NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
18
and of their lack of the necessary musculature. In most of these flies, however,
there are certain processes on the distal ends of these organs which certainly
bear a superficial resemblance to teeth and have been repeatedly described as
such, though I am not aware of any detailed description of the method by
which they might be supposed to act. It is just in this situation that one
would expect to find a filtering mechanism, and it is worth while to examine
these structures in some detail to see if they fulfil the requirements.
Considered with regard to the armature of these organs the flies of this
group fall naturally into three divisions, the first including Simulium,
Ceratapogon, and P hlebotomus, the second the Tabonidæ, and the third the
mosquitoes. These will now be examined in detail and compared with one
another.
In Simulium (figures 15 and 14) the labrum-epipharynx and the hypo-
pharynx, which are flattened and spatulate organs, terminate in gently rounded
margins without any point, the middle portion being in fact almost straight.
In the labrum-epipharynx there is on each side of the middle line a rather
thick longitudinal band of chitin, extending nearly but not quite to the tip,
the two bands lying on either side of the food canal. On this baud there is a
row of fine non-pigmented hairs, somewhat irregularly arranged and extending
in an oblique line to the point where the lateral border joins the distal one.
They are directed forwards, inwards, and towards the opening of the food canal,
and those which are the most proximal in position are a little longer than
those at the distal end. At the extreme distal end of the organs there are two
sets of extremely minute hooks or teeth, one set on each side of the opening,
and projecting beyond the distal margin. On account of their small size and.
position it is very difficult to be certain of the exact number and arrangement
of these structures, which could only be examined satisfactorily in a prepara-
tion mounted with the long axis of the organ in the same line as the micro-
score, and the figure must be taken as approximate only. There appear to be
three teeth on each side, each having the shape of a rose thorn, and radiating
outwards from their bases. One on each side projects definitely beyond
the distal margin. It is somewhat difficult to find a function for these peculiar
structures ; they cannot be used as cutting teeth, for they are incapable of
movement on the labrum, having neither joint or muscles so far as one can
see, neither do they appear at all suited for use as a filter. Possibly they are
used to fix the organ in position when a suitable depth has been reached. The
edge of the labrum-epipharynx on either side of these teeth is very thin and
difficult to define in cleared preparations, and is membraneous and probably
flaccid ; it bears a few minute hairs similar to those on the thickened portion
of the surface.
19
The hypopharynx terminates distally in a border which is parallel to that
of the labrum-epipharynx. In this case, however, the organ is chitinous right
to its extremity, and is highly modified. The margin on each side of the'
alivary duct is broken up into innumerable minute elongated processes, which
form a fine fringe like the frayed edge of a piece of cloth, all the processes
being directed distally. This fringe occupies the entire extent of the rounded
portion of the distal end, and terminates where the lateral borders become'
parallel to one another. On the posterior or inferior surface of the hypopharynx
there are a few small hairs similar to those on the anterior surface of the
labrum, but smaller, and arranged in a loose row near the margin. These arise
from the inferior or labial lamina of the organ.
In Ceratapogon (figures 6 and 7) we find a similar arrangement, but the
serrated border is found on both organs. The labrum-epipharynx is gently
rounded at its distal end, and has on each side of the curved distal margin a
row of five flattened and blunt pointed processes. These are simply produced
by indentations of the margin, and have no thickened bases such as are found
in true cutting teeth ; they point directly in the long axis of the proboscis
and have no fine serrations on their edges ; one side of the process is not
thinner or sharper than the other. Between the two most internal ones of the
two rows there are two other pairs of processes, set on each side of the extreme
apex of the organ, where the food canal opens to an exterior. These are
rounded, about twice as long as thick, and with slightly constricted necks,
which are bent a little backwards so that the rounded ends of the processes
hang over the opening of ,the food channel. The two innermost of the two
sides are more distant from each other than any of the other processes, and
internal to their bases there is a pair of minute thickenings in the chitin, and
proximal to these a medial oblong thickening situated a short distance proximal
to the tip of the organ.
The serrated margin of the tip of the labrum-epipharynx is not in the same
line as that of the rest of the lateral borders, but is separated from it by a deep notch
situated a short distance proximal to the proximal process. This gives the labrum
a somewhat barbed appearance. It will be noted in the figures that, if we
deduct the width of the slightly thickened margins which project beyond the
notch on the two sides, the width of the labrum is almost the same as that of
the hypopharynx, and the serrated margin of the former will therefore project.
beyond the margin of the latter in the natural superimposed condition of the
parts.
The hypopharynx resembles the labrum-epipharynx in outline, but, as
indicated in the last paragraph, it is slightly smaller. Its margin is also
serr ated in a manner very similar to that of the labrum-epipharynx, there D
20
being five indentations and five processes on each side. At the point where the
salivary duct opens to the exterior there are four small finger-like filaments,
which arise from the lower lamina and project distally. The upper or anterior
surface of the organ is marked by five longitudinal dark patches, which
commence at the bases of the processes in the margin, and pass inwards towards
the salivary duct. The appearance suggests that the surface is traversed by
oblique longitudinal grooves, with ridges between them, and. that the dark
patches represent thickened portions of the chitin, but the parts are too small
to permit one to be certain of the point.
The parts in Phlebotomus are rather simpler. The labrum-epipharynx
is produced to a narrower point, the curve of the lateral margins at the distal
end being concave rather than convex. The narrowed portion is divided up
into a number of minute processes, coarser than those of the hypopharynx of
Simulium, but much finer than those on that of Ceratapog on ; the two spine
at the extreme apex are rather thicker than those at the sides, and project
forwards almost parallel with one another. The hypopharynx has its edge
serrated in a similar manner, but the indentations are not quite so deep or so
numerous. In its outline the hypopharynx is not quite so much pointed as the
labrum-epipharynx, and some of the processes on its margin overlap its edges
in the natural condition of the parts.
In these three flies we find, then, that the distal ends of these organs are
split up into rows of more or less elongated and pointed processes, varying in
shape, but all exhibiting the same general structure, and all directed forwards
in line with the proboscis. With the single exception of those on the labrum-
epipharynx of Simulium they bear no resemblance whatever to biting teeth
such as are found in the mandibles and maxillae of these flies, nor do they
resemble those found on the inner surfaces of the labella of the blood-sucking
Muscids. Apart from the facts already insisted on, that a suitable motive force
and articulation are entirely absent, the nature of the margin, when it is examine
ed in a well cleared preparation, is a sufficient indication that it is not primarily
intended for cutting purposes. On the other band, the arrangement is ad-
mirably adapted to serve as a filter or sieve, by which large particles can be
prevented from entering the canal.
The groove on the under surface of the labrum-epipharynx does not
extend to the extreme tip of the organ, but terminates a short distance from it
by broadening out into a flattened sheet of softer tissue which is not distin-
guishable from the anterior lamina, the space between the two being gradually
reduced until they come in contact with one another ; in other words, the
epipharynx does not extend the full length of the combined organ as a
separate structure, but is reduced in size and fused with the labrum to form a
21
more or less flattened and tongue-like end. Normally this lies in front of the
flat apex of the hypopharynx, and the opening of the food canal is therefore
not a circular one, but a transverse fissure between two superimposed plates.
The figure of the ends of the two organs given in Newstead's paper on
phlebotomus is most suggestive. He shows that there is a slight interval
between the two, caused by a slight forward curve of the labrum-epipharynx
and a corresponding backward curve of the hypopharynx. The " teeth " on the
former organ are directed slightly backwards, and those on the latter slightly
forwards, so as to meet one another and even to overlap to a slight extent, thus,
filling up the interval which would be left by the curvature of the two ex-
tremities. The blood has to pass through the opening guarded by these
processes before it can reach the circular portion of the lumen of the canal, and
it is evident that the presence of these spines must be a most efficient means
of stopping any particle too larger for the lumen from entering it. I have
seen a similar appearance in a preparation of Ceratapogon amounted on its side,
but the evidence of Professor Newstead's drawing is all the more convinc-
ing in that it appears to be simply an instance of accurate observation and
drawing, for he does not refer in the text to the possibility of the mechanism
being a filtering one.
I think it probable that the filtering mechanism provided by these spines
is not altogether a passive one, but that the space between the two flattened
tips can be varied to a limited extent by means of the muscle situated at the
base of the labrum. Probably in the resting condition the two plates are in
contact with one another and the spines more or less interlocked, at least in
the case of Phlebotomus ; in other words the mouth of the fly, or rather the
prestomal orifice, is closed. When the piercing parts are introduced into the
skin as the wound is made, the labrum is raised up a little by means of its
muscle, and the potential opening converted into an actual one through which
the blood can enter. It is at least very suggestive that the epipharynx and
hypopharynx show no sign of fusing together to form a complete tube, in any
of these flies, indicating that a certain degree of movement between the two is
advantageous for the insect, whereas one would expect that a complete tube
would afford greater stability than is provided by the simple apposition of the
two organs; since the epipharynx and the hypopharynx are of identical origin,
both being outgrowths from the same part of the stomodæum, one would expect
such a fusion if a simple tube to function as a food channel were all that were
required, and such fusion does actually occur in the upper portion of the canal
in the more highly specialised biting flies of the Muscid group.
The Tabanidœ (figures 18 and 19) show a different type of apparatus, in
which the filtering mechanism is not quite so obvious. Here the labrum-epi-
D 2
22
pharynx is a much stouter and broader organ than the hypopharynx, and it is
also slightly longer, so that the tip of the hypopharynx falls a little short of the
other stylets. The apex of the labrum-epipharynx is broad and truncated, and
the lateral borders are quite free from serrations such as are found in the flies
already discussed. On the distal margin there are three sets of tubercles,
arising from slightly raised areas, and projecting beyond the distal margin.
The two lateral tubercles, which are comparatively large, consist of ten or
twelve short stout peg-shaped processes, and the middle set of two rows of five
each. Their exact number and arrangement is rather difficult to make out an
account of their position, the posterior ones being overlapped by the anterior.
The tip of the hypopharynx is distinctly narrower than that of the labrum-epi-
pharynx, and is therefore overlapped by it. The two, however, resemble one
another in shape, and, like the labrum-epipharynx, the hypopharynx is devoid
of any armature at its sides. In some of the large species, such as Tabanus
striatus, a distinct flange can be seen at the opening of the salivary duct,
and in some preparations the projecting edges of this flange look very like teeth
set on either side of the opening. It is easily seen in fresh preparations that
they are not teeth, and that the lat eral borders of the organ are only very thinly
chitinised, so that they do not present well defined edges, but are on the
contrary somewhat wavy. The flange at the mouth of the salivary duct bears
a few extremely delicate hairs, possibly sensory in function.
In these flies there is therefore no structure which could act as a cutting
weapon, and we conclude that they have only to do with the ingestion of food.
The tubercles at the apex of the labrum-epipharynx will act as a protecting
fringe to prevent the ingress of large particles from the front and sides, but can
be by no means so efficient as the well developed fringe found in Phlebotomus
or Ceratapogon. The hypopharynx in this case probably plays the most im-
portant part in the mechanism. Its tip and sides are extremely thin and are
very easily bent, from which it is justifiable to conclude that in the natural
condition they are flaccid. Now Stephens and Newstead, in their description
of the mouth parts of Stomoxys, make the suggestion that the soft tip of
the hypopharynx functions as a valve to prevent the ingress of large particles
into the mouth, and I think that the same thing happens in the case of the
Tabanidœ. In the natural position of the parts the rounded tip of the hypo
-pharynx lies just behind that part of the labrum-epipharynx which bears
the tubercles, and since both organs are flattened at the tip they will be
normally in contact with one another, except when the labrum is elevated by
the muscle at its base. The negative pressure created in the food canal by the
dilator muscles of the sucking apparatus will tend to draw the two portions of
the sucking tube together, and will act with the greatest effect on that part
23
which is the most flaccid and the least supported, that is, it will tend to close
the prestomal orifice by drawing the apex of the hypopharynx towards the tip
of the labrum-epipharynx. The organ is sufficiently rigid to prevent this hap-
pening to such a degree as would completely close the orifice, and one must
conclude that there is a nice adjustment between the flaccidity of the tip of the
hypopharynx and the force tending to approximate it to the labrum-epipharynxe,
so that an aperture of the correct size is left between the two.
In the mosquitoes the mechanism by which ingress of large particles is
prevented is similar to that in the Tabandœ, but on account of the small size
and delicate nature of the parts it is much more difficult to follow. The distal
end of the labrum-epipharynx (figure 17) varies a good deal in the different
forms, but so far as I am aware it never bears any armature which could be
used for cutting or piercing. In shape it differs a little from any of the forms
so far considered, for the main portion of the labrum-epipharynx is always
rounded to the extent of almost four-fifth of a circle, and the transition from
this round tube to the flattened tip gives the organ the appearance of being
obliquely truncated, something after the shape of a quill pen. In Nuttall
and Shipley's monograph on Anopheles maculipennis the tip of the organ is
shown to be provided with two pairs of fine tooth-like processes, one pair at the
extreme point and another pair a little distance behind them. These do not
occur in all species of Anopheles, and I have not found them in any species of
Culex which I have examined, even in Culex concolor, where they would be
easily seen if present on account of the large size of the parts. In a species of
Joblotia I have found the distal margin indented so as to produce four teeth on
each side, the indentations being deep but the spaces between the teeth ex-
tremely narrow. There seems to be a considerable amount of variation in the
method of termination of this organ, but all forms agree in the essential points,
viz., that there are no cutting teeth and that the end of the organ is flattened
and fairly thick and rigid.
The tip of the hypopharynx presents a characteristic appearance in the
large forms. If one examines the organ in the fresh state under a magnifica-
tion of at least a thousand diameters and with as small a diaphragm as possible,
one sees that the organ does not terminate in a well-defined point as ordinarily
depicted, but in a very thin and apparently membraneous expansion, with a
gently rounded margin (figure 9). In view of what has already been said with
regard to the probable mechanism in the Tabanidœ the mechanism in this case
will be obvious. The extremity of the hypopharynx lies a little above that of
the labrum-epipharynx, so that when the sucking action commences blood can
only enter from the posterior aspect, and in doing so will impinge on the soft
tip of the hypopharynx and tend to drive it against the more rigid labram-epi-
24
pharynx, the stream of blood being aided by the negative pressure in the food
canal, which will also tend to draw the two organs together. The entrance to
the food channel is not a fixed and rigid one, but varies in size according to the
distance between the labrum-epipharynx and the hypopharynx, this being
regulated by the muscle at the base of the labrum. The adjustment is a very
fine one, for the more powerfully the pharyngeal muscles contract the more
will the two parts of the tube be drawn together, and more the valve-like end
of the hypopharynx will tend to close the aperture.
A word is necessary with regard to certain minute spines found in the in-
ernal surface of the groove of the labrum-epipharynx. I have only found
these in Tabanidœ and in Simulium, but it is likely that they also occur in the
other members of the group. They are short, stout, and gently curved struc-
tures, arising from expanded and globular bases which lie in the substance of
the chitin of the epipharynx, and are seen as clear spaces when the structure is
examined in optical section. The base of each of these spines is perforated
where it projects into the space between the labrum and the epipharynx, and
with careful focussing a fine canal can be seen in the lower part of the free
portion of the hair. They closely resemble the spines described in a correspond-
ing situation in Stomoxys by Hansen and by Stephens and Newstead, and
one is inclined. to regard them, from their situation, as organs of taste, though
of course there is no direct evidence as to their function. Possibly it may be
through them that communication between the prestomal orifice and the muscle
of the labrum is established for the purpose of regulating the flow of blood.
Their occurrence in such a widely separated flies as Simulium and Stomoxys is
suggestive.
The mechanism of the labium appears to have escaped the notice of
workers on this subject. It is generally stated that when the mosquito feeds
the labium becomes bent backwards like a bow, and the labella separated to
allow the piercing parts to pass between them, but no mention is made of
the forces which bring about this change in position, and one is left to
assume that the labium is simply pushed back because it cannot enter the
skin. The process can be readily watched in the mosquito, and is faithfully
depicted in Nuttall and Shipley's drawing. In all the other flies without
exception the same thing must take place, since the labium and labella are
always long enough to conceal the mouth parts in the condition of rest.
There is no reason to suppose that there is any such haphazard bending as
that indicated above, for the labium has its intrinsic muscles, some arising
within the head capsule and some within the cavity of the organ, which are
inserted into its integument at the distal end and into the labella. These
are familiar from the many diagrams of the cross-section of the proboscis
25
of the mosquito, and are of course figured in the classical papers of Meinert
and Hansen. In most Nematocerous flies the labium and labella are
separated from one another by a loose joint in which there are lateral trans-
verse bars of chitin, very conspicuous but not described in detail in the
mosquito, which are probably homologous with the furca in the muscid flies,
and some of the muscle fibres can be traced to these in the large mosquitoes.
We have here all that is required for the retraction of the labium and the
eversion of the labella which can be seen to take place when the fly feeds.
The muscle fibres arising within the cavity of the labium and inserted into
these transverse rods evert the labella to allow the piercing parts to enter the
skin, exactly as is the case in Tabanus where the labella are large and impor-
tant structures with a function of their own with regard to feeding. The
retraction of the labium and the bent position it assumes are the result of
the contraction of its muscles and of the peculiar structure of its wall. The
mentum of the mosquito is a long chitinous gutter, straight or slightly carved,
and of fairly uniform diameter throughout ; its anterior open side is filled
in by a soft membrane homologous with the labial gutter of other flies. The
chitin of which it is composed is not, as generally represented, a continuous
sheet, but is in its upper portion composed of a large number of separate
narrow transverse plates, set very closely together and united by the mem-
braneous ground work. These are very minute, and are only revealed by
the examination of well-cleared preparations under a high magnification. They
are found at just that portion of the labium which is bent acutely when
the piercing parts are inserted into the skin, and the arrangement is, of
course, one with which one is familiar in other situations in which a certain
amount of rigidity has to be combined with flexibility.
The Sucking Chambers.
The nomenclature of the sucking apparatus in the Diptera is unfortu-
nately in a most confused state, and it will be well before going further to fix a
terminology and to point out the significance of the terms employed by
different writers. It is agreed by all that the sucking apparatus is formed
from the first part of the stomodæum, and is therefore an invagination at the
anterior end of the fly, lined with chitin which is continuous with the exoske-
leton. In all the Orthorrapha this invagination is dilated into two chambers,
each of which, in the species which have been examined closely, is provided
with a set of muscles which pass between the outer sides of the walls of the
chamber and the walls of the head capsule. By means of these muscles
the chambers can be dilated, so that their potential cavity becomes an actual
one, into which the fluid food is sucked by a simple pumping action. The two