NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
21
labella has been so very much diminished with the adaptation to a blood-suck
ing habit, its extent has been reduced to the minimum, so that it now exists
merely as a narrow sheet connecting the external wall with the ridge of chitin
formed by the fusion of the bases of the teeth. There is of course no trace of
the pseudo-tracheal channels, but, as I hope to show in a future memoir, there
is reason to believe that the petiolated blades are derived from the rings of the
p s e u d o - t r a c h e æ , a n d m o r e o v e r c o n t i n u e t o f u l f i l t h e f u n c t i o n o f t h e c h a n n e l s
so far as their filtering action is concerned.
We have next to consider the fixed portion of the internal wall. This is
evi dentl y deri ved from the di scal scl eri te, but the al terati on has proceeded to
such a degree that it will not be advisable at the present juncture to attempt
to separate it into its component parts, since its homology only becomes evi-
dent when we trace it through the intermediate forms. The term discal sclerite
may then be accepted provisionally, in place of the nomenclature used by the
authors cited above.
The discal sclerite is composed of two plates of chitin, lying in the antero
- p o s t e r i o r p l a n e , u n i t e d t o o n e a n o t h e r b e l o w , a n d w i t h t h e i r i n n e r s u r f a c e s
concave towards one another in their middle thirds. The distal margins of
these plates are articulated to the teeth, and the proximal ends to the labial.
gutter. Its structure is so complex and at the same time so important in con
- n e c t i o n w i t h t h e m e c h a n i s m o f t h e p r o b o s c i s t h a t i t m u s t b e d e s c r i b e d i n
detail.
When a well-cleared preparation mounted on its side is examined, the
teeth, discal sclerite, and the end of the labial gutter can be readily seen
through the thin external wall of the labellum (Fig. 4, Plate I). One
obtains a still better view if by a fortunate dissection one contrives to remove
the external plate of one side and to leave the rest of the structures in sitû.
From this aspect one sees that the apices of the teeth form a line parallel to the
distal margin of the labella, and that the ridge of chitin formed by their united
bases lies about the junction of the distal and middle thirds of the labella,
measured from the furca. The remaining two-thirds are occupied by the discal
sclerite, which is closely apposed to the bases of the teeth in front and extends
to the end of the labial gutter behind. It does not occupy the whole of the
vertical depth of the labella, but only about one-third, and is situated much
nearer the upper than the lower border. Its shape when seen from the side is
as follows. It is roughly oblong, its long axis being in the long axis of the
labella ; the distal end is wider than the proximal, and is parallel to the distal
margin of the labella, that is to say, it is directed obliquely downwards and
backwards. It is also slightly concave downwards to correspond with the con
- v e x i t y o f t h e d i s t a l m a r g i n o f t h e u n i t e d b a s e s o f t h e t e e t h , w h i c h a r e s o c l o s e l y
D 2
22
pressed against it that the two appear united ; there is however no chitinous
union between them, and they are readily separated by dissection in potash
preparations. The upper or anterior border of the sclerite is straight for its
distal two-thirds, and in its proximal third dips suddenly downwards so as to
leave a shallow notch, in which the lateral portion of the end of the labial
gutter rests. The lower border of the sclerite is concave downwards, but its
two ends lie equidistant from the border of the labellum. The proximal end
is most irregularly shaped, on account of the complexity of the joint between
the sclerite and the end of the labial gutter ; its ventral third projects upward
beyond the dorsal portion, and is bounded by a thickened ridge of chitin con
- t i n u o u s w i t h t h e c o n c a v e l o w e r b o r d e r o f t h e s c l e r i t e . T h i s t h i c k e n e d p o r t i o n ,
equal to about one-third the total width of the sclerite, terminates abruptly
by bending again forwards, and from the slight projection thus formed a few
strands of fibres can be seen to pass towards the lower angle of the sclerite in
front, thus separating the lower portion of the sclerite, bounded by its concave
margin, from the remaining portion. The part of the posterior border above
this projection is ill-defined, as the chitin is here thinner than elsewhere. It
has already been stated that the upper border dips suddenly downwards in its
posterior third, where it is overlapped by the lateral part of the labial gutter ;
at its proximal limit it dips again downwards to join the projecting portion
just described, and so forming a sharp angle. This angle is situated exactly
in line with the sharply cut away distal end of the lower part of the labial
gutter, and by careful focussing one can see that the sharp end of the gutter,
which projects forward beyond the angle, in the manner already described,
lies between the two sides of the discal sclerite, in fact, it projects into the
interior of the sclerite for at least one fourth of its length ; the ends of the
lateral and lower divisions of the labial gutter are at the same level, but the
former is outside, the latter inside, the sclerite.
One more point is well brought out by examination from the side. The
wall or the sclerite is not of equal thickness throughout, but is thickened along
the upper and lower borders, and at the articulation with the labial gutter.
There is also an apparent thickening along the distal margin, but it is difficult
to make out whether this is a part of the discal sclerite or if it is the ridge at
the bases of the teeth. The thickening along the lower border, and the up-
turned end. of the posterior border continuous with it, form a conspicuous hook
- l i k e p i e c e , t h e s i g n i f i c a n c e o f w h i c h w i l l b e s e e n l a t e r .
The discal sclerite when seen from the side appears to be a flat structure,
and this is confirmed when we examine it from the front. It is not, however,
a simple flat plate, as has already been hinted, but is composed of two plates
with a narrow interval between them. This will be better understood if we
23
describe the appearance as seen in a cleared preparation mounted anterior side
uppermost.
In the first place one must note that the two labella are not in contact
with one another, but that they diverge from behind forwards, leaving a com
- p a r a t i v e l y w i d e f i s s u r e b e t w e e n t h e m . T h i s i s , I t h i n k , t o b e r e g a r d e d a s a n
a r t i f a c t , d u e t o t h e s h r i n k a g e i n t h e m a k i n g o f t h e p r e p a r a t i o n ; t h e t w o
l abel l a are i n al l probabi l i ty cl osel y apposed to one another i n the natural
condition of rest. Allowing for this in the interpretation of the appearance
seen, the two sides of the distal sclerite appear from this point of view as
parallel or slightly diverging ridges, the edges of the plates. By careful
focussing one can make out that the two plates are concave on their opposing
surfaces, bounding a cavity open only on the anterior surface, and probably
closed in the natural condition of the parts by the apposition of the anterior
edges of the plates (Fig. 7, Plate II). The joint between the sclerite and
the labial gutter is seen by careful focussing in a well-cleared preparation.
At the most anterior (dorsal) point one can distinguish the two lateral rods
derived from the end of the labial gutter, overlapping the most proximal
portions of the upper borders of the sclerite, and lying a little to their outer
side. At a lower level the projecting pointed end of the lower part of the
labial gutter comes into view (Fig. 8, Plate II), and it is seen that the poin
corresponds to the middle third of the floor of the gutter ; when seen from
above it is not unlike a J pen in shape. At either side of the base of this point
the edges of the gutter are transverse. It is clear from this that the middle
portion of the gutter does actually project into the cavity between the two walls
of the sclerite. At this level there comes into view a stouter piece of chitin,
at the distal end of the sclerite, which marks the point where the two labella
are united on the posterior surface, and by focussing a little lower than this
(Fig. 9, Plate II) one sees that it forms the point of a long and narrow triangle,
the base of which is situated proximal to the joint between the labial gutter
and the sclerite. The base, which is a well marked transverse band, lies at a
lower level than the apex, and, as one may infer from the fact that the lateral
portions come into focus before the middle portion, is itself so bent as to be con
- v e x d o w n w a r d s . T h e s i g n i f i c a n c e o f t h i s a p p a r e n t t r i a n g l e w i l l b e e v i d e n t b y
comparing it with the lateral aspect. It is, in fact, the lower triangular portion
of the distal sclerite. The pointed apex is the most distal limit of the lower
border of the sclerite, a point where the two lateral walls are attached to one
another, and the base is the upturned edge of the lower border, which when
seen f rom t he si de appears as a hook-l i ke proj ect i on.
To confirm the above observations it is necessary to examine the labella
in serial section.
24
Figure 14, plate III, represents a section through the distal portion of the
labella, at a point where the two are still separate. The external chitinous
plates of the two sides form an oval outline, with the long axis in the vertical
plane. Inside this there are seen the two sets of teeth parallel to one another
in the long axis of the labella, and separated from one another by a narrow
interval, which, by the way, is probably an artifact due to shrinking. At the
upper end the teeth are united to the external plate by a strong membrane,
which is thrown into numerous folds ; at the lower end the junction is less
definite, being formed: by loose strands of fibres. At the lower end of the
section one sees some of the rod-like hairs cut into irregular but recognisable
fragments, and some of strands of yellow chitin which are probably the stalks
of the petiolated blades. The free portions of the teeth are shaded lightly, and
show an irregular outline clue partly to fractures and partly to the teeth of the
two sides not being cut in exactly the same plane ; in the sections they are
of a bright canary yellow colour, like that of the piercing stylets of the
Nematocerous Diptera. The more deeply shaded portion into which the teeth
merge i s stai ned a deep purpl e by Del afi el d' s hæmatoxyl i n, and i s of a fi brous
nature. It forms a continuous band which unites the bases of the teeth to the
edge of the discal sclerite, and it is by virtue of the flexibility and strength of
this band that, as will appear later, the teeth are retained in position and yet
can be turned completely backwards on the edge of the sclerite.
Sections distal to this have much the same appearance, except that they
show the fringe of hairs on the distal border of the labella, and in these one
can note the central canal which perforates them, and the enlarged base, which
bears the same relation to the hair as the basal joint of the antenna does to the
rest of the appendage. The rod-like hairs do not show a central canal, nor can
one recogni se i n them the groove whi ch they appear to have when exami ned i n
cleared preparations.
As the teeth are traced upwards the darkly stained band of fibres at their
bases becomes more regular in outline until it forms a continuous band on
either side of the labella, and after a few sections the bands of the two sides
unite on the lower side. The membrane on the external aspect of the labella
then becomes continuous from side to side and the labella are open only on the
anterior aspect.
A few section above this we have the appearance represented in figure 15,
Plate III. The teeth have disappeared, and in place of them we find only the
discal sclerite, composed here of two lateral plates in the vertical plane, united
at the lower end only. In the upper and lower thirds the two plates are almost
parallel to one another, but in the middle they diverge so as to present opposing
concave faces, limiting a circular space. In the interval between the two at
25
the lower end there appears a separate piece, oval in outline, pointed at its
ends, and almost filling up the interval between the two lateral walls in this
situation. In the next few sections this piece enlarges, becomes more rounded,
and is more and more closely approximated to the lateral plates, until it fuses
with them. The distal end of this free piece is situated at the point where the
two labella fuse with one another on the posterior surface, and it can be seen
in this situation in cleared preparations. As it broadens out distally the two
lateral halves become a little separate from one another, and a small amount
of loose cellular tissue appears in the space.
In the sections proximal to this point there is a rapid change in the
appearances, once we pass to the upper end of the discal sclerite. Figure 16,
Plate III, represents a section just behind the point where the free median
plate has fused with the lateral plates. We see here that the external plate has
undergone a considerable change and now presents an irregular outline, marked
by two furrows on either side of the middle line ; the median portion of the
sclerite is now fused to the lateral portions, and shows some loose cells in its in
- t e r i o r . T h e t w o p l a t e s o f t h e s c l e r i t e a r e n o w d i v i d e d i n t o a n . u p p e r a n d a
lower portion. A few sections distal to this the division commences at the
upper limit, and passes down as we progress from the labella to the labium.
In the middle of the lower portion of the section there appears another free
piece anterior to the one already noted, as shown in the figure. We have
therefore two lateral and anterior portions, a U-shaped lower or posterior plate,
and a middle piece fused in the angle between them, and now another piece of
solid yellow chitin anterior to this. The two lateral pieces are the ends of the
upper portions of the labial gutter, the anterior piece in the middle is the end
of the middle portion of the gutter, and the rest belongs to the discal sclerite.
As we pass upwards from this point the discal sclerite becomes steadily reduced
in size while the labial gutter comes to occupy more and more of the section.
In the next figure (Fig. 17, Plate III) we see that the anterior middle piece has
increased in breadth and in depth, and the vertical extent of the lower U-
shaped piece has become much less, and at the same time it is wider. The
gutter rests on two notches on the inner sides of the sclerite, which is here
rather V-shaped. In the next figure, which represents the next proximal
section, the middle portion had definitely assumed the shape of the gutter, and
has become connected with the ends of the lateral portions, while it is embraced
by the now rather wider part of the sclerite. Still higher up the sclerite be
comes much thinner and recedes behind the gutter, where it appears as a wide
arch. A few sections higher up it appears only as a narrow arched hand behind
the gutter, the two portions of which remain quite separate from one another.
Above this it ceases, and at the level of the furca (Fig. 21, Plate IV), the two
26
halves of the gutter unite with one another by bands of fibres. Proximal to
this the keel of the gutter begins to appear and the section passes through the
ventral sclerites.
In the distal portion of the labella, then the food canal commences between
the teeth. As we shall see later, the vertical portion of the interval between
the two sides at this part of the proboscis does not exist in the state of action,
but the true permanent canal begins at the distal sclerite, about the point
depicted in figure 23, Plate IV. Its commencement is bounded by the two
sides of the discal sclerite, and above this by the labial gutter. The labrum
-epipharynx and hypopharynx do not appear in the section till we reach a point
just proximal to the thick ends of the venial sclerites.
The Hæmat ocœl e of t he Label l a.
So far only the inner and the outer walls of the labella have been described.
Distal to the point of fusion on the inferior surface each labellum has a complete
inner and outer wall, as shown in figure 14, Plate 3, and throughout the labella
are separated on the anterior surface of the labium. The space seen in the
sections between the inner walls of the two labella in the position of rest is
probably an artifact due to the different degrees of shrinkage between the
c h i t i n o u s a n d me m- b r a n e o u s p o r t i o n s ; t h e t wo i n n e r wa l l s , n o r ma l l y i n c o n t a c t
with one another in the resting state, correspond to the pseudo-tracheal mem
-brane of the house fly. The potential space between them represents, as will
be seen presently, an invagination of the distal end of the proboscis, which can
be done away with by the retraction of the outer wall. Between this inner
wall and the chitinous outer wall there is a space, which is continuous with the
hæmatocœle of the labium and through it with that of the rest of the body ;
this space, as shown in the sections, which are accurate drawings traced with
the aid of a camera lucida, contains a number of large and irregularly shaped
cells, which are the corpuscles of the blood of the fly. In the drawing they are
depicted as actually seen in the preparations, and show obvious shrinkage and
deformity, due of course to the drastic treatment which is necessary to get even
tolerably good sections of such a resistant and impermeable region. Most of
the cells are free in the space, while others, which are probably of different
origin, appear to be attached to the inner and outer walls. The number and
distribution of these cells differs a good deal in different preparations, but they
are generally most numerous in the distal part of the labella. From the point
of view of the parasitologist, the existence of this blood space in the labella is of
some importance, on account of the opportunity it offers for the passage of any
parasite which once becomes free in the body cavity to pass into the wound
made by the fly. The only obstacle to be encountered is the thin inner wall of
27
the labellum. The connection of this blood space with the mechanism of the
proboscis will be discussed later, but it should be noted here that this very small
and definitely bounded space represents the wide interval between the inner
and outer wall which is found in the labella of the non-biting flies, when the
labella are distended.
The Mechanism of Feeding.
Having now described the anatomical structure of the proboscis, we are in
a position to consider the mechanism by which it acts in the making of the
wound and in the absorption of the blood. It will be evident that such extra
-ordinarily complex structures as are found in the labella must have an equally
complex method of action, and it will be well at the outset to state that since
the difficulties in the way of observing the actual working of the organ, or
producing the actions by artificial means, have proved insurmountable, the
explanations offered hereafter are in the nature of working hypothysis, and do
not pretend to be a complete account of all the factors which may come into
play when the insect feeds. So many remarkable features have appeared in the
course of the investigation that one hesitates to assume that the subject is
exhausted, and although what has been found agrees in the main principles
with what occurs in other flies, one must be prepared to find that further study
will reveal yet more complications.
So far the whole description of the anatomy has referred to the parts in
the position of rest, and all the sections are drawn from specimens fixed in
that position. Sometimes, however, one can succeed in fixing a proboscis in
another position, which, there is reason to believe, is that into which it passes
during the process of feeding and, as I hope to show, it is the change from the
one position to the other that it is the essential act by which the wound is made.
It will be necessary to describe the appearance of the labella in this position
(Fig. 13, Plate II) ; the attitude taken up by the haustellum and the rostrum
has already been dealt with.
The specimens which I have obtained fixed in this position were got by
pressing forcibly on the head while the fly was immersed in the fixative, and
mounting without clearing. I have found that the action of the potash in dissolv-
ing the soft structures always results in bringing the proboscis back to a position
of rest, and that unfortunately precludes any detailed examination of the chitinous
parts. It enables one to assume, however, that the position is attained, and
retained by the action of the muscles, and that when the muscles relax, as hap-
pens when they are dissolved by the potash, the position of rest is again assumed.
In the position of action, as we may provisionally term it, the appear-
ance of the labella is totally changed The internal surface becomes external,.
E