NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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specialisation than seen in Stomoxys, thus corresponding with the external
characters. Notwithstanding the similarity of the two forms, however, I
propose to give a detailed description of Lyperosia, for my observations differ
in many points from those of the authors cited, and my view of the mechanism
is so essentially different that a complete account will be necessary to ensure
lucidity. The homology of certain of the parts with those of Musca will be
pointed out in passing, leaving the details of this interesting branch of the sub-
ject to be dealt with in a further paper, which I hope to publish shortly.
Methods.
The parts were studied first by the examination of cleared preparations, in
order to become familiar with the chitinous skeleton of the proboscis. It was
found convenient to place a large number of specimens in potash solution at
the same time, and to remove them at intervals, so that preparations showing
different degrees of transparency could be obtained. Dissection is most con-
veniently done after the preparations are cleared in clove oil, for many of the
parts, for instance the labella, are so small that one runs great risk of losing
them if they have to be subjected to any manipulation after removal from the
rest of the organ. It is so difficult to mount such small objects in the desired
position that it is best to mount a large number at the same time and prefer
-ably on the same slide, so as to increase the chances of getting the desired point
of view. Dissection was performed under a Zeiss binocular microscope, with a
magnification of from twenty to forty diameters. Carefully ground needles
with various shaped points are essential. The dissection of fresh specimens is
of the greatest importance in the study of the mechanism, for one often can
decide what is the function of a muscle by pulling on it when one cannot be
absolutely certain of the exact point of its attachment. The examination of
flies recovering from a dose of ether is also very instructive, as one can actually
see the same movements of the proboscis as are performed during the act of
feeding, though they are, of course, convulsive and irregular.
For serial sections the combined paraffin and celloidin method was used.
It is best not to attempt to cut a whole series from one preparation, but to cut
away boldly one end of the specimen, so as to allow the fluids to penetrate
freely. The chitinous walls themselves are practically impervious to the ordi-
nary reagents, and it is advisable to perforate them with a needle at some un-
important point, in addition to making a free opening at one end. Bles's
fluid was used as the fixative, followed by rapid dehydration in several changes
of absolute alcohol; the shorter the time chitinous objects are left in absolute
alcohol the more likely one is to get unbroken sections.
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General Account.
The proboscis (Fig. 1, Plate 1) consists of three parts, corresponding to
those found in Musca, and more or less freely moveable on one another. The
rostrum is hidden from view on the under surface of the head in the position
of rest, and is much smaller than that of Musca. The haustellum, or proboscis
proper, on the other hand, is elongated and narrowed, and projects horizontally
forward in front of the head. It is not fixed in this position, but is capable of
forward and backward movement, and elevation and depression, in exactly
the same manner as that of Musca, though to a smaller extent. It consists
of the usual parts, viz. , the labrum-epipharynx, hypopharynx, and labium, the
latter forming the main bulk of the haustellum and concealing the other two in
a groove on its upper surface. The labium is typically spindle shaped. ; its.
greatest breadth, about one-third of the total length, is situated at its proximal
end, this portion being termed by Stephens and Newstead the " bulb." At
the distal end the labium is divided into a pair of labella, which are completely
separated from one another in the middle line in front and distally, but united
in the proximal part of their posterior surface. They project beyond the labrum-
epipharynx and hypopharynx. The wall of the rostrum is entirely membra-
neous and flexible, that of the haustellum chitinous and rigid, except at the
place where the labium divides into the two labella, the chitin being here
partly replaced by a soft membrane. That portion of the labellar wall which
is external in the position of rest is chitinous.
The palps arise from the anterior surface of the rostrum about its middle.
They are clavate, slightly flattened from side to side, and have a few short
stout bristles scattered over the surface, and a coat of very fine recumbent
hairs. Their length is about equal to that of the haustellum. At the base of
each palp there is a small area extending downwards in which the membrane
is chitinised. In the resting position the palps lie in front of and in contact
with the proboscis, but when it is in use they move forwards and upwards so as
to become almost perpendicular to it.
It will be well to refer here to the relation of the several parts of the pro-
boscis to one another in the position of rest, and to the range of movement as
seen in the living fly, comparing Lyperosia with Musca as regards these points.
The most obvious difference between the two is that while in Musca the
proboscis is completely retractile, in Lyperosia the haustellum in the posi-
tion of rest remains completely visible and in a horizontal plane. The labella
are not folded backwards on the lower (or posterior) surface of the haustellum,
but remain fixed in the same line with it. The joints between the head and
the rostrum and between the rostrum and the haustellum may be compared to
the human shoulder and elbow, if one imagines the shoulder joint to be
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capable of movement only in the same plane as the elbow joint. In Musca the
position of rest is equivalent to that in which the upper arm is bent backwards
horizontally behind the body, and the forearm flexed so that the two are in
contact ; in Lyperosia the amount of flexion of the upper joint is not quite so
great, and the haustellum, which remains in a horizontal plane, makes an
angle of about sixty degrees with the rostrum. The difference in appearance
is simply due to the increased length of the haustellum in Lyperosia, and to
the incompleteness of the retraction.
From this position movements can be performed which are in every respect
similar to those of Musca, except that they are more limited in extent. The
proboscis is extended by the straightening out of the two joints in the same way
that the arm can be extended from the position indicated above until it is
thrust out in front of the body. The two joints act together, but not always
to the same degree, so that the tip of the proboscis may come
to lie in any position within a quadrant of a circle the centre of which is
the joint between the rostrum and the head, and the radius the line between
this and the tip of the proboscis when fully extended ; the upper limit of
the quadrant is the horizontal line in which the proboscis lies in the position
of rest, and the lower limit in the vertical line, that is, in the position of
full extension in Musca. The range of movement from the periphery of the
circle is of course limited to a distance less than twice the length of the
rostrum. In the act of feeding the position occupied is about midway between
the upper and posterior sides of the quadrant, and a short distance from
the periphery, or, in other words, one of incomplete extension of both joints,
whereas in Musca and in Philæmatomyia there is complete extension, a posi-
tion only occasionally seen as an agonal phenomenon in Lyperosia. By
means of these two joints the fly is enabled to move its proboscis about on
the skin of the host, while selecting a suitable place to insert the biting parts,
and in doing so it is assisted by the lateral and rotatory movements of the head
and neck. It will be obvious at once that the whole arrangement of joints is
adapted to ensure mobility, and that the proboscis is by no means the fixed
organ it is sometimes assumed to be. It follows also that there can be no ques-
tion of vis a tergo in the method of making the wound. The mechanism of
these movements will be discussed when the anatomy of the parts has been
described.
The Rostrum.
The rostrum has the shape of an inverted and truncated cone, the length
of which is slightly greater than the breadth. The relative lengths of the
rostrum and haustellum are approximately as one to two, while in Musca the two
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parts are about equal. In the position of rest the rostrum is retracted and can
only be seen when in profile. Its wall is entirely composed of a tough but
flexible membrane, attached above to the margins of the aperture in the head
capsule, and below to the upper end of the labium and to the broadened upper
end of the labrum. When the proboscis is fully extended the wall of the
rostrum is smooth and taut, and presents four surfaces, anterior, posterior, and
two lateral, but when retraction takes place the organ collapses and the wall is
thrown into numerous folds, mainly on the posterior surface.
The rostrum is, strictly speaking, a part of the head which can be protrud-
ed, and is not composed of the true mouth parts. The internal structures
consist of the pharynx, the salivary duct, certain important tracheal sacs, and
the muscles which act upon the joint between it and the head and on the haustel-
lum in the movements of extension and retraction. All these correspond with
similar structures in Musca, but show certain modifications which must be noted.
The pharynx is contained within a chitinous fulcrum which is propor-
tionately smaller than that of the non-biting forms, and narrower in proportion
to its length. The lateral plates arise from the middle third of the lateral
borders of the posterior plate, and their upper borders, which are much thicker
than the rest of the lateral area, pass almost directly forwards to the anterior
arch, which therefore lies more nearly opposite to the middle of the posterior
plate than in Musca. The upper cornua are rather long, and turn slightly
forwards at their terminations. The anterior plate of the pharynx, which with
the posterior plate encloses the potential cavity of the pharynx, presents an
interesting peculiarity in the arrangement of the sensory hairs. There are two
sets, a lateral one corresponding to those described by Kraepelin in Musca, and
a median set situated on the sides of the ridge in the middle line. The lateral
set consists of eleven hairs, the position of which is easily seen under a low
power on account of the circular clear areas on which they are set; they are
arranged in pairs, but only irregularly so, for those of the left side are slightly
distal to those on the right and the right hand member of the third pair is
absent. The first (distal) pair is much larger than the others, and the second
pair smaller, and placed nearer to the first than to those of the third; the median
set are small and are just visible as clear spaces at the sides of the ridge, though
of course on the opposite surface. There are two nearly opposite one another
between the first and second pairs of the lateral set, and three others at irregular
intervals proximal to this. This irregularity in structures which one would
expect to be symmetrically arranged is rather remarkable.
The lower end of the fulcrum (Figures 11 and 12, Plate II), at the inlet of
the pharyngeal cavity, is very different to that of Musca or Calliphora, being
specially adapted to receive the duct which connects the pharynx with the
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food canal in the haustellum. The lateral diameter of the fulcrum contracts
until a four sided channel is formed, bounded in front and behind by the
anterior and posterior plates of the pharynx, and at the narrowest point the
anterior and posterior walls of the channel cease, while the lateral walls expand
downwards and diverge from one another so as to form a sort of funnel, incom-
plete in front and behind, the concavity of the plates increasing as they
diverge.
As described above, the length of the fulcrum is proportionately less than
it is in Musca, and there is a considerable interval between the lower end of the
pharynx and the end of the labrum-epipharynx. This interval is bridged over
by a flexible duct, which is continuous with the food canal at the distal end of
the haustellum, and terminates above between the lateral funnel-like expan-
sions at the end of the pharynx, which I have just described. This duct cor-
responds in position and in function to the hyoid sclerite of Calliphora, and to
the " chitin kapsel " of Kraepelin. Something of the kind appears to occur
in all the flies of the Muscid group, and it will be of advantage to
adopt for it a term which will be applicable to all. In the
above flies, which are certainly more primitive than Lyperosia, it resembles a
miniature pharynx, having the form of a cavity, communicating in front with
the food channel between the labrum-epipharynx and the hypopharynx, just
behind the point correctly regarded by Kraepelin as the mouth of the fly. It
corresponds exactly with the first of the two cavities of the sucking apparatus
of the Nematocerous Diptera, except that it has no dilator muscle and can take
no active part in the mechanism of feeding. As it is always sound to name a
structure common to many different forms from its appearance in the simpler
rather than the more highly specialised forms, I suggest the name " buccal
cavity " for this structure, a term which has the double advantage of referring
precisely to its position and emphasizing its homology with the corresponding
part in the more primitive Diptera.
The buccal cavity in Lyperosia (Figure 1, Plate 1, and Figure 30, Plate
VI), then, has the form of a tube, which extends from the distal end of the
haustellum to the inlet of the pharynx, and has length equal to about one-third
that of the fulcrum. In the resting position it is bent forwards at a sharp angle,
the upper half being nearly vertical and in line with the pharynx, the lower
half horizontal and in line with the haustellum; in the extended position of
the proboscis the tube becomes straight. The structure of its wall illustrates an
arrangement commonly met with in insect anatomy in situations where
strength and flexibility are required in the same structure. It consists of two
laminæ, and internal and an external one. The internal lamina is soft and
membraneous for most of its length, and is attached to the chitinous duct
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which is formed by the coalescense of the labrum-epipharynx and the
hypopharynx at the distal end of the haustellum. At the upper end, where
the duct is enclosed by the funnel-shaped expansion of the lower end of the
pharynx, the membrane is replaced by thin but rigid chitin ; in this situation,
of course, no flexion takes place. The outer lamina resembles, when seen from
the side, a coarse tracheal wall. It consists of about thirty incomplete rings,
very much thicker behind than at the sides, and open in front ; the open space
between the rings in front diminishes towards the pharynx, and the terminal
rings appear to be quite closed. The thickness of the posterior portion of the
rings is so great that they come into contact with one another, and so have,
when seen from behind, the appearance of a continuous band with a series of
transverse ridges. The lateral portions of the rings in the lower half of duct
terminate in minute forks, though those of the one side do not interlock with
those of the other as do the more regularly arranged rings in the pseudotracheal
channels of the house fly. The arrangement is admirably adapted to maintain
the lumen of the soft inner duct, and at the same time to allow of flexion
without occlusion.
The salivary duct (Figs. 30 and 31, Plate VI) lies in the middle line behind
the pharynx and the buccal cavity. The distal portion is a simple chitinous
tube with no cellular lining or differentiation of the wall, but after a short
course this is replaced by a series of chitinous rings similar to that of the sali-
vary duct of Musca. A short distance above the junction with the hypopharynx
there is a small valve, acted on by a delicate muscle, precisely corresponding to
that Musca.
The tracheæ of the rostrum play an important part in the mechanism of
extension of the proboscis. There are two large lateral ones, rather of the nature
of air sacs than true tracheæ, on either side of the fulcrum. In dissections of
fresh flies one can often separate out these as white sausage-shaped bodies, filled
with air ; in transverse sections they appear as irregularly crenulated circles
much reduced below their natural size on account of the extraction of the air ;
at the lower end of the rostrum they are reduced to the ordinary size and struc-
ture of tracheæ, and pass into the cavity of the labium.
The musculature of the rostrum is very difficult to study in so small a fly,
and I have not succeeded in dissecting out all the separate bundles of fibres ;
sections give one little help. The main bundles are however distinct enough,
and do not differ from those described by Kraepelin in Musca. It is indeed
remarkable how closely the arrangements correspond in the two forms. It will
be convenient to discuss the separate movements and their musculature together.
Starting with the proboscis in the position of rest, we have to consider first the
nature of the mechanism by which the rostrum is extended on the head. The
method by which this is accomplished in the house fly has been dealt with with
such conclusiveness by Kraepelin that it would be unnecessary to go inte the
matter further than to say that there is nothing to indicate that the action is
different in this fly, is it were not that the most important muscle of the rostrum
has had assigned to it by Hansen the function of extension, in his description
of Stomoxys. According to Kraepelin, extension of the rostrum on the
head is accomplished by the distention of the air sacs of the rostrum, and
this is easily demonstrated, as I have pointed out in the case of Philæ-
matomyia, by pressing on the head with a needle. The rostrum is in fact
blown out in the same manner that the top of the puparium is forced off at the
time of emergence of the imago. Now there is a pair of large muscle arising
from the internal surface of the head cavity just in front of the aperture to the
margins of which the wall of the rostrum is attached, and inserted into the
posterior cornua of the fulcrum, and therefore tending to pull the fulcrum for-
wards. According to Hansen, the course of this muscle is a backward and up-
ward one, so that its contraction causes the fulcrum to rotate on the fixed point
formed by the anterior margin of the aperture, and thus extends the rostrum
on the head, an exactly opposite view to that taken by Kraepelin, who regards
it as rotating the fulcrum in such a way that its over end passes backwards,
that is, as a retractor of the rostrum, acting in association with another pair
which will be referred to below. The direction of rotation really depends on
the relation of the fixed point on which the rotation takes place to the line of
action of the muscle ; if the fixed point at any period. of the contraction of the
muscle is below the line of the muscle, then the rotation must take place in a
backward and upward direction, and retraction will result. If, on the other hand,
the fixed point is above the muscle, then the lower end of the fulcrum must
pass forwards and the result will be extension. The action of this muscle might
be compared to that of a piston rod acting on a wheel : the direction in which
the wheel will revolve is determined by the initial position of the wheel, and if
the rod were to be exactly across the middle of the wheel there would, theoretic-
ally, be no rotation at all, but the whole wheel would be pulled bodily back-
wards. In this case, however, the " fixed point " is not really a permanent one,
but can be altered by the action of other muscles tending to raise the lower end
of the fulcrum, so that the line of action of the muscle is by no means a constant
factor. Kraepelin recognised this, and regarded the two movements of rotation
and upward displacement as taking place together, the point on which the rota-
tion occurred being constantly displaced upwards and backwards by the action
of another muscle. It is quite possible that the divergent views held by
Kraepelin and. Hansen may each represent a part of the truth, and that this
muscle may function as both an extensor and a retractor of the rostrum, the
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direction of rotation being determined by the action of other muscles ; but this
is somewhat unlikely. The point depends really on the exact relation of the
upper end of the fulcrum to the aperture in the head cavity in the position of
rest, an anatomical detail on which it would be rash to speak with certainty.
The retraction of the rostrum takes place, like the extension, by means of
rotation of the fulcrum on a fixed point formed by the apposition of the anterior
arch of the fulcrum to the anterior boundary of the head capsule, and this is
brought about by the contraction of the muscle discussed above. The pair of
muscles which act in conjunction with it arise in the interior of the head, above
the occipital foramen, and pass downwards through the whole length of the ros-
trum to be inserted into the upper end of the labium posteriorly. In
addition to these comparatively large and definitely separated muscles, there are
several other shorter bands of fibres, apparently corresponding to some at least
of those described by Kraepelin and Hansen, but so small that it is impossible
even in sections to make out their precise points of attachments. Some of these
doubtless aid in the retraction of the rostrum, while others, inserted more ante-
riorly, appear to flex the haustellum on the rostrum.
The extension of the haustellum on the rostrum is accomplished mainly by
a well-defined pair of muscles corresponding exactly to those described by
Kraepelin as the " strecker der russel" They arise from the lower end of the
fulcrum and pass upwards and outwards to the expanded upper ends of the
labral apodemes, and will therefore thrust the labrum-epipharynx, and with it
the labium, downwards and into line with the rostrum. The reverse action is
brought about by a pair of muscles passing between the upper end of the
fulcrum and the distal end of the apodemes, and lying in the front of the
rostrum ; flexion is also probably assisted by some of the ill-defined groups of
fibres situated, as indicated above, on the sides of the rostrum, some of which
are inserted into the membraneous wall.
The Haustellum.
The haustellum of this fly is a much more solid and compact organ than
that of Musca, a much greater degree of rigidity being necessary for the efficient
working of piercing mouth parts. The wall is almost entirely chitinous, and,
as in Stomoxys, the labrum-epipharynx and hypopharynx lie concealed in a
deep and thick " labial gutter." The external appearance has already been
described.
The labrum-epipharynx is an elongate slip of yellow chitin of equal dia-
meter throughout the most part of its length, thereby differing from that of
Musca, which tapers from base to apex. Its upper surface is evenly rounded,
and its lower surface deeply grooved to form, with the hypopharynx, the food