NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
36
of the labium, the lateral portions of which curve inwards and forwards so as
to almost meet in front. The labial gutter lies out of sight between the edges
of the mentum, and the membrane uniting the two is of very small extent.
There are no large pigmented macrochætæ on the mentum, and the few hairs
which are present are short and inconspicuous.
The labella have undergone a complete transformation from the type seen
in Musca, and have become converted into a pair of small compact and par-
tially fixed organs, the sole function of which is to pierce a hole in the skin of
the host. In consequence of the loss of function the pseudo-tracheal membrane
has disappeared, and it is only in Hœmatobia, the first member of the group,
that traces of it can be recognised. The hæmatocœle, or space between the
inner and outer walls of the labella, being no longer required to contain enough
blood to distend the large oval lobes, has become very much reduced in size,
and all that remains of the walls is sufficient to cover the teeth and the inter-
dental armature in the position of rest. Both joints of the labella are consoli-
dated; the anterior joint is limited in movement, but the posterior one permits
of an even greater range of movement than in Musca .
The teeth are not, as one might expect, larger than those of Philœmia-
tomyia, but the mechanism by which they are brought into play is a much
more efficient one. All the teeth of one side are attached to one another by
their bases, and the ridge of chitin so formed lies parallel with the edge of the
discal sclerite, though it is not continuous with it. The interdental armature
persists ; it varies with the species in matters of detail, but is always attached
to the prestomum in a manner similar to that in which the Y-shaped strands of
chitin terminate in this region in Musca . Certain accessory structures, probably
sensory, also become. prominent.
The discal sclerite appears in a new form, which is not, at first sight, at
all comparable with that of either Musca or Philœmatomyia insignis . Closer
examination, however, reveals their essential similarity and homology. In all
three flies it has the form of a funnel-shaped collar of chitin, surrounding the
prestomum, and articulated with the ridge of chitin formed by the united bases
of the teeth in front and with the labial gutter behind. The funnel is com-
posed of two flattened plates, roughly oblong in shape, with the vertical dia-
meter greater than the antero-posterior, and the distal border concave. They
are continuous on the anterior borders with the median fissure between the
labella, and are united ventrally. The space between them, which appears
empty in sections, is oval in shape, and broader in the middle than at the
upper and the lower angles. These flattened lateral portions represent the
labellar rods. On the ventral side there is a third piece, always more conspicu-
cus in sections than in cleared preparations, which binds the two lateral plates
37
together and forms the floor of the funnel. This represents the axial
apophysis, a term which was first applied by Stephens and Newstead to this,
piece in Stomoxys , and subsequently used by me to indicate its homologue in-
Philæmatomyia insignis. The ridge of chitin at the bases of the teeth is.
closely apposed to the concave distal border of the sclerite, and is firmly bound.
down to it by a narrow band of fibrous tissue. The ridge is also attached by
a fibrous band at the point where the lateral walls and the axial apophysis
fuse together at the anterior ventral angle. The posterior end of the sclerite
i s
irregular in shape, and articulates with the end of the labial gutter in a.
manner which differs slightly in the three forms, but in all cases is such as to
indicate that the range of movement at this joint is a very small one, not in
the least comparable to that in either Musca or Philæmatomyia , but more
marked in Hæmatobia than in Stomoxys. The sclerite has, in fact, become
fixed in the position of extension, as seen in Philæmatomyia when the labeller
rods are in line with the labial gutter and the axial apophysis directed straight
downwards. Normally the labella are directed a little backwards from the long
axis of the labium, and are brought into line with it when the parts are in use,.
in the same way that the discal sclerite is erected in Musca.
As a consequence of this fixation of the anterior joint, the muscles which act.
upon it are reduced, together with the extent of their surface of origin. The
anterior and median set of muscles is reduced to a very much smaller size than
is found in Philæmatomyia , and the keel of the labial gutter from which it arises
is diminished to a commensurate amount progressively in the three flies of the
series. it is interesting to note that with the shortening of the arm of the
lever to which this muscle is attached there is a corresponding decrease in the
distance between its origin and insertion, for we find that the keel of the gutter,
which in Philæmatomyia is attached to the upper portion only, is in these
flies situated at the lower end, and that it becomes less extensive and more
distal in position progressively with the other changes. The labial gutter itself,
having no longer to bear the strain of a powerful pair of muscles, and since it is
to a large extent replaced as a supporting structure by the mentum, reverts to its
original function of protecting the labrum-epipharynx and hypopharnyx, and is
composed of chitin little thicker than that of the mentum ; the thickness, how-
ever, varies according to the amount of muscle attached to it, and is much greater
in the region of the keel and at the extreme lower end, around the articulation ,
than further up. At the same time the distinction between the lateral and the
median, portions of the gutter remains well marked at the distal end, and they are
here moulded so as to form a more or less complex joint with the discal sclerite.
The posterior articulation, between the fork of the mentum and the furca,
new becomes the most important joint in the proboscis, and is specialised to a
G 2
38
high degree. In the case of Philæmatomyia both the joints probably come
into play; the anterior one allows a backward rotation of the labellar rods on
the end of the labial gutter, so that the teeth can sweep backwards across the
skin, while the posterior one allows the teeth to be pulled outwards, traction
being exercised on the inner walls of the labella by means of the intimate
attachment of the furca to the outer wall. In this series of flies the first action
is restricted to the straightening of the labella on the labium, and all the cut-
ting is performed through the last joint, the various parts of which are modi-
fied and strengthened in accordance with their increased functional importance.
The actual line of the joint has become displaced backwards, so as to lie be-
t ween t he f ur ca, and t he pr oxi mal por t i on of t he ar m of t he f or k.
The posterior margin of the lower end of the mentum is not notched as in
Philæmatomyia, but shows at most a shallow groove. On either side of this
there is a thick rod of chitin, having the shape of an elongated wedge with its
base directed distally and its most prominent angle turned outwards; this
corresponds to the proximal division of the arm of the fork. The furca is
reduced in size in accordance with the greatly reduced diameter of the labella ;
it is a wide and thick arch, the middle portion of which articulates with the
thickened ends of the arms of the mental fork. On each side of it there is a
short thick rod of chitin, closely pressed against the furca laterally, and having
its proximal end only slightly separated from the fork of the mentum; this is
the distal joint of the fork, which in Philæmatomyia lies parallel with the
furca when the labellar rods are rotated into a position corresponding to that in
which they have become fixed in the Stomoxydinæ.
The outer wall of the labella is almost entirely composed of a sheet of
rather thin chitin, which is intimately connected with the extremity of the
furca on each side. It reaches to the distal edge of the labella, and there bears
a fringe or more or less conspicuous hairs, those of the two sides intermingling
when the labella are closed. At the margin it gives place to a softer and more
flexible membrane, which occupies the inner surface and bears the teeth and
connected structures, and is continuous with the thicker fibrous tissue which
unites the teeth to the discal sclerite. In this way the teeth are connected
much more directly with the furca than in any of the flies previously
considered.
The great increase in the strength and importance of this joint has brought
about other changes in the structure and relations of the parts. Most import-
ant of these is the great increase in the size of the muscles which act on the
posterior joint, and which are homologous with the posterior and lateral mus-
cles, or retractors of the furca, in Musca . As the mentum is now narrowed to
a point, the muscles are displaced to the upper end, which is dilated into the
39
"bulb" to accommodate them, and they are connected with their insertions by
long tendons. The tendon of insertion is spread out after the manner of an
aponeurosis. In addition to its main insertion into the lateral arm of the
furca, it sends numerous fibres to the external wall of the labellum, and to the
distal portion of the fork of the mentum, which is displaced forwards and fused
with the labellar wall as stated above.
The mechanism of the proboscis has been dealt with in the description of
Lyperosia , and the other species show no divergence from that fly except in
very minor details. It will be evident from that account, bearing in mind
what occurs in Musca and in Philæmatomyia, that the efficiency of the cutting
action depends entirely on the great increase in the strength of the posterior
articulation between the labium and the labella, and in the muscles which
act upon it. The anterior joint is on the contrary limited to such movement
as will place the teeth in the position for the next cut, and the muscles which
act upon it, not having to work against the same resistance, are of much
smaller size. A further point which is of some importance in considering the
mechanism both of these flies and of Glossina is that there is a membraneous
interval on the lateral aspect of the haustellum in the neighbourhood of the
joints, which, while it permits of free backward rotation of the furca, would
also detract from the rigidity of the proboscis as a whole, were it not for the
fact that the labial gutter reaches its greatest thickness at this point.
The labrum-epipharynx and the hypopharynx are remarkably little altered
from the Musca type. As a consequence of the narrowness of the groove in.
which they lie, they are narrower than those of the non-biting flies, and as from
the same circumstance they are much. better protected, they are more slender
and less rigid. The labral apodemes are also somewhat shorter, corresponding
with the reduction in size of the rostrum as a whole, and are not quite so thick.
The most important feature of the food canal is that the fusion of the two lips
of the groove in the epipharynx takes place at a higher level in these flies than
in Philæmatomyia , which shows that there is a tendency for it to become con-
verted into a closed tube, a continuation into the proboscis of the stomadeaum
as a closed canal, instead of as dorsal and ventral outgrowths. On the other
hand, the separation of the lower lamina of the hypopharynx from the labial
gutter is maintained as in other blood-sucking flies.
Hæmatobia (Plate III).
The proboscis of Hæmatobia presents a most interesting feature in the
possession of the remains of a pseudotracheal membrane, and certain other
peculiarities which indicate that it represents a relatively early stage in the
40
evolution of the blood-sucking proboscis. I am indebted to Captain Patton of
this Institute for named specimens of Hæ matobia irritans and. to Mr. Howlett
for specimens sent from Pusa. Unfortunately the genus does not appear to
occur in this neighbourhood and lack of fresh material renders my description
somewhat more meagre than it would otherwise have been.
The relative lengths of the rostrum and the haustellum are practically the
same as those in Stomoxys, but the difference in shape is greater than the
measurements indicate, for the thickest part of the labium is at the junction of
the upper and adjacent fourth, from which the proboscis narrows evenly to the
furca, while in Stomoxys the greatest thickness is well above this line, and
distal fourth of the labium is practically of a uniform diameter.
In the position of rest the labella are bent distinctly downwards on the end
of the labium, much more so than is the case in Lyperosia or Stomoxys, and
are distinctly wider than the adjacent part of the labium; they also bear a
fringe of moderately large macrochætæ along the distal margin. All these
points indicate a closer relationship with the Musca type. The keel of the
labial gutter, though not so thickly chitinised as that of Philæmatomyia, is
almost of equal extent, and is present in the upper two-thirds of the labium,
reaching in the upper part almost to the mentum; it thus provides a con-
siderable area for the attachment of the anterior set of muscles. The gutter
itself is strong and deeply pigmented, and at its lower end (Plate V, figure 25)
the separation between the lateral and the median portion is well marked.
The articulation between it and the discal sclerite is obviously one which allows
of a greater range of movement than can take place in Lyperosia . The lateral
portions of the labial gutter are produced into rounded processes, which are
inclined slightly backwards (downwards in the position of rest, the labium
being horizontal) and extend beyond the median portion of the gutter. They
lie in an L-shaped depression at the upper and anterior angle of the sclerite,
and are in contact with it when the labella are straightened on the labium, but
separated from it when the labella are in the position of rest. The bottom of the
median groove in the gutter is produced beyond the rest of the median portion
and fits into the space between the lateral walls of the sclerite, through which
it can be seen in well cleared preparations. The labella, through the discal
sclerite, can therefore be straightened on the labium, and maintained in the
extended position by the pressure of the upper border of the lateral wall of the
sclerite against the lateral portion of the labial gutter. When in this posi-
tion the median portion of the gutter projects between the two walls of the
discal sclerite, and. will afford additional support. The discal sclerite itself pre-
sents no differences from that of Stomoxys , except that the axial apophysis is
more pear-shaped, and that its pointed end projects further forward.
41
The outer wall of the labellum is partly membraneous and partly chitinous.
The chitinous portion is in the form of a plate shaped like a cockle-shell; the
rounded edge of this forms the lower border of the labellum, and bears some
of the hairs forming the terminal fringe ; the upper end, which corresponds to
the hinge of the shell, is thickened and drawn out, and is closely fused with the
extremity of the furca. The upper portion of the posterior side of this plate is
also attached to the lower border of the furca by a thin chitinous prolongation,
so that the whole outer wall of the labellum is firmly bound down to it, and
must follow it when it is drawn upwards. This plate corresponds in position
to the space on the inner wall occupied by the teeth, which can be seen by
focussing through it in a cleared preparation.
The rest of the outer wall is membraneous, and is continuous with a mem-
braneous area, on the anterior and lateral aspects of the end of the labium,
which extends upwards above the furca for a distance equal to the length of
the labella. The membrane is quite different in appearance to any that is met
with in the flies already described ; it consists of a transparent, homogeneous, and
structureless groundwork, in which there are placed small plates of thin chitin,
irregularly oval in shape and set close together, though not actually touching
one another. At its boundaries it merges indefinitely with the attenuated
edges of the chitinous plates, and at the border of the labella it is continuous
with the inner wall.
At the distal border of the labella there is on each side a row of five finger-
like flaps, passing distally beyond the margin, and continuous with its outer
wall. They are processes of the labella, and it is on their inner surfaces that
the channels of the pseudotracheal membrane are to be found; they will be
dealt with in connection with the structures found on the inner wall of the
The teeth are five in number on each side. Counting from the dorsal or
anterior side, the second is the largest, the first and third equal in size and
slightly less than the first, and the third and fifth distinctly smaller, the fifth
being the smallest of the set. The largest tooth is about one-fourth the length
of the labellum. Each tooth is conical in shape and about twice as long as it
is broad ; the distal end is produced to a blunt and slightly curved point, just
below which there is a notch in the margin of the tooth, the edge proximal to
the notch forming a second cutting point. The notch is situated on the dorsal
borders of the first and the second teeth, on the internal border of the third,
and on the posterior or ventral borders of the fourth and fifth. On the inter-
nal surface of each tooth, that is to say, on that surface which is turned out-
wards when the teeth are everted, there are a number of peculiar secondary
teeth (Plate III, figures 18 and 19). These are situated about the middle of
42
the surface, and are arranged in rows across the tooth. Each is a small oblong-
or oval eminence, the distal end of which is free from the surface of the large
-tooth and forms a cutting point. All the teeth are united at their bases by
chitinous prolongations, the union of the first two and the last three to one
another being the most firm. When flattened out by the pressure of the
coverslip, as in the preparation drawn (Plate IV, figure 22), the bases of the
teeth are in the same straight line, but when the proboscis is mounted whole-
it is seen that in the position of rest they are arranged in a curve which cor
responds with the line of the distal margin of the discal sclerite.
The interdental armature consists of a series of lanceolate leaf-like blades.
closely resembling the " petiolated blades " described by Stephens and New-
stead. Like those of Stomxys , they are extremely delicate objects, containing
little or no pigment. Their surface is finely granular, and the stalks from which
they arise are thicker and flatter than those of Stomoxys . They are arranged as
follows :—There are three sets, similar to one another, between the first and
second, second and third, and third and fourth teeth. Each of these consists
of a central stalk, from each side of which three blades arise and pass forwards,.
those of the two sides diverging from one another. The terminal pair of blades
lie on either side of the pseudotracheal channel. External to the first tooth
there is a set of smaller and narrower blades, five pairs on a common stalk, the
terminal blades also surrounding the termination of a channel. External to
these there are two stouter elongate filaments of chitin arising at the base of
the first tooth. Between the fourth and fifth teeth there is a common stalk
bearing only two pairs of smaller blades, the diameter of which is little
greater than that of the stalk. External to the fifth tooth there is another
stout stalk which bears two pairs of blades, and arises from the ridge of chitin
at the base of the tooth. On account of the extremely heavy pigmentation
of the teeth and their close approximation I have not been able to ascertain
whether the middle sets of blades arise from the teeth or from the membrane
of the inner wall of the labellum.
Between the petiolated blades, and situated opposite the apices of the teeth,.
there are certain prominent hairs, the position of which should be noted.
These correspond to those termed by Stephens and Newstead the "rod-like "
h a i r s ; t h e y a r e s h o r t a n d s t o u t , a r i s e f r o m p r o m i n e n t r a i s e d a n d e x p a n d e d
bases, and terminate in truncated ends. They are darkly pigmented. and con-
spicuous, slightly curved, and about five or six times as long as thick. The.
appearance of a groove is probably due to the presence of a central canal and
not to an actual groove. They are arranged as follows:—One opposite the first
tooth, two opposite each of the second, third, and fourth teeth, and one opposite,
the fifth tooth; in addition, there are two smaller ones placed between the small
43
petiolated blades external to the first tooth and the extreme anterior strand of
chitin. These rod-like hairs occur in all the biting flies dealt with in this
paper, except the first two species of Philæmatomyia , and presumably have
some special sensory function connected with the biting habit.
The membrane forming the inner wall of the labellum, as far forward as
the attachment of the two distal petiolated blades to the common stalk, is trans-
parent, homogeneous, and apparently structureless; its existence is only shown,
in cleared preparations, by the way it holds the parts together. But distal to
this it consists of a membrane of the same nature as that which forms the non-
chitinised parts of the external wall, and is produced beyond the distal border
of the labella as the inner wall of the finger-like flaps already referred to. The
structure of these flaps is as follows:
—
Each flap is roughly triangular in shape, and consists of two walls, an
inner and an outer. The outer wall is continuous with, and of the same struc-
ture as, the membrane which forms part of the external wall of the labellum,
and which dips in between the two lobes at the anterior median fissure. The
squamæ in this region are thinner and less sharply defined than elsewhere,
and are all more or less pointed and distally directed. Those at the extreme
tip of the flap are turned inwards towards those of the opposite side. In the
middle line of each flap there is an area over which they are deficient, corre-
sponding in position and extent to the pseudotracheal channel on the inner
wall. The inner wall of each flap is continuous with the homogeneous and
structureless layer of tissue which lies subjacent to the teeth, the line of junc-
tion between the two being situated at the level of the distal border of the
chitinous plate in the external wall of the labellum. The distal portion, that is
to say, that which is opposite to the outer wall of the flap, is composed of a
membrane similar to that of the outer wall, but the squamæ are here so dis-
posed as to form a shallow channel, the representative of the pseudotrachea.
The plates are only found in the area which corresponds to that on the outer
wall in which they are absent, so that the distinction between the two layers
of the flap can only be made out by careful focussing in a preparation which
has not been compressed by the coverslip, and by using a high magnification.
The channel is formed by a series of thin transversely elongated plates, set side
by side and fitted loosely to one another; each plate is curved to such an extent
that the channel, in cross-section, would be approximately semi-circular, and the
lateral extremities of the plates are alternately bifid and narrowed. At the
distal end the channel is turned upon itself in such a way that the last three
or four of the plates are in the same line as those proximal to them, or
nearly so. Opposite to the termination, which is not at the distal end of
the flap, but some distance away from it on account of the bend at the