NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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ends of the labial apodemes and the anterior arch of the fulcrum, and act in
the reverse direction to those which are inserted into the expanded ends of the
apodemes.
These two joints provide a very wide range of movement, and the distal
end of the labium may be moved to any point within a segment of a circle the
centre of which is situated at the point of attachment of the rostrum to the
head, and the radius the length of the organ in the extended position. The
upper limit of this segment is in the horizontal plane, a little below the line of
the long axis of the body ; the posterior limit is the vertical line between the
epistomal orifice and the surface on which the fly is feeding. The joints do
not permit of any lateral or rotatory movement.
The movements of the labella are somewhat complex, but as these organs
become fixed and greatly altered in the blood-sucking flies, a brief note will
suffice here. The movement is of two kinds, produced by different forces, and
brings about very marked changes in the form and position of these organs.
In the normal resting position the labella are compressed, and their two walls
are separated from one another by only a small amount of cellular tissue. The
two lobes are folded backwards on the end of the haustellum, rotation taking
place at the joint between the end of the labial gutter and the discal sclerite,
and between the fork of the mentum and the furca. The distal margins of the
outer walls of the two labella are in contact with one another, and the oral
surface is completely concealed. When they are required for use they are
distended with blood from the hæmatocœle of the labium, which is of course in
direct communication with the rest of the body cavity, in such a way that their
internal surfaces are forced apart, and the outer walls rotated outwards and
upwards as the space between the internal and the external wall of each
labellum is increased. At the same time the discal sclerite and the furca are
rotated by muscular action, so as to bring the expanded surface, which has now
been formed by the two internal walls, into a position at right angles to the
rest of the proboscis in its extended position.
The muscles which produce these actions are found in the blood-sucking
flies in very much the same arrangement as in Musca. There are three main
groups, two longitudinal and one transverse. The transverse muscle is com-
paratively small, and is found only in the lower part of the labium ; it runs on
each side between the internal surface of the mentum and the labial gutter,
and by reducing the transverse diameter of the labium tends to drive the blood
into the labella when they are to he distended. The anterior and median of
the two groups of longitudinal muscles, arising from the mentum in its upper
part, is inserted into the labellar rods of the discal sclerite, and on contraction
rotates them backwards at the joint formed by its attachment to the ends of
I
17
the labial gutter. The posterior and lateral group of muscles becomes of very
great importance in the more highly specialised flies. It arises together with
the retractors of the discal sclerite, and is inserted into the arms of the furca.
As one pair of these muscles is inserted into the anterior and the other into the
posterior of the two sclerites which connect the labella with the labium, the
two are antagonistic ; when they act I ogether they will spread out and stretch
the labellar walls, and when they act independently one will erect the labella
and the other, the one attached posteriorly, will bring the labella to the position
of rest. Both acting at the same time will retract the labella, and will there-
fore antagonise the transverse muscle, which by driving the blood distally will
tend to distend them. The distension of the labella is not, however, due
entirely to the action of the transverse muscles, but is helped by the muscles of
the abdomen acting as in respiration ; the longitudinal muscles of the labium
have to antagonise the transverse muscles to the extent of driving the blood
from the oral lobes, when the proboseis is to be retracted.
The evagination and. invagination of the labella in Musca should be parti-
cularly noted, for it furnishes the clue to the mechanism of the biting
apparatus of the blood-sucking flies of this group, in which the undoing of the
evagination between the walls of the labella, by which the internal surfaces
become external, constitutes the act of biting.
The prestomal teeth of the house fly are probably used to scrape food
surfaces, and are moved by the alternate contractions of the anterior and
lateral muscles, which cause the discal sclerite to rotate in an anterior and
posterior direction on the pivot formed by its attachment to the labial gutter.
When they are in use the posterior portion of the wall is drawn backwards out
of the way by the muscle attached to the furca ; as only the distal half of each
tooth is free from the membrane, they cannot be very efficient weapons, but in
view of the homology existing between them and the powerful teeth of other
flies their method of action is interesting.
The Development of the Blood-Sucking Habit.
While the domestic species of Musca are mostly omnivarous, it is a matter
of common knowledge that they are especially attracted by animal juices,
either excrementa or exudations from the living body. Any slight cut or
abrasion of the skin is certain to be sought out, and in the case of sick animals,
too weak to protect themselves with their tails, flies collect in enormous
numbers, especially in tropical countries, to feed on the exuded fluids. The
prestomal teeth are probably sufficiently powerful to scrape away coagulated
serum so as to enable the fly to reach the moisture beneath, though not to cut
through living tissue. It is not unreasonable to infer that, in the ordinary
•
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course of evolution and differentiation of species, certain forms arose which had
a special taste for blood and at the same time specially strong prestomal teeth,
by means of which they were able to scrape an abraded surface more effectively,
and, in a further stage, to make a wound on unbroken skin. Once such a
habit was started it would be only a matter of time until the fly became
confirmed in this manner of obtaining food, and in succeeding generations the
whole structure of the proboscis would become adapted for this specialised
function, the separate stages of the process remaining as distinct forms, more or
less predominant and widespread according to the degree of perfection of their
adaptation to their environment.
It so happens that these theoretical considerations receive confirmation
from the study of the habits of some members of the genus. There are at least
three flies, and probably many more, which, although their teeth are no more
powerful than those of Musca nebulo, are nevertheless confirmed blood-suckers,
and probably have no other food. These are Musca corvina, Portchinski,
Musca pattoni, Austen, and Musca convexi frons, Thompson. The two latter
are both common cattle flies in Madras, and have the peculiar habit, first
pointed out by Captain Patton, of feeding on the blood which exudes from the
wounds made by true biting flies, most frequently associating for this purpose
with Philæmatomyia insignis, the commonest biting fly in this district. Both
these flies are much larger than Philæmatomyia, and it is very remarkable to
watch them, often half a dozen at a time, collect round the smaller fly as it is
in the act of feeding, and endeavour to thrust their proboscides near the
wound. As soon as the biting fly has finished its meal and flown off, one of
the others takes its place and at once settles down to feed, only leaving the
wound when it is as fully gorged as possible ; both these flies resemble true
biting flies in taking at one meal the maximum quantity of blood that they
can contain.
These three flies, then, are examples of an early stage of the development
of the blood-sucking habit ; closer observation of the habits of other species in
other parts of the world will probably reveal many instances of this peculiar
form of parasitism. The matter is one of considerable economic importance,
for such flies are as capable of transmitting blood parasites as those on which
they rely for food.
The presence of even minute prestomal teeth is worthy of consideration
when dealing with the question of trypanosome transmission, for it has been
repeatedly shown that the discharge from ulcers, which are so constant a
N. B.—Since writing the above, another species which has the same habit has come to light. This fly,
which has the additional peculiarity of being larviparous, has been described by Captain Patton and myself under
the name of Musca bezzi, in a paper in the Indian Journal of Medical Research, Vol. I, No. I.
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feature of many trypanosome diseases, contain large numbers of trypanosomes.
Any such delicate organism would die after a very short exposure to the air and
to the tropical sun, but flies with teeth sufficiently strong to scrape away the
protective clot so as to reach the moisture underneath would be quite capable
of transmitting live trypanosomes from one animal to another.
The Structural Changes in the Proboscis.
Before considering the different forms in detail it will be of advantage
to indicate briefly the directions in which the structural changes, which are the
response to the change in the habit of life, are to be looked for. All the
modifications can be traced to the development of the prestomal teeth,
the increase in the size and importance of which has rendered necessary
a corresponding increase in the stability of the structures which support them,
and in the strength of the muscles which bring them into play. At the same
time the proboscis is altered in shape in such a way that it has actually become
a piercing organ, pointed at the distal end and able to enter the skin of
the host, in some cases to a considerable depth ; and as the piercing parts
are limited to the haustellum, this part of proboscis has become elongated, until
it may be, as in Glossina, more than three times the length of the rostrum.
The muscles which actuate the biting apparatus, greatly increased in size,
are collected towards the posterior end of the labium, and connected with their
insertions by tendons. At the same time the pseudotracheal membrane,
no longer of use to the fly, has disappeared. The anterior of the two joints by
which the labella in Musca are articulated to the labium is fixed more or less
firmly in that position in which the labella are in a line with the labium. The
discal sclerite is separated into two parts, the axial apophysis and the labellar
rods, to the latter of which the teeth are attached by means of a projecting
flange. The rods themselves are expanded into plates forming an incomplete
collar round the prestomal orifice. The furca, to which the powerful muscles
which move the teeth are attached, is greatly thickened, and takes part in a
much more definite articulation. The fork of the mentum is resolved into its
two parts, the distal one of which is moved forwards and outwards and takes
little or no part in the articulation. The most obvious change is in the posi-
tion of the proboscis in the resting attitude. On account of its increased length
the haustellum can no longer be concealed when it is retracted, and projects
conspicuously in front of the head.
The pseudotracheal membrane of Musca, as pointed out by Graham-Smith,
fulfils a special function in filtering the food of the fly so as to prevent
the ingress into the food canal of particles too large for its lumen. In
the biting flies the membrane no longer exists as such, but some sort of filter is E
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as necessary in these flies as in the non-biting ones, for on account of the
method by which the wound is made there must be many particles, torn off by
the teeth, sufficiently large to block the canal between the epipharynx and
the hypopharynx if they gained access to it. This is prevented by the
development, from the remains of the rings of the pseudotracheæ, of a series of
processes, which are attached between the teeth and are swept through
the wound in advance of them to keep clear the surface to which they are
applied.
The Genus Philaematornyia.
The genus Philæmatomyia was created by Austen in 1909, for one species,
insignis. The characteristic feature of the genus is the presence of a circlet of
teeth at the apex of the proboscis, between the labella. In other respects
it closely resembles Musca, and unless the teeth are noticed there is nothing
about the general appearance of the proboscis to suggest that the fly is
a blood-sucker ; it is probable that the above species, which appears to be
widely distributed and common where it occurs, had previously failed to
attract notice on account of its innocent and inconspicuous appearance.
Since the description of the above species two others have been found, viz.,
Philæmatomyia lineata, Brunetti, and gurnei, Patton and Cragg. Both these,
and especially the latter, appear to be rare flies, at least in Madras, where
insignis is extremely common. All three are of exceptional interest, as they
represent separate and early stages in the evolution of the blood-sucking
proboscis. In all the proboscis is of the simple Musca type, is completely
retractile within the head, and possesses a pseudotracheal membrane : the
relative proportions of the rostrum and haustellum are the same as in Musca,
the labella are only a little reduced in size, and the hairs and macrochætæ
are similarly distributed and fairly abundant. It is obvious that such a
proboscis cannot be simply thrust into the skin of the host, and, as will be seen
when the mechanism is discussed, the wound made is a scarification rather than
a puncture. The teeth are used, in fact, very much in the same way as
those of a non-biting Musca.
The structure of the proboscis being for the most part identical with
that of Musca, it need not be described in detail, and we may proceed at once
to consider the modifications which are found. They are all in connection
with the teeth, the distal sclerite, and the labial gutter ; it will be convenient
to deal with each fly separately.
Philæmatomyia lineata.
The evidence with regard to the blood-sucking habit of this species requires
confirmation. In Brunetti's original description it is stated that Dr. Annandale
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has seen it gorged with blood, but this is not absolute proof that it can
itself make an effective wound, for it may have the same habit as Musca pattoni
and convexi frons. It is not common in Madras, and neither Captain Patton
nor myself has been able to confirm Dr. Annandale's observation, or to ascertain
if it can make a wound for itself, though we have frequently seen it on cattle.
The teeth (Plate IV, fig. 24) are shaped like rose thorns, and are attached
to the membraneous inner wall of the labella by expanded bases, as thorns
are attached to a stem. When seen in profile it is evident that only the
distal half of each tooth is free from the membrane ; the middle portion is com-
posed of a slightly expanded base ; the proximal portion of the tooth is flatten-
ed, adherent to the membrane, and distinctly divided in its long axis, so that
it consists of two lateral ribs with a thinly chitinised interval between them ;
these are articulated to a flange of the discal sclerite.
There are four of these teeth, similar to one another in size and shape,
situated at the prestomal orifice, and alternating there with the terminations of
those pseudotracheal channels which open directly into the prestomum. The
pseudotracheæ do not extend quite so far inward as do the corresponding ones
in Musca, but terminate at the level of the distal portion of the attached bases
of the teeth. On each side of each channel there is a pair of narrow flattened
chitinous strands, which unite with those of the opposite sides to form a thicker
strand at the level of the separation of the tooth from the membrane. This
strand is continued between the teeth to near the attachment of the inner wall
of the labellum to the discal sclerite, there being one such strand between each
two teeth and one on the proximal side of the proximal tooth. At the distal
side of the four large teeth there is a smaller thin slip of chitin, which appears to
represent a rudimentary tooth. It is situated at the point where the anterior
collecting channels terminate, and is contiguous with the irregularly shaped
plates of chitin which replace the rings of the collecting channel in this
region.
The discal sclerite, though retaining the general shape and appearance of
that of Musca, shows evidence of the commencement of important changes.
In the first place, it is much stouter and more heavily pigmented than in the
non-biting species, to correspond with the increased size of the teeth and the
necessity for more powerful muscles. The racket shape is modified by the wide
separation of the labellar rods, with the result that the sclerite is V-shaped, the
point corresponding to the junction of the parallel rods with the loop in Musca
being marked by a prominence on the external surface.
The separation of the discal sclerite into two portions is evident even
at this early stage. If a well cleared preparation (Plate V, fig. 31), care-
fully decolourised with chlorine, be examined under a high power, it will be seen
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that there is a definite fissure separating the anterior fourth from the posterior
three-fourths, commencing on the external surface a little below the pointed
apex and running backwards and slightly outwards towards the inner surface.
The portion anterior to this fissure is the axial apophysis, and that posterior
to it the labellar rod; the importance of this distinction will be seen later.
The axial apophysis is Y-shaped, the long arm of the Y, free and
forwardly directed, being the most distal part of the sclerite; the lateral
arms are only separated from the labellar rods by the fissure described
above. The distal part is not straight, but is bent forwards at a right angle,
so that its extremity lies in approximately the same plane as the teeth when
extended for use; it is interesting to note that in decolourised preparations a
median fissure separating the axial apophysis into two lateral halves can be
made out.
The labellar rods consist of three parts, a posterior part in which the
muscles are inserted, an anterior part which unites with the axial apophysis,
and a tooth plate. The posterior portions, composing about half the length of
the sclerite, are wedge-shaped and have their pointed ends directed outwards
and backwards; their broad ends form prominent tuberosities on the outer
sides of the sclerite. The anterior part is thinner and somewhat flattened from
side to side. The tooth plate is a narrow flange of thin chitin, arising from the
whole length of the outer edge of the anterior portion of the sclerite and follow-
ing its curve; it is continued in front of the fissure separating the axial apo-
physis from the labellar rod, but at the fissure it turns directly forwards as an
isolated spur, which reaches to about the level of the tip of the apophysis. It is
from this spur that the rudimentary tooth arises. Posteriorly the flange is
continued inwards and upwards across the tuberosity, the most posterior portion
projecting inwards towards that of the other side. When seen in profile the
tooth plate presents a sickle-shaped free border, the anterior and posterior
extremities of which are placed at the terminations of the two longitudinal
collecting channels of the pseudotracheæ. The bases of all the teeth and the
terminations of the central pseudotracheæ fit into the concavity, and the
proximal ends of the teeth and the supporting strands of the pseudotracheæ
are attached around its margin. In this way the tooth plates of the two sides
form a sort of cup into which the bases of the teeth fit.
The labial gutter of this species differs from that of Musca in the greater
thickness and pigmentation of its chitin. The lateral rods are definitely
demarcated, and articulate with the labellar rods at the junction of the anterior
and posterior portions. They can be traced almost to the proximal end of the
mentum, lying on either side of the labrum-epipharynx. The hypopharynx is
free from the gutter except at the extreme upper end.
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The pseudotracheal membrane does not differ from that of Musca. There
are, however, only eighteen channels, whereas twenty six seems to be the
average number in non-blood-sucking forms.
The mechanism of the proboscis appears to be identical with that described
for Musca nebulo. The teeth are brought into play by the contraction of the
muscles inserted into the furca, the requisite resistance and adaptation being
obtained by the muscles inserted into the labellar rods. When in use the
labellar rods are placed more or less perpendicular to the labium, and the
teeth, turned outwards by the tightening of the membrane to which their
bases are attached, are arranged in a half moon with the axial apophysis
in the centre, not in a circle as stated by Brunetti. They may either be
drawn across the surface in an antero-posterior plane by the movement of the
labellar rods on their attachment to the labial gutter, or they may cut by the
simple eversion of the labellar walls, which turns them outwards and at the same
time draws their points across the skin.
Philæmatomyia gurnei.
In a recent paper written in conjunction with Captain Patton of this Insti-
tute the discovery of this species was briefly chronicled, a detailed description
being deferred in the hope of obtaining more material. Unfortunately this
hope has not been realised, and up to the present time the only specimen in
our possession is the one from which the original description was given.* It
happened, however, that this specimen was caught in the act of sucking blood
from a cow, and had its abdomen distended with blood, so that there is no
doubt regarding its habits. The proboscis was removed from the head,
cleared in potash, and partially dissected so as to display the teeth and con-
nected structures. The discal sclerite, separated from the labium by the rup-
ture of its tendinous attachment to the end of the labial gutter, was flattened
out and compressed, in order to display the teeth of both sides, and in doing
so the attachments of the axial apophysis to the labellar rods were fractured
and the pseudotracheal membrane and part of the outer wall of the labella,
torn. The microphotograph (Plate V, fig. 32) represents the appearance
after this dissection.
The teeth are much stouter and shorter than those of lineata, though they
resemble them in general configuration; their greatest breadth is about one-
fourth the length. There are five principal and three rudimentary teeth on
each side, arranged in a similar manner to those of lineata; the large teeth are
* A series of this species has been obtained while this Memoir was in the press. A detailed description
appears in the Indian Journal of Medical Research, Vol. I, No. 1. Examination of farther specimens has
confirmed the above account.