NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
44
end of the channel, there is a minute, short and stout hair set on a thicker
base.
At the proximal end of the flap the flat plates which form the groove are
replaced by rounder rods very similar in appearance to those of ordinary
pseudotracheal channels. The transformation takes place gradually and
regularly, and there is no doubt that the plates in the region of the flap and
the rings between the petiolated blades are the same structure in different
forms. The termination of the pseudotracheal channels in the region of the
teeth is abrupt and simple, and there are no T-shaped rings, or rings split in
half, such as have been described in the previous forms. The terminal rings
differ from the rest only in being produced into U-shaped loops.
There is no doubt that this structure represents a pseudotracheal mem-
brane, and is homologous with that found in Musca . In order to ascertain
definitely to what degree the function is retained it would be necessary to
examine fresh specimens, but it appears likely that the channels do actually act
in conveying the fluid to the prestomum. The space between the two walls of
the flap is evidently a part of the hæmatocœle, and will therefore be distended
by the blood of the fly as in Musca . It was shown in the case of Lyperosia
that the teeth are returned into position for cutting by the distension of the
hæmatocoele. In the case of Hæmatobia this distension will also result in
the approximation of the two ends of the elongated transverse plates, in the
same manner as that in which the pseudotracheal channels are closed in
Musca . Possibly the two ends interlock, leaving only the interbifid spaces
open as in the non-biting Muscids.
The occurrence of this structure in a fly with a piercing proboscis is very
remarkable, and has an especial interest as indicating the source of the pseudo-
tracheal channels. Evidently these, in Musca and its allies, are mere thicken-
ings of the wall of the labella, and are similar in origin to the squamæ in
Stomoxys and Hæmatobia.
Stomoxys.
The anatomy of the proboscis of Stomoxys has been described in great
detail by Stephens and Newstead, and I may say at once that as regards the
details of structure and armature there is little to add to their account, my
dissections having in the main confirmed their results. Their work on Stomoxys,
however, was done after an examination of Glossina, the most complex and
highly specialised of the series, and the true homology of the parts, which can
only be understood by proceeding from the simplest forms to the more complex,
is not brought out in their paper. The terms used by them are descriptive and
45
provisional only, the establishment of a definite nomenclature having been post-
poned pending further research. The account therefore lacks cohesion, and is
difficult to follow unless one has the actual preparations before one. It will
therefore be necessary to explain the relation of the terms used by them to
those used in this paper. In so far as they are applicable to structures not
already named in Musca, their terminology has been retained and used in the
previous papers of this series.
To commence with the labium: the whole of the chitinous wall as seen
externally, including the bulb, I regard as the mentum, homologous with that
of Musca; the two joints of the fork at the lower end are separated from one
another, the proximal one being the support of the furca on each side, and the
distal one pressed forward against the furca. The former of these is called the
" ventral sclerite," the latter the " lateral sclerite of the fork." For the labial
gutter and the keel I have used their terms, which are equally applicable to the
corresponding parts in other flies. In the labial gutter they distinguish the
lateral borders of the terminal portion concerned in the articulation as the
" dorsal hooked sclerite " and the base as a separate sclerite, which unites with
the lateral ones proximal to the articulation. In their description of the labella
there is a good deal of confusion, owing partly to their having placed too much
reliance on the appearances seen in sections, and to their not having recognised
the nature of the joint. Their figures are so accurate and clear that the confu-
sion of the text is to a large extent removed when one compares them with dis-
sections. Their tooth plate ( k) corresponds to that part of the sclerite which
I have termed the labellar rod; the axial apophysis, as before, represents the
tip of the discal sclerite which has become displaced upwards. The petiolated
blades constitute the interdental armatures; the " fork " is the furca, and
should not be confused with the fork of the lower end of the labium. The
tendon which is shown in their sections is the tendon of the muscle of the bulb,
which pulls the teeth outwards and actuates the cutting mechanism.
With regard to the mechanism of the cutting apparatus my view, as ex-
plained in connection with Lyperosia, differs from theirs. I can find no joint
in the whole of the proboscis which could permit of a lateral rotatory move-
ment of the teeth, and the transverse muscles at the lower end of the labium
fulfil a function entirely different to the one suggested by them. So far as one
can gather from their paper these authors do not appear to have had access to
fresh material for dissection, a most serious handicap, for without that, and a
considerable quantity of it, it is extremely difficult to make sure of points such
as these. The action of the muscles of the bulb can only be fully realised by
dissecting out the bulk of the muscle and making traction on it while holding
the labium firm at a lower point on the other side, thus imitating the natural
H
2
46
contraction of the muscle. This is a difficult manœuvre, and one requires a
large number of fresh specimens to demonstrate the effect beyond doubt.
The proboscis of Stomoxys conforms to the general description already
given; all the points which indicate an advance in specialisation are more
marked in this fly than in Hæmatobia . The labium is more pointed, the bulb
confined more to the upper part, and there are fewer and smaller hairs at the
discal fringe and at the sides of the mentum, points which indicate that the
organ is more adapted for piercing. As showing the greater fixation of the
anterior joint, the labial gutter is not so well marked and its keel is confined
to the distal part of the labium; the labella are not so large as those of
Hæ matobia and are placed more nearly in the same long axis as the labium in
the position of rest. The joint between the end of the labial gutter and the
distal sclerite is a closer one, and the two parts are united by a strand of
chitin which passes between the distal ends of the lateral portions of the gutter
and the dorsal part of the sclerite, thus limiting still further the amount of
movement between the two, already small by reason of the nature of the joint.
The teeth and the other structures on the inner wall of the labella, which have
been described in great detail by Stephens and Newstead, resemble generally
the arrangement in Hæmatobia, but some structures described by these authors
have not been recognised either in Hæmatobia or in Lyperosia . The external
wall of the labella is much less chitinised, and the membraneous portion
between the truncated end of the mentum and the furca is more extensive, and
is marked by folds indicating its disposition when the furca is drawn
upwards .
Lyperosia.
While one can readily believe that Stomoxys has had for its immediate
ancestor a Hæmatobia- l i k e
f o r m , i t i s c l e a r t h a t
L y p e r o s i a
i s n o t d e s c e n d e d
from the Stomoxys stem, but from a common ancestor of the two, although
the relation between the three flies is a very close one. Perhaps when the
opportunity occurs to examine some of the other genera of Muscid biting flies
forms will be found which will explain more clearly the positions which these
flies ought to occupy in the group.
In the greater length of the haustellum in proportion to that of the
rostrum, and in the much more typical spindle shape of the labium, we have
clear indications that this fly is more perfectly adapted for piercing the
skin than is Stomoxys . But in other details, small in themselves but
significant when taken together, the adaptation appears less perfect. The
palps approximate more to those of Hæmatobia than to those of Stomoxys,
a point on which systematists lay some stress. The distal border of the
47
labella has a fringe of macrochætæ which are distinctly more prom-
inent than those of Stomoxys , though not so conspicuous as those
of Hæmatobia. The external walls of the labella are far more chitinised than
those of Stomoxys, though the chitin is thin and. the plate which it forms is in-
terrupted by the presence of longitudinal fissures, which will to a certain
extent compensate for the rigidity. The form of the discal sclerite, however,
shows clearly that Lyperosia is not directly related to either Hæmatobia
or Stomoxys, and at the same time suggests that it is more specialised than
either. in its general shape and relations it conforms to the type already de-
scribed, but, when seen in section, it is found to be a much more complex
structure, the relation of which to the mechanism of the joint it is by no means
easy to determine. The change is limited to the axial apophysis, which
has lost the solidity of the corresponding part in the other flies, and consists of
a shell of chitin with cellular tissue inside. The proximal end is turned
upwards behind the end of the labial gutter in a curious manner, and appears
to take some part in the mechanism of the joint, though neither muscle nor
tendon can be found attached to it. The joint between the labial gutter and
the discal sclerite is of the most complex description, and can obviously permit
of only a slight degree of movement.
Glossina.
Glossina is as widely separated from the flies previously considered as
regards the structure of the mouth parts as it is in its other anatomical
features and in its life history, and there is at least as wide a difference between
its proboscis and that of Lyperosia as there is between Lyperosia and Musca
in this respect. Nevertheless we find that the great changes in the anatomy of
the parts are but an extreme instance of the adaptation of the Muscid.
proboscis to a blood-sucking habit, and there is little doubt that the proboscis
of this fly is but a modification evolved from the Musca type.
The anatomy of the proboscis has been minutely described by Stephens
and Newstead, but these authors have not touched on the subject from the
point of view of this paper, and have simply described the appearances as seen,
with a suggestion regarding the mechanism. My remarks on this fly will
resolve themselves into an interpretation of the homology of the parts as
described by them, and to a consideration of the mechanism of the proboscis as
a whole, dealing in more detail, however, with the structure of the rostrum,
which is not touched on in their work. My material has been limited to some
dried specimens of Glossina sub-morsitans, for which I am indebted to Professor
Newstead, and one specimens of Glossina palpalis, placed at my disposal by
48
Captain Patton. To make the most of such limited material the
specimens, after clearing in potash, each for a different period, were
mounted whole in different positions, and after examination replaced in clove
oil, dissected, and remounted in balsaam. I have found no differences from the
description or figures in either palpalis or sub-morsitans.
The proportionate lengths of the rostrum and the haustellum as given on
page 45 are somewhat misleading, as the position in which the proboscis
is held is a much more definitely fixed one than in the other flies, and it is not
possible in cleared preparations to stretch the two divisions out into the same
straight line. In the figures given by Stephens and Newstead and by Hansen,
the haustellum is shown to be about three and a half times as long as the
rostrum, The rostrum is short and compact, and is almost entirely occupied
by the fulcrum, the space available for air sacs between its posterior plate and.
the limiting membrane being very small. The fulcrum is much more regular in
shape and more evenly chitinised than in any of the previous flies. Seen
in profile it has the shape of a triangle with three approximately equal sides
;
the posterior side is gently convex backwards, the anterior convex forwards in
the upper part, where it is produced into a sharp angle, and concave below;
the upper side is concave, but is not produced in the definite cornua at the
posterior angle as in Stomoxys. The walls of the fulcrum are equally chitinised
throughout, and from this fact and from the absence of ridges for muscle
attachment, and from the lack of cornua at the upper and posterior angle, one
would infer that the movements of the rostrum on the head, though of the same
nature as those of the other flies, are reduced to a minimum; and that the
adaptation of the position of the proboscis for feeding is carried out by move
ments of the body of the fly and by the joint between the rostrum and the
haustellum. The chitin of the anterior arch occupies a much larger proportion
of the anterior surface of the rostrum than is the case in the other forms,
The buccal cavity approaches more to the type seen in Musca than to that
of the Stomoxys group. It consists of a small chamber with chitinous posterior
and lateral walls, formed from a small transverse plate, the up-turned sides of
which are united anteriorly by a membrane. I am not able to say whether it
has a dilator muscle or not. It is connected with the epipharynx in the same
-way as in Musca. The membrane which unites the buccal cavity with the
pharynx is a very short one, and from the structure of this part of the proboscis
it is evident that there is a little movement at the joint.
The briefest examination of the haustellum shows that the whole of it has
become transformed into a piercing organ, and that the separation of the labella
from the labium, though clearly marked, has not the same significance as it has
in the Stomoxys group. All the structures have become most profoundly
49
modified, the armature has become a most complex one, and the nature of the
joints has altered so completely that it is difficult to say in some cases exactly
what they represent or at which point the movement takes place. The funda-
mental changes on which the appearance of the proboscis depends are the pre-
dominence of the labial gutter over the mentum in the narrow part of the pro-
boscis; the fusion of the discal sclerite with the end of the labial gutter, resulting
in the total disappearance of the anterior joint; and the conversion of a part of
the outer surface of the labella, and a part of the surface of the labium itself,
into a cutting apparatus. All this is but an extreme stage of what one sees
going on in the evolution of the other flies.
To take these changes in detail : The bulb of the proboscis is evidently
the same as that of Stomoxys, and may at once be termed a part of the
mentum. But at the distal end of the bulb the thick and pigmented chitin of
which it is composed gives place to a very much thinner sheet which is flexible
and membraneous, and which cannot contribute to the rigidity of the organ. In
the middle line it is provided with a bilateral row of peculiar spines, which extend
right to the commencement of the bulb. (S. &amp; N., figure 2, i, ii) Sections at
any point between the labella and the bulb show that the wall of the mentum
is a very thin one; on the other hand, the labial gutter is very well developed,
and it undoubtedly constitutes the main support of the proboscis, thus reversing
the conditions found in Musca. The membrane which unites the two is short,
but in the sections figured it is shown as thrown into irregular folds, which
are very noticeable in my preparations. They run obliquely upwards and
inwards, not in the same way in all the preparations but varying in different
positions of the labella, in such a way as to indicate that the labial gutter and
the mentum do not always occupy the same position in relation to one another.
The only trace of the keel of the gutter that is to be found is a thin slip
of chitin extending less than half the distance between the gutter and the men-
tum in the bulb. In the middle line between the two lateral halves of the
gutter there is a deep pocket-like indentation in which the hypopharynx
lies.
The posterior joint, between the labium and the labella, has undergone
most profound changes, and it is no longer possible to recognise the constituent
parts as seen in Stomoxys. But by reverting to the appearance in Musca, and to
the essential nature of the parts, one is able to trace the homology fairly closely.
The furca in Musca is nothing more than a thickening of the external wall
of the labella on each side; in the biting flies allied closely to Musca it
becomes transformed into a strong bar because the important muscles of
the bulb are inserted into its lateral arms. In Glossina we find a reversion to
the more primitive type. The muscles of the bulb are apparently not inserted
50
by one thick tendon, but are spread out over a considerable area of the
external wall, and the necessity for a thick rod therefore does not exist.
There is instead a transverse plate, which is united with a longitudinal one.*
The latter may, without any great stretch of the imagination, be recognised as
formed by the fusion of the two rods which form the fork of the mentum, the
ventral sclerites of Stephens and Newstead. These authors have pointed
out that the lateral arms of this T-shaped piece are variable in position
according to the degree of eversion of the labella, exactly as one would expect
if they represent the arms of the furca. A considerable amount of the muscle
of the bulb is probably inserted into the region referred to as the " dark
ventral chitinous area,"
The anterior joint has disappeared, and if the sections are traced
downwards it will be seen that the walls of the labial gutter become directly
continuous with the inner walls of the labella, just as the outer walls of
the mentum are continuous with their outer walls. There is, however, a
thicker portion shown in their figures 15 to 19, which corresponds to the position
of the discal sclerite; the hypopharynx lies at the upper end of this thickening
in the position of rest. The situation of the thickening of the internal
walls corresponds in position to that of the " dark area " on the outer walls.
The interpretation of the mechanism of the cutting apparatus offers no
difficulties. The teeth resemble in shape, though not in arrangement, those
of the preceding examples, and the " fans " distal to them are evidently
the homologues of the petiolated blades of Stomoxys, and therefore of the
pseudotracheal rings of Musca. The teeth are united at their bases by the
three pieces of chitin which bear the rasps, in a manner corresponding gener
-ally with that in which the teeth of Stomoxys are attached to the tooth
plate; whether the rasps really represent the united bases of the teeth or are
a detached part of the discal sclerite it is not possible to say. The " jointed
chitinous rods " appear to be the rod-like hairs, which, in order to maintain
their position in advance of the teeth during the act of cutting, have passed
through the wall of the labellum in the manner described.
The mechanism of the proboscis is evidently the same as that of the
Stomoxys group. But here there is an important point which should be noted
in the interpretation of the drawings. I pointed out in connection with
Lyperosia that the appearance of the labella in the everted position in cleared
preparations is not the same as that in fresh dissections, for, when the muscles
are dissolved by the action of the potash, the parts relax ; it is impossible to
get cleared preparations which show the true position of the teeth during the
* An examination of more material has convinced me that the longitudinal and transverse portions of this
T-shaped rod are not continuous with one another, but are united by a narrow strip of membrane. This serves
to hinge them together, while permitting of free movement.
51
act of cutting. The same applies to Glossina, and I have not the least doubt
that the lateral arms of the " fork " are turned much further backwards, and
the teeth much more fully everted, than is shown in figure 8 (S. and N.).
The chitin between the " dark area" and the bases of the rasps is evidently
thin and membraneous, and it is here that the eversion takes place when the
teeth are brought into action.
There is a membraneous area between the end of the labial gutter and the
fork, as is found in the other flies. But in the case of Glossina, the entire
mentum, with the exception of the bulb, is drawn upwards when the teeth are
everted, and the membrane between it and the labial gutter is thrown into the
folds already mentioned. The posterior surface of the mentum is armed with
spines which would appear to have some cutting action, and it is probable
that many of the spine and tubercles so minutely described are really cutting
organs accessory to the teeth and rasps, and that not only the inner wall of the
labellum, but its outer wall and the outer wall of the labium also are a part of
the cutting apparatus. The whole is actuated in the same way as in Stomoxys,
by the contraction of the muscles of the bulb, the long tendons of which are so
conspicuous in the sections. But instead of a simple eversion of the teeth the
whole of the inner wall of the labellum, its outer wall, and part of the mentum
are drawn up, until the thickened part which represents the discal sclerite is at
the tip of the proboscis. When this has occurred the labrum-epipharynx and
the hypopharynx are at the point of puncture, and are able to draw in the blood.
The disproportion between the length of these two organs and the total length
of the labium and the labella is less than that seen in Stomoxys, and this corre-
sponds with the shorter space between the tip and the softer portion of the
inner wall at which the eversion takes place. The short excursion of the cut-
ting surface is more than compensated for by its greater lateral extent, for the
armature extends from the rasps to the bulb, and includes portions of the inner
and outer walls of the labella and also of the posterior surface of the
mentum.
The labrum-epipharynx and the hypopharyx are altered in such a manner
as one would expect. On account of the more pronounced stylet-shape of the
haustellum as a whole, they are reduced in diameter and fit much more closely
into the groove in the labial gutter. They are more firmly united to one
another, so as to constitute almost a closed tube, and the labrum is closely fitted
to the labium by means of an elaborate arrangement of interlocking teeth,
Between the labrum and the epipharynx a small comma-shaped interval can
be seen in the region of the bulb, in the situation in which the fan-shaped
muscle is found in Musca.S t e p h e n s a n d N e w s t e a d s t a t e t h a t t h e l a b r u m , b y
which term they designate the combined organ, fuses at the upper end with