NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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channel. On cross section it is seen to consist of two laminæ, the outer one,
which represents the labral element, having a heart-shaped outline, while the
inner lamina, which is the epipharynx, is oval. The two unite at the borders
of the groove, and enclose between them a U-shaped space in which there is a
small amount of loose cellular material, but no fan-shaped muscle similar to
that found in Musca and Philæmatomyia. At the upper end of the channel the
walls become much thicker and the space between them at first increases, so
that the whole organ is a little broader than elsewhere and the food channel of
a slightly greater capacity. Immediately beyond this point the channel under-
goes transition into a closed duct, in the following manner. The outer and in-
ner laminæ approach one another until they come in contact, and fuse to form
one thick layer of chitin, the space between them being entirely obliterated ;
at the same time the edges of the groove formed by the epipharynx approach
one another and unite, so that closed canal is produced. This point is
therefore the true mouth of the fly, and lies anterior to the position of the
mouth in Musca. The canal is at first oval in outline, being broader in its
transverse diameter than in its vertical one, but in a short distance it becomes
circular. The duct is produced upwards a short distance beyond the end of the
labrum, and projects into the rostrum. Here it joins the duct already des-
cribed as the buccal cavity.
The labral apodemes are so similar to those of Stomoxys that they call for
no remark. They are attached to small pits on the lateral aspect of the
labrum at its thickened upper end, the method of attachment being such as to
permit of movement inwards when the apodemes are brought into line with the
haustellum in the movement of extension. In the resting position the angle
between the two apodemes is about fifty degrees ; in the position of extension
they are almost parallel.
The distal end of the labrum-epipharynx (Fig. 3, Plate 1, and Fig. 23,
Plate 4) is modified in a very curious manner, which all the more calls for a
detailed description in that no explanation of its function can be offered,
though the structure is obviously a purposeful one. When seen from above, the
lateral edges approach one another in a gentle curve, to form a point like
that of a two-edged sword. The area between the converging margins and
that portion of the blade immediately proximal to it has a totally different
appearance to the rest of the blade. The upper surface remains as before
described, an evenly rounded one, but on the under surface there is developed
a conspicuous elongated elevation, about four times as long as it is broad, a
little narrower in the middle than at the two ends, and equal in width at its
narrowest point to one half the width of the whole organ. When seen in sec-
tion the organ has at this point a complete rounded outline, with no trace of a
11
groove. The upper half of the section is more pointed than the lower, and the
two are separated by a prominent groove, due to the fact that the raised
portion of the under surface is constricted at its base. At the proximal end
of this elevation the groove on the under surface of the rest of the labrum-
epipharynx ceases abruptly in what is apparently a cul de sac, the margin of
the raised portion being slightly concave backwards. This margin is situated
at the junction of the middle third of the elevation with the upper third, and
the elevation proximal to it consists of raised flanges on either side of the
termination of the groove, which end abruptly as a pair of blunt tubercles
projecting into the food channel. Proximal to these tubercles the margins of
the groove show a slight concavity, and thereafter run parallel to one another
in the ordinary manner.
The distal end of the elevation extends to within a very short distance
from the extreme tip of the organ ; at its broadest point it reaches almost to
the limit of the converging margins. Beyond it, and reaching to the distal
margin, there is a hollowed out tongue-shaped depression on the under surface.
The lateral margins of this are raised and overlap the central portion in the
middle third, and converge to wards one another so as to form a narrowed
portion which might be compared to the handle of a racket. The narrowed
portion projects into the elevation of the under surface for nearly one-third
of its extent, and has at a point just distal to its termination a pair of
short curved tubercles set on either side of a slight expansion of the lateral
diameter.
It will be seen that the groove of the epipharynx falls considerably short
of the end of the organ, being separated from it in fact by the whole length of
the elevation. At first sight one is apt to think that the purpose of the tongue-
shaped depression at the extreme end must be to direct the food into the
channel, and that the elevation is due to the union of the two lateral margins
of the epipharynx in the same way that they are united at the upper end of
the organ, and that the elevation is a hollow one through which the food
passes. Examination of preparations under a very high power tends to con-
firm this view, for distinct clear space can be seen by focussing down through
the elevation. Sections, however, do not confirm this, for they show only one
continuous outline, with a little shrunken cellular tissue inside it, and no trace
of even a membraneous inner tube, and it is clear that the food channel
does actually end at the concave transverse ridge in the upper third of the
elevation.
The only suggestion that occurs to one as regards the function of this
remarkable structure is that it, in connection with the hypopharynx, acts in
some manner as a valve to regulate the flow of blood into the food channel. It
C
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can have nothing to do with the making of the wound, for the tip of the organ
is far too thin and flexible to have any perforating power, and moreover—and
this is an important point which will be referred to later—the labrum-epi-
pharynx does not reach to the tip of the proboscis.
The sensory hairs of the labrum-epipharynx resemble those of Musca and
Stomoxys. There are two opposite to one another at the level of the tubercles
which project into the lumen at the distal end of the canal, and several others
at irregular intervals along the course of the canal. Each consists of a short
stout cylindrical base, projecting into the lumen of the channel and a very fine
hair arising from the apex of this. The hairs are all bent downwards in the
direction of the canal.
The hypopharynx (Fig. 23, Plate 4, et seq. ), like the labrum-epipharynx,
is a much finer and more delicate organ than that of Musca. It is a thin flat
slip of yellow chitin, containing in its middle line the salivary duct, and also
serving by its apposition with the labrum-epipharynx to close in the food
channel. Its distal end, which falls a little short of that of the labrum-
epipharynx, is quite simple ; it is not produced to a point, but terminates in a
very much attenuated margin, in the middle of which the salivary duct opens.
At the upper end of the haustellum the hypopharynx fuses with the labial
gutter and disappears as a separate organ, only the salivary duct being left.
Where the lateral portions of the organ cease the walls of the duct become
much thicker and also become adherent to the closed canal formed by the
union of the lateral margins of the epipharynx (Fig. 29, Plate 6). The
salivary duct thus projects at the upper end of the haustellum below the com-
mencement of the buccal cavity.
The Labium.
The labium consists of two separate parts, the mentum and the labial
gutter, which together form a cylindrical chamber enclosing the muscles and
other structures. The shape of the labium in Lyperosia is intermediate be-
tween that of Stomoxys and that of Glossina, that is to say, the " bulb " is more
pronounced than that of the former, and the whole not quite so pointed as that
of Glossina. Distally it is divided into two labella, which differ so much from
the rest of the labium that they constitute a distinct organ.
The mentum (Fig. 1, Plate 1) composes by far the largest part of the
labium. It has the form of a spindle, composed of a plate bent to the shape,
and evenly rounded except on the anterior surface, where there is an interval
in which lies the labial gutter. The truncated upper end is continuous with
the membraneous wall of the rostrum, and the lower end with the external
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wall of the labella. The groove on the anterior surface is a deep one, the edges
of the mentum turning backwards as well as inwards, and leaving an interval
between them, in which the labrum epipharynx and the hypopharynx lie.
There is no thickened lateral border such as is found in P h i l œma t o my i a .O n
the external surface there are a few scattered short bristles, irregularly
distributed. The whole length of the organ shows a series of transverse ridges
and furrows, deeper and more numerous at the narrower part of the spindle,
similar to the " striations " in Stomoxys; a s w i l l b e s e e n l a t e r , t h e s e a r e o f g r e a t
importance in the mechanism of the proboscis. The lower end. of the mentum
is obliquely truncated from in front backwards and upwards, in such a way
that the distal portion of the posterior border is in advance of the anterior
border to an extent equal to the diameter of the labium at this point.
On the posterior surface of the distal end of the mentum there is a pair of
small but stout chitinous sclerites, termed by Stephens and Newstead the
ventral sclerites (Fig. 2, Plate 1). They are homologous with the forked
rods at the end of the mentum in Musca. Each is wedge-shaped, with the
broad end of the wedge directed distinctly, and forming a support for the
U-shaped rod which separates the labium from the labella This rod, termed
by Stephens and Newsteacl the " fork," is the homologue of the rod commonly
known as the furca in Musca. It has the shape of a wide U, and embraces
the posterior half of the labium at its junction with the labella. In the resting
position of the parts it is directed obliquely downwards and forwards across the
external surface of the labella, reaching a little more than half-way across the
surface. The middle portion of the furca is moulded to articulate with the
ventral sclerites, on to which it is pressed by the action of the muscles inserted
into it. A short distance external to the ventral sclerites, there is on each side,
proximal to the furca, a small rod of chitin, attached to the edge of the reced
ing margin of the mentum, and closely approximated to the posterior border
of the furca, though not fused with it. This represents the second or distal
division of the fork of the mentum as seen in Musca, w h e r e i t i s a s l o n g a s a n d
similar to the first division, and only separated from it by a slight thinning of
the chitin. The furca and its "lateral sclerites" are not internal structures,
as st at ed by St ephens and Newst ead, but are t hi ckeni ngs of t he ext ernal
wall of the labella, the articulation lying between it and the ventral
sclerites.
The distal end of the mentum has been described as being obliquely trun-
cated, the posterior portion, which articulates through the ventral sclerites with
the furca, being produced beyond the anterior. In this way a space is left
between the border of the mentum and the furca, where there is no chitinous
wall to the labium. This interval is filled in by a membrane which is attached
c2
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to the border of the mentum, and anteriorly becomes continuous with the
external wall of the labellum; it has developed in it two thin chitinous plates,
r o u g h l y o v a l i n s h a p e , a n d p l a c e d o n e b e h i n d t h e o t h e r o n t h e a n t e r i o r b o r d e r
of the organ. The plates are mere thickenings in the membrane, and have no
chitinous attachments, and are much thinner than the chitin of the wall of the
labium.
The labial gutter forms the bottom of the groove in the anterior surface of
the labium, and supports the labruna-epipharynx and the hypopharynx. In
its general features it resembles that of Stomoxys ; it is, of course, the homo
- l o g u e o f t h e s h a l l o w c h i t i n o u s g r o o v e o n t h e a n t e r i o r s u r f a c e o f t h e l a b i u m . o f
Musca , but on account of the great development of the muscles of the bulb it
has come to have additional functions. It is a stout chitinous groove, U-shaped
in the distal portion, but becoming more V-shaped. towards the upper end. It
extends the whole length of the labium., from the termination of the labrum
and the formation of the closed portion food canal to the level of the lateral
portion of the furca in the position of rest. As for the most part of its course
it lies between the overlapping edges of the mentum it cannot be seen even in
cleared preparations, but at the distal end it projects beyond the anterior border
of the mentum and forms the anterior boundary of the membraneous space
between the furca and the truncated end of the mentum, and can easily be
distinguished through the thin wall in this situation. Throughout its length
its lateral borders are attached to the adjacent borders of the mentum by a
stout membrane, which, however, is of very limited extent, its width being just
sufficient to fill in the sides of the space occupied by the labrum-epipharynx—
a marked contrast to the corresponding membrane in Musca, which is very
loosely arranged. At the distal end of the proboscis it is attached to the furea
by means of a continuation of the same membrane, in the triangular space
already referred to.
The sheet of chitin projecting into the cavity of the labium from the po
s - t e r i o r s u r f a c e o f t h e l a b i a l g u t t e r , a n d a p t l y t e r m e d b y S t e p h e n s a n d
Newsteacl the " keel," is present also in this fly, in a slightly modified form.
It commences a little distance behind the end of the gutter, as a ridge equal
in height to the depth of the groove, and of about the same thickness as
the rest of the gutter. This extends for about one-fourth of the length
of the gutter, and then becomes thinner, and approaches the base of the gutter
again. The keel does not, however, cease at this point, but broadens out again
and passes still deeper into the cavity of the labium as the latter becomes of
greater diameter towards the bulb, until it almost reaches the posterior surface
of the mentum. This upper part of the gutter is only a very thin sheet of
chitin, easily bent in dissection, and difficult to see in cleared preparations.
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the mentum ; these do not correspond to anything found in Musca, and it
is impossible to say what their origin may have been.
The only other structures to note in the labium are the nerves and tracheæ.
As might be expected in so small a fly, one cannot trace them to their ultimate
terminations, but there is one important point to note with regard to the
t r a c h e æ . F r o m t h e e n d o f t h e t r a c h e a l s a c i n t h e r o s t r u m t o t h e e n d o f t h e
proboscis the only tracheal structures to be found are a small pair of simple
t r a c h e æ , s m a l l e r i n d e e d t h a n o n e w o u l d e x p e c t t o f i n d , s i t u a t e d o n e i t h e r
side of the keel of the labial gutter. These become very small in the distal
part of the proboscis, and can only be distinguished in the labella by follow
- i n g t h e m u p i n s e r i a l s e c t i o n s . T h e r e a r e n o t r a c e s o f a n y t h i n g r e s e m b l i n g
an air sac. The nerves pass down the cavity of the labium together with
t h e t r a c h e æ , a n d a r e r e m a r k a b l y l a r g e . T h e y l i e e x t e r n a l t o t h e t r a c h e æ ,
one on each side, and at the upper end of the proboscis the two are of about
equal si ze, but as they pass down the probosci s the tracheæ di mi ni sh i n si ze
much more than the nerves, so that at the junction of the labium and the
labella the trachea appears in sections as a small clear area on the posterior
surface of the nerve. It is evident that though the labella require a large
suppl y of nerves, most of t he t racheæ are dest i ned f or t he suppl y of t he
muscles in the labium, and it will be a legitimate inference from this that
the labella are very highly sensitive structures, and that the reflexes on which
the control of the biting parts must depend are highly efficient.
The Labella.
The structure of the lab ella is so extraordinarily complex that it would
be almost impossible to describe it intelligibly were it not that one can trace,
in some at least of the parts, a homology between this fly and the simpler,
or at any rate the better known, non-biting flies. It will appear in the course
of the description that according to my interpretation of the anatomy and
physiology of the proboscis, that of Lyperosia is not essentially different to
that of Musca , but is a modification of it, all the parts found in M u s c a
being present here and fulfilling for the most part the same functions.
Regarded from this point of view the first thing that strikes one is the
great reduction in the size of the labella. The transverse diameter of the
two labella is the same as that of the narrowest part of the labium, and the
vertical diameter only a little greater. Secondly, the labella, in the position
of rest, are not folded backwards on the labium, but are extended in the same
line with it, and are, as a matter of fact, incapable of being moved into the
position of rest of Musca. Further, the fringes of macro-chaet which are
so conspicuous in the non-biting flies are not present in Lyperosia.
The labella are oval in shape, slightly longer than broad, and deeper in
the vertical diameter than in the transverse. They are completely separated
from one another on the anterior surface by a narrow fissure, which is con
-t i nuous wi t h t he groove on t he ant eri or surf ace of t he l abi um. The f i ssure
extends round the blunt apex to the posterior surface, but about midway
between the tip of the proboscis and the furca it terminates, and from this
point the two are continuous with one another. Each labellum consists of
an external and an internal wall, united at the side of the fissure, except
where the two posterior surfaces are continuous with one another. The external
wall on each side consists of thin plates of chitin separated from one another
by narrow fissures, which are filled in by membrane, the arrangement
being adapted to ensure rigidity and at the same time to permit of move
-ment . The i nt ernal wal l i s composed of t he bi t i ng t eet h, t he di scal scl eri t e,
and the membrane uniting it to the external wall.
When seen in cross section (Fig. 14, Plate III) the external wall of the
labella appears as a oval ring of chitin, the long axis being directed
vertically ; the ring is incomplete on the anterior surface throughout the
length, and in the distal half of the posterior surface. When examined in
cleared preparations the plates of the two sides, convex externally, show
certain well-defined fissures. The largest of these is on the posterior surface,
and commences on each side a short distance external to the middle line and
just distal to the furca ; from this point the two run parallel to one another
up to and a little beyond the point where the two posterior walls diverge
at the fissure. Proximal to the commencement of these fissures and an
-terior to the furca the chitin of the external wall is very thin and semi-
membraneous. At the side of each plate there is a similar membraneous
interval posterior and distal to the end of the furca. On the anterior border
of each labellum the continuous plate ceases gradually and is replaced by
a thin membrane loosely arranged, and of course continuous with the mem
- b r a n e b o u n d i n g t h e t r i a n g u l a r s p a c e b e h i n d t h e f u r c a . T h e m e m b r a n e i s
of the same nature as the one in the same situation in. Stomoxys, that is to
say, it is composed of a clear and transparent (in cleared preparation) ground
s e t wi t h s ma l l s q u a mæ a r r a n g e d s o t h a t t h e c l e a r s p a c e s b e t we e n t h e m f o r m
a f i ne net work. The squamæ or t hi ckened areas i n L y p e r o s i a
a r e h o w e v e r
much smaller and less conspicuous than in Stomoxy.
The distal limit of this external plate is not well defined, but merges
imperceptibly into the membrane of the inner wall. It presents a gently
rounded contour, on which there is a row of ten short bristles arising from
small raised bases, and forming a terminal fringe to the proboscis. Except
for a few much finer and shorter hairs, two pairs of which are situated just
19
in front of the lateral end of the furca, and two more pairs, one internal and
external to the membraneous interval on the posterior surface, near the
termination of the median fissure, the external surface is naked.
The internal surface of each labellum consists of two distinct portions,
one of which is freely moveable, the other being united with its fellow of
the opposite side. The distal moveable portion corresponds to the inner
surface of the labellum of Musca, w h i l e t h e f i x e d p a r t i s c o m p o s e d o f t h e
discal sclerite. It will be necessary therefore to describe these two parts
separately.
The free portion of the internal surface of the labellum extends obli
- q u e l y f r o m t h e p r o x i m a l l i m i t o f t h e m e d i a n f i s s u r e o n t h e p o s t e r i o r s u r f a c e ,
forwards and upwards to the anterior surface, so that it occupies rather less
than half the area of the labellum as seen from the side. It is entirely occupied
by the biting teeth and their accessory structures, and is connected with the
external surface by the thin and transparent membrane which forms the actual
internal wall of the labellum in this situation.
The teeth (Fig. 6, Plate II) are very formidable weapons for so small
a fiy. There are in all eight on each side, three of the set being small and
inconspicuous. All are roughly oblong in shape, slightly bent in an upward
and forward direction, and with one exception, are bifid at the free end.
Counting from the anterior or upper end of the set, the third tooth is the
largest, the second and fourth equal in size and little smaller than the third,
the fifth and sixth are of the same size and are smaller than the fourth, while
the first is smaller still. The seventh and eighth teeth are rudimentary. The
seventh resembles the others and is about the half the length of the sixth ;
the eighth, which is concealed in the angle formed by the two sets of teeth,
is very small and its shape is difficult to be certain of. It is not bifid, but
pointed, and has on each side of it two deep notches. All the teeth are
united to one another at their bases by means of a stout but narrow ridge
of chitin, into which the bases merge, so that the arrangement is not unlike
that of the teeth of a comb. The ridge of chitin is gently curved along its
proximal margin, in such a way that the cutting edges of the teeth are brought
into the same straight line, the longest arising from the deepest part of the
curve, the short ones from its two ends. The eighth and seventh teeth do not
come into line with the others. When in sitût h e c u t t i n g e d g e s o f t h e t e e t h
form an oblique line running from the front backwards and downwards, in the
same direction as the end of the discal sclerite.
The accessory structures of the teeth may for convenience be termed
collectively the interdental armature . That of Stomoxys has been elaborately
described by Stephens and Newstead and the appearances seen in Lyperosia D