NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
28
blood-sucking habit. Notwithstanding the fact that the last member of the
group is a voracious blood-sucker, and has, so far as I have been able to ascertain
by a close observation of its habits and by repeated examinations of its intestinal
contents, no other food than blood, there is no change in the external appear-
ance of the proboscis, and the existence of biting parts may be unsuspected
until they are revealed by dissection ; the proboscis retains its retractility, and
is withdrawn in a typical Musca-like manner when not in use.
The changes from the Musca type fall into three groups ; those dependent
on the increase in the size and in the functional importance of the teeth, those
in the pseudotracheal membrane, and. certain modifications in the general form
of the proboscis. These latter are inconspicuous and will be referred to later in
connection with the more marked changes of a similar nature found in the
St omoxyclinæ and in. Glossina.
In addition to the increase in size, the teeth show a tendency to an ap-
proximation of their bases. As this occurs, the flange of the discal sclerite
which supports them develops into a cup, formed by the flanges on the
two sides and situated at the distal end of the sclerite, into which their
'bases fit. In Musca the proximal ends of the teeth are attached directly
to the chitinous margin of the prestomum, but in the last two species they
are quite free from it, being attached only to the membrane on the inner
wall of the labellum, and held in position by the flange ; this facilitates
eversion.
The discal sclerite is separated into its two parts, the axial apophysis and
the labellar rods, to an increasing degree in the three species ; at the same
time the parts change their shape and their relations. Comparing the two
extremes, as seen in Musca and in insignis, we find that there has been a back-
ward displacement of the axial apophysis, or, what is the same thing, an
elongation of the labellar rods, and that at the same time the attachment of
the teeth to the discal sclerite has passed from the sides of the sclerite to the
distal end. In Musca the teeth are attached to that side of the sclerite which
is distal when the oral lobes are distended for use and the sclerite perpendi-
cular to the labium, but insignis they are situated at the end of the
sclerite, which can only be distal in position when the labellar rods are in the
same line as the labium. In each case the teeth are arranged around the
incomplete chitinous ring which is formed by the discal sclerite at the pre-
stomum, but in Musca this is only directed downwards, in which position it
must lie when the fly feeds, when the labellar rods are parallel to the food
surface and therefore perpendicular to the labium, while in. insignis the orifice
is only open in a position at a right angle to this. There is therefore an
alteration in the position of the discal sclerite, when the teeth are in use,
29
which amounts to a right angle, and we have to consider the causes which have.
brought this about.
It was stated in the account of the mechanism of the proboscis of
Philæmatomyia insignis that the teeth cut by eversion, this being effected by
the muscles inserted into the furca and acting on the teeth through traction.
on the inner wall of the labella. Now the relation of the size of the teeth to the
extent of the inner wall is very different in Musca and in insignis, for in the
former the pseudotracheæ are functional and the teeth rudimentary, while in
insignis the membrane is reduced in size and complexity, and the teeth very
large. It follows that cutting by eversion will be much more effective in
insignis than it could be in Musca, though it probably does occur in the latter
form. But the teeth of Musca can also be brought into action by alternating
contractions of the anterior and posterior muscles, which rotate the discal
sclerite in opposite directions on its point of attachment to the labial gutter,
and such a movement will evidently be more effective than simple eversion
on account of the greater range of movement of the teeth. Probably in Musca
the two go on together. The teeth are first everted, the membrane drawn
away, and the sclerite moved on its fixed point so that the everted teeth scrape
the surface. As in succeeding generations new forms were evolved, the
larger size of the teeth would make the act of eversion a more effective one in
the scraping operation. But at the same time displacement of the teeth
towards the distal end of the sclerite would be most advantageous to the fly, for
it would shorten the arm of the lever to which the muscle is attached, and
lengthen the one which bears the teeth, in this way giving to the cutting appar-
atus an increased facility for rapid action. We find then that in the three
species of Philæmatomyia, contrasted with Musca, the teeth show a progressive
increase in size, closer approximation of their bases, and displacement towards
the distal end of the sclerite. This is associated with a difference in the relation
of the labellar rod to the labial gutter in the position of action, for in Musca
the discal sclerite can only be rotated on its fixed point until it comes to a per-
pendicular position, whereas the sclerite of insignis can pass back until the
labellar rods are in line with the labium, a position which would be of no ad-
vantage to Musca , but which gives a much longer range of excursion to the
teeth in lineata and in gurnnei , in the former of which the sclerite can certainly
be brought more into line with the labium than it can in. Musca. At the
same time the joint between the sclerite and the end of the labial gutter has
become consolidated, until in insignis we find the two parts definitely moulded
to one another as a regular articulation. But in the latter species, the sclerite
being in the same line as the labium, the teeth in their final terminal position,
and the pseudotracheal membrane considerably reduced in extent, the
F2
30
action of eversion of the teeth, brought about in the manner described, will
be a most effective one. It is obvious that excursion through a short arc is
more suitable for piercing, while a comparatively wide sweep of the teeth,
such as would be obtained by rotation of the sclerite on its fixed point with
the teeth in the everted position, is the better for scraping a surface. It is
probable that scraping by rotation of the sclerite as a whole in an antero
-posterior plane, through an angle of half a right angle or thereabout, with the
teeth in the everted position, is the predominant factor in Musca and perhaps
in P hilæmatomyia lineata, while in the higher form which is exemplified in
insignis the labellar rods are erected once for all at the commencement of the
act of feeding, and the wound actually made by rapidly repeated eversions of
the teeth.
In spite of the changes in the form and position of the sclerite the relations
between it and the labella remain unaltered. The inner wall is attached to
the borders of the sclerite as in Musca, and the interval between the two
labellar rods corresponds to the median fissure on the anterior surface, between
the two lobes. In order that the prestomal orifice may be directed distally, how-
ever, the labellar rods must be in line with the labium, not perpendicular to it.
It should be remembered that the prestomum is not a closed ring, but is simply
the point where the labellar walls converge to each other, and that it is defined
by the discal sclerite. In Musca the opening through which food passes is at a
right angle to the long axis of the sclerite, and insignis in its long axis, but in
each case the interval between the labella is only closed in by the apposition
of the two inner labellar walls.
The modifications of the labial gutter and the posterior joint are an
obvious result of the method of action. The teeth and the discal sclerite are
increased in size, and therefore require greater strength in the gutter which
supports them the increased weight and the more forcible movement demand
greater consolidation of the joint and increased muscle, and this latter factor
necessitates an increase in the area of muscle attachment, a demand ful-
filled by the development of the keel. The posterior joint and its muscles
also show an increase in strength and stability, in accordance with the greater
importance of the muscles which evert the teeth.
Considerable interest attaches to the origin of the interdental armature,
which, from its persistence in all the Muscid biting flies, is evidently an
essential part of the biting apparatus. The key to its homology is to be found
in a comparison of the two early forms with one another and with Mu sca , and
in the relation between the degree of development which it exhibits and the
extent of the pseudotracheal membrane. It was stated in the description of
Mu sca that the method of termination of the channels of the pseudotrachee
31
- was variable, and the large amount of variation there is in even closely allied
species is indeed remarkable. The variations are slight and. difficult to make
out, it is true, but it is just in such a labile structure that one would expect
to find new developments. The usual method has already been described;
the rings adjacent to the sclerite become more open, and develop a median
T-shaped projection which is turned towards the prestomum. It is not
:actually attached to the chitinous margin, and on either side of it there are
thin slips of chitin, sometimes simple, sometimes branched in au irregular
manner, which pass from the sides of the open gutter at the termination of the
channel towards the prestomum, and terminate either in very close proximity
to it or by actually fusing with it. In some cases there appears to be a sort of
loose joint between the two. These strands of chitin lie between the teeth and.
on either side of the gutter at their terminations; they are evidently of the
same nature and origin as the rings of the pseudotracheæ, and are almost
certainly formed by the splitting in the middle line of the terminal T-shaped.
rings. Now in Philæmatomyia lineata we find similar but much more
conspicuous strands, arising from a common stalk between each pair of teeth,
and dividing into four filaments terminally, two of these passing to each side
of the end of the gutter. In gurnei, some of the last rings are divided into
two, and pass towards the sclerite ; outside these, and attached near the chitinous
margin, there are two definite leaf-like blades, distinguished from the strands
by their flatness, but otherwise disposed in precisely the same way. These are
turned slightly outwards from one another at the distal end, where they lie on
the sides of the terminal gutter of the pseudotrachea. In insignis the serrated
blades, although elaborate, are essentially of the same nature as those seen in
gurnei, and like them are derived from the proximal rings at the terminations
of the pseudotracheal channels ; each of the blades in each bunch represents a
modified strand of chitin such as those in gnrnei, attached to a common stalk,
and the complexity of structure is more apparent than real. A comparison of
the serrated blades of insignis with the chitinous strands which occur in the
prestomal region of the blow-fly, as figured by Graham-Smith (his figure 8,
e.g.) shows at a glance how the transformation has occurred.
The origin of the interdental armature from the pseudotracheal channels is
all the more interesting when one considers the function which they fulfil in
the two forms. As pointed out by Graham-Smith, the pseudotracheal channels
of the blow-fly act as a filter to prevent the ingress into the mouth of particles
too large for the food canal, and as I have shown in the case of L yperosia,
which is provided with an arrangement similar to that of insignis, the blades
are swept across the wound in advance of the teeth at each eversion, and will
thus keep the surface clear while the wound is being made. When it is
32
sufficiently large for the blood to flow, all the fluid which enters the prestomum
must first have found its way through the network formed by the blades,
which form a sort of brush at the extremity of the proboscis, an arrangement
which must act as a most efficient filter. Thus, though the rings of the
pseudotracheal channels have undergone a great change of form, the function
is retained.
The affinities of the three species, so far as can be judged from the struc-
ture of their mouth parts, are as follows :—lineata is closely allied to Musca,
while gurnei represents a much greater advance, and is probably the first stage
at which the fly can make a wound for itself. Insignis is much further
removed from gurnei than that fly is from lineata , as shown by the definitely
erect position of the labellar rods in the position of action and the high degree
of development of interdental armature. So far these are the only three
species known, but it is highly probable that more will be found, as the first
species described, insignis, has proved to be so common and so widely distributed
since it was first differentiated from Musca . New species will be awaited with
great interest, for among them one may expect to find forms showing interme-
diate stages, in which the teeth will not be quite terminal in position, and the
discal sclerite not so much thickened nor quite in line with the labial gutter
in the position of action. Such forms may also be expected to show stages
in the evolution of the interdental armature midway between gurnei and
i nsignis.
The Stornoxys Group.
The next three flies, Hæmatobia , Stomoxys and Lyperosia, are included
in the Stomoxydinœ. The proboscis of Lyperosia , perhaps the most specialised
of the group, has been described at length in a previous memoir, and in their
main features the other two are so similar that no general description of them
is necessary. The three flies studied in the present research do not present a
connected series comparable with that formed by Musca and the genus Philœ-
matomyia, but there are certain points which appear to indicate that the order
in which they are placed above is that representing their respective degrees of
specialisation. There is a wide gap between these flies and those of the genus
Philœmatomyia, but doubtless future research will reveal the existence of
intermediate forms which will show the course of the evolution more precisely
than it can be seen at present.
In view of the close similarity of these genera with regard to the struc-
ture of their mouth parts, a general account of the form of the proboscis in
its relation to the Muscid type will be given first; the details in which they differ
from one another, so far as they are cognate to the subject, will be given later.
33
All the changes from the Musca type are directed to one end the produc-
tion of a piercing organ, to enable the fly to pierce the skin of vertebrate
animals in order to suck their blood. The fundamental difference between
the flies in this group and those of the genus Philæmatomyia is that they
actually do pierce the skin, whereas Philæmatomyia only scarifies it.* The two
essential features in such a piercing organ are that it should be rigid and that it
should terminate in a point. It is in these directions that the proboscis in the
S tomoxydinæ is altered from that of Musca.
Rigidity is obtained by the consolidation of the rostrum and the haustel-
lum, the reduction in size of the former, and the closer linking of the two by
means of the buccal cavity. The haustellum is greatly strengthened by the
increase in the extent to which its wall is formed by the mentum, which curves
forward so as almost to meet in the middle line in front, leaving only a narrow
membraneous interval between it and the labial gutter. The teeth are also
more closely united to one another, though still equally free to move in the
socket formed by the tooth plate. The alteration in the attitude of the pro-
boscis. which is the most striking feature, is merely due to the increased
length of the haustellum, which can no longer be folded on the lower surface of
the head, but projects horizontally forward when in the position of rest.
The movements of proboscis, though not so conspicuous as those of the
house fly, are essentially the same, and have practically the same range. They
are used to bring the proboscis from its horizontal position of rest into a more
or less perpendicular position when the fly is feeding. On account of the al-
teration in the relative size of the haustellum and the rostrum, however, the
latter is more or less fixed in the position of extension, and when the fly is at
rest the two parts of the proboscis are at a right angle to one another; the
rostrum is apparently capable of being moved in a forward direction to a
greater extent than in Musca, and one can at times see the whole organ extend-
ed and pointing in a direction anterior to the perpendicular. One may note
in passing that the arrangement of these joints is quite the least advantageous
possible for the propagation of any force from behind, and that it is ana-
tomically impossible that the piercing parts should be inserted by means of the
muscular power of the body of the fly; the proboscis is merely lowered, into
the wound as it is made.
Rigidity is also produced by the consolidation of the anterior joint between
the discal sclerite and the labial gutter, by the partial fixation of the labella in
a position which corresponds to that of extension in Musca, and by the con-
* This does not mean that the proboscis of Philæmatonyia is necessarily a less efficient organ than that of
Stomoxys and its allies. One may express the difference by saying that the one scrapes a hole in the skin,
the other bores a hole.
34
solidation and modification of the posterior joint between the furca and the
mentum.
The alteration in the shape of the proboscis is practically confined to the
haustellum. This is always much broader in the upper than in the lower part,
and tapers to a point anteriorly. The labella are so much reduced in diameter
that they are little if at all wider than the narrowest part of the haustellum, and
thus offer no obstacle when the proboscis enters the skin. The macrochætæ
which are so conspicuous in Musca are reduced in number and in size,
both at the distal margin of the labella and on the sides of the mentum.
The alteration in the appearance of the proboscis depends mainly on the
progressive enlargement of the haustellum and diminution in size of the ros-
trum. This change in the relative proportions of the parts of the proboscis is
a fundamental one, and can be traced through the whole group from. Musca to
Glossina. It forms a reliable index of the correct position of the several
members of the group. The following are the proportionate lengths of the
rostrum and haustellum in all the flies considered in this paper. In the case
of the Stomoxydinæ the measurements were made along the posterior plate of
the fulcrum from the upper to the lower cornua, and from the upper limit or
the mentum to the tip of the labella, following the curve of the organ. In
M usca and Philæmatomyia, an equivalent measurement of the haustellum
was obtained by adding together the length of the labial gutter and of that por-
tion of the discal sclerite which is distal to the articulation, including the teeth.
The figures represent an average of several observations on different specimens;
on account of the impossibility of obtaining a series of preparations mounted
in exactly the same way they are to be regarded as roughly approximate
only.
Ros t r um.
Haustellum.
Musca nebulo
...
. . .
... 1
to
. 712
Philæmatomyia gurnei
...
... 1
to
. 733
Philæmatomyia lineata
...
... 1
to
. 777
Philæmatomyia insignis
...
... 1
to
1.411
Hæmatobia irritans
...
...
1
to
1.86
Stomoxys calcitrans
...
. . .
1
to
1.91
Lyperosia minuta
...
...
1
to
2.53
Glossina submorsitans
...
... 1
to
3.70
As will appear later, the progressive increase in the length of the haustel-
lum is directly proportionate to the degree of fixation of the joint between the
discal sclerite and the labial gutter, and to the degree to which the haustellum
is drawn out to a point.
The rostrum , in addition to the reduction in size and the limitation of
movement, is a much more firm and rigid structure than that of Musca; the
35
investment of the fulcrum in the membraneous wall is closer and the air sacs
posterior to the fulcrum, by the distention of which the rostrum is pushed
forward, are not nearly so conspicuous. The lateral plates of the fulcrum ex-
tend further forward, and the membrane uniting them on the anterior surface
is thicker and merges indefinitely into the chitin at the sides. On the other
hand, the anterior arch is not thickened into a stout bar for muscle attachment,
and the whole structure is more closely fixed to the head than in the species
previously considered.
The buccal cavity has undergone a remarkable transformation. Elon-
gation is foreshadowed in the case of P hilæmatomyia insignis ; in the
Stomoxys group we find that it has gone on to such an extent that the cavity
has entirely lost its, original shape and structure, and is converted into
a duct with specialised walls, connecting the food canal in the haustellum with
the pharynx. It is essential that this duct should be capable of flexion with-
out occlusion of the lumen, for these flies feed with the joint between the
haustellum and the rostrum a little flexed, and not with the whole proboscis
fully extended as in Musca. When the organ is in the resting position the
buccal cavity forms a tube at the lower end of the rostrum, the upper end of
the tube being in line with the pharynx and the lower end with the haustellum,
so that there is a point about its middle portion where it is flexed at a right
angle. To ensure that flexion will not result in the occlusion of the lumen
the wall of the tube is composed of two parts, an inner and outer; of these
the inner consists of a simple tube of thin but rigid chitin in the upper half,
where it is in line with the pharynx, and below this of a softer membrane.
Outside this there is a series of transverse rings, thicker behind than in front,
and approaching one another in the middle line behind so closely as to appear
to be fused with one another. These rings will thus keep open the lumen
of the tube should it be flexed during the act of feeding,* whereas the food
channel of Musca is probably blocked in the flexed position by the approxima-
tion of its walls at the buccal cavity. In all the Muscid biting flies the
buccal cavity is merely a channel, dilated in the early forms and tubular
in the higher, which connects the food canal in the haustellum with the
pharynx. It takes no part whatever in the sucking action,
The haustellum , instead of the loose cylindrical arrangement of chitin and
membrane seen in Musca, is here a compact and almost entirely chitinous
spindle, tapering gently from the expanded upper portion, known as the "bulb",
to a point. The entire surface as seen externally is composed of the mentum
* This arrangement of chitinous rings has another and more important object, viz., to prevent occlusion of
the channel when a negative pressure is brought about in the pharynx, into which it opens, during the act of
sucking.
The negative pressure in such a fly as Stomoxys, which fills itself to repletion within a few minutes, must
be much greater than in the pharynx of intermittent feeders such as Musca .