distal ends
of the labium is so shortened that the labrum-epipharynx and the hypopharynx
come to be at the same level as the end of the discal sclerite. They are thus in
a position to form the tube up which the blood can be sucked, and the hypo-
pharynx can transmit its saliva directly into the wound. We have next to
consider how the parts pass from the position of action thus attained to the posi
-tion. of rest, in order to be able to repeat the movement. There is obviously no
muscle which can directly invaginate the end of the labella, and undo the
furrowing of the mentum, and we must look for some indirect method by
which this can be accomplished. Here a comparison with Musca helps consi-
derably. In Lyperosia, as in Musca , there is a well developed transverse
muscle, situated, as already described, in the distal portion of the labium, and
the simple acceptance of the similarity of function between this and the
corresponding one in the non-biting flies provides a rational explanation of
32
the method by which the labella are returned to the position of rest. In
Musca, according to Kraepelin, the contraction of this muscle drives the blood
into the space between the inner and outer walls of the labella, and thus
assists in distending them so as to bring the pseudotracheal membrane into
action. In. Lyperosia the relations of the labellar walls to the labium are
considerably altered on account of the fixation of the organ in a definite
position, the backward and forward rotation of the discal sclerite being very
limited, but the distension of the intralabellar space can still occur, and is
brought about by the same mechanism as in Musca, that is to say, respiratory
movement, aided by the transverse muscle. The result in this case is that
when the blood is forced down into the labella the invagination at their distal
end is reproduced, and the two internal walls come again in contact with one
another, and at the same time the contraction of these fibres, by reducing
the transverse diameter of the labium, must increase the longitudinal diameter,
and thus undo the furrowing of the wall. The eversion of the teeth,
which is the essential feature of the action of the proboscis, is thus
accompanied by evagination of the labella, and the passage back again to
the position of rest by a corresponding invagination evagination being
a reduction, and invagination an increase, in the length of the labium.
The fixed point from which the labellar walls can be forced out, and back
to which they can be drawn, is the distal end of the discal sclerite, to which
they are attached.
There remains only to consider the mechanism by which the discal sclerite
is returned to the position of rest. Again a comparison with Musca affords
valuable assistance. We have seen that the discal sclerite is articulated, in
a most complex manner, with the end of the lanial gutter, and that there
is in. Lyperosia a small bundle of muscle fibres which correspond with the
anterior and menial set in Musca, the retractors of the discal sclerite. The
exact point of insertion of these fibres, which have no terminal tendon, is
difficult to make out, but they can be traced to the proximal end of the
sclerite. Whether they are actually inserted into the sclerite or to the
membrane in its vicinity will not affect the action, for the fibres are definitely
anterior to the larger muscle of the bulb, and must act on the joint in an
opposite direction. The joint between the discal sclerite and the end of the
labial gutter having then been flexed by the indirect action of the retractor
muscle of the furca, it can be straightened again by the anterior muscle fibres,
and these two actions follow one another each time the teeth are drawn through
the wound at the eversion and. closing of the labella. Eversion of the teeth is
of course carried out against the resistance offered by the skin of the animal on
which the fly is feeding, and consequently the muscles which bring it about are
33
very highly developed, while the closing of the teeth is carried out without any
special resistance, and the muscles are not more developed than in Musca. We
may suppose that after each contraction the labella are thrust a little deeper as
the wound is made, until blood is reached. When this occurs the proboscis is
probably held in the fully retracted position, so that the labrum-epipharynx and
the hypopharynx are in the most advantageous position for collecting the blood.
34
REFERENCES.
AUSTEN, E. E.—A Monograph of the Tsetse-Flies. London, 1903.
CRAGG, F. W.—Studies on the Mouth Parts of the Blood-Sucking Diptera. No. 1,.
Philæmatomyia insignis, Austen. No. 2, Some observations on the
Morphology and Mechanism of the Parts in the Orthorrapha. Scienti-
fic Memoirs by the Officers of the Medical and Sanitary Departments
of the Government of India, New Series, Nos. 57 and 58, Calcutta,
1913.
HANSEN, H. J.—The Mouth Parts of Glossina and Stomoxys. In Austen's Monograph,
vide supra.
KRAEPELIN, K—Zur Anatomie and Physiologie des Russels von Musca. Zeit. f. Wissen-
schaftliche Zoologie, Vol. 39, 1883.
STEPHENS, J. W. W., and NEWSTEAD, R.—The Anatomy of the Proboscis of Biting Flies
I. Glossina. Memoir XVIII, Liverpool School of Tropical Medicine,
1906. II. Stomoxys, Ann. of Trop. Med. and Paras., Vol. 1, No. 2,
1907.
35
EXPLANATION OF REFERENCE LETTERING.
a. a., axial apophysis, or ventral portion of the discal sclerite.
a. f,, the fissure between the two labella on the anterior or dorsal surface.
ap., apodeme of the labrum.
b.,the bulb..
b. c., the buccal cavity.
c.,blood cell, in the hæmatocœle of the labellum.
.c. p., the distal chitinous plate in the membraneous area of the labella.
c. p., the proximal ditto.
c. s., the cul de sac at the end of the labrum-epipharynx.
d. ph., the dilator muscle of the pharynx.
d. s., the discal sclerite.
e. f., the external fringe of hairs on the labella.
ex. h., extensor of the haustellum.
ex. p., external plate of chitin in the labellar wall.
ev.,elevated area on the ventral aspect of the labrum-epipharynx.
f.,the fulcrum.
f.',the anterior arch of the fulcrum.
f.",the funnel-shaped lower end of the fulcrum.
f. b.,strong fibres uniting the upper end of the labial gutter and. the lateral borders of the
mentum.
f. c., the food canal.
f. t., fibrous tissue uniting the teeth to the discal sclerite.
fu.,the furca.
fu. f., fibrous band connecting the two lateral arms of the furca internally.
hy., hypopharynx.
k,,keel of the labial gutter.
lb.,labella.
1. ep., labrum-epipharynx.
1. g., labial gutter.
1. g.' dorsal portion of the labial gutter, forming the sides of the groove.
1. h.,
labral hairs.
. 1 r.,lateral rods, correspnding to the distal portion of the fork of the mentum in Musca
m.,membrane forming the wall of the rostrum.
m. a., membraneous area between the chitinous walls of the labella and the mentum
m ,the transverse muscle of the labium.
m.",the anterior and mesial set of muscles, which act on the joint between the discal
sclerite and the labial gutter.
m ,the retractor muscle of the furca, with its tendon.
m. a., membraneous area between the labium and the labella.
mt., the mentum.
n ,,nerve.
p ,the palp.
p .,the distal petiolated blades.
p ,the small proximal set of petiolated blades. F
36
ph.,
the pharynx.
r.,
the ridge of chitin which connects the furca with the external chitinous plate of the
labella.
r. r.,
retractor of the rostrum.
r. h.,
rod-like hairs.
s. d.,
salivary duct.
t.,
teeth.
tl., t. 6., the first and sixth teeth, counted from the dorsal side.
tb.,
tubercles at the distal end of the labrum-epipharynx.
tr.,
trachea.
v. sc., ventral sclerites.
PLATE I.
Figure 1.—The proboscis in profile, drawn from a potach preparation. The position is one of
slight extension, and only the structures of one side are drawn. Note the
extent and position of the membraneous area near the furca, and the transverse
ridges and furrows on the mentum.
Figure 2.—A view of the posterior surface of the labella, to show the furca and its attach-
ment to the external wall of the labella on each side, and its articulation with
the ventral sclerites.
Figure 3. —The ventral aspect of the labrum-epipharynx at its distal end, to show the curious
nature of the t ermination of the food canal, which ends in the cul de sac. c. s.
Figure 4.—The end of the labium and the labella in side view. The middle portion of the
external wall is shown as cut away to disclose the distal sclerite situ, with
its attached teeth.
Figure 5.—Outline of the labial gutter at its distal end, where the lateral dorsal and the
middle ventral portions are separated.
PLATE I.
Fig. 1.
Fig. 5.
Fig. 4.
Fig. 2.
Fig. 3.
F. W. C. DELET
photo–Engraved & printed at the Offices of the Survey of India, Calcutta, 1912
PLATE II
Figure 6.—The teeth and their accessory structures, dissected off the discal sclerite. Note
that, although the teeth differ in size, their cutting ends are brought into line by
the curvature of the attached ends.
Figure 7.—The labella as seen in a well cleared preparation, mounted dorsal side uppermost.
In this drawing the portions nearest the microscope were in focus,
Figure 8.—The same preparation, at a slightly lower focus. Here the ventral portion of the
labial gutter is shown, and the lateral arms of the furca, which arch forwards
from the posterior surface so as to come into focus.
Figure 9.—The same preparation, at a lower focus. Here the furca is seen from the front,
and the whole length of the triangular axial apophysis is in view.
Figure 10.—The distal end of the labial gutter, as seen in a dissection. Note the separation
of the dorsal and ventral portions.
Figure 11.—The distal end of the fulcrum, showing the f unnel-like expansion and the buccal
cavity.
Figure 12.—The proximal end of the food canal in the haustellum, showing the tube formed
by the fusion of the epipharynx and the stomadeal portion of the hypopharynx
as it projects from the haustellum into the rostrum.
Figure 13.—The labella in the position of action.
Fig. 11.
Fig. 12.
Fig. 13.
PLATE II.
Fig. 6.
Fig. 9.
Fig. 10:
Fig. 7.
Fig. 8.
F. w. C. DELET
photo.-Engraved :&: printed at the Offices of the Survey of India. Calcutta, 1912
PLATE III.
Figure 14.—Transverse section through the labella in the position of rest, at the level of the
teeth, The space in the middle between the labella is an artifact due to shrink
age. The irregularity in the outline of the teeth is due to the slight obliquity
of the section.
Figure 15.—A transverse section a little higher up than the last figure, passing through the
discal sclerite and the tip of its axial apophysis. Note the fusion of the
two posterior walls of the labella at this point.
Figure 16.—A transverse section above the level of the teeth. The ends of the labial gutter
are seen in the upper part of the section, projecting in front of the discal
sclerite as seen in figure 4, Plate I. The axial apophysis is here united to the
rest of the discal sclerite, occupying the ventral angle. Note the strong
bands of fibrous tissue which unite the two external walls in this situation.
Also compare the size of the tracheæ and nerves with those in figure 24,
Plate V.
Figure 17.—The next proximal section. Only the joint arrangements are shown in this and
the next three figures. Note the disappearance of the axial apophysis and the
appearance of the ventral portion of the labial gutter, which is however separated
from the dorsal portion by the intervention of the dorsal and lateral part of the
discal sclerite.
Figure 18.—The next section, showing, the widening of the labial gutter, and the approach of
the ventral and lateral portions. The ventral portion of the gutter rests on
two lateral projection of the discal sclerite.
F. W. C. DELET
Photo.- Engraved &c printed at the Offices of the Survey of India, Calcutta, 1912
PLATE IV.
Figure 19.—A transverse section a little higher up. Here the discal sclerite is very much re-
duced in size, and has passed altogether behind the labial gutter.
Figure 20.—The next section. This has passed through the upturned end of the discal
sclerite, as seen from the front in figure 8, and from the side in figure 4.
Figure 21.—Section through the region of the furca. Note that the discal sclerite has com-
pletely disappeared, that the two portions of the labial gutter are united by
fibrous tissue, and that the keel of the labial gutter is not present at this point.
There is a well-marked band of fibrous tissue passing across the labella at the
level of the furca.
Figure 22.—Section a little higher up than the last, showing the first appearance of the keel.
The dorsal half of the labium at this point is bounded by membrane only.
Compare other drawings of this area, figures 1, 4, 7, 8, 9, The ventral
sclerites make their appearance at this level.
Figure 23.—Section at the level of the first appearance of the labrum-epipharynx and the
hypopharynx. This section passes through the ‘elevation’ on the tip of the-
labrum-epipharynx.