NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
the wound almost at the angle at which one ordinarily uses a knife. The
action is very like that of a circular saw acting through a short arc.
With the exception of the mosquitoes, all the members of this group have
well developed mandibles, and the variation found in the different forms is
remarkably small. In Tabanus (figure 4) the serrations are extremely fine,
although the blade itself and the edge bearing the serrations are quite strong.
In Simulium (figure 2) the serrations are much larger and. are continued round
the tip of the blade to the external border. In Ceratapogon (figure 3) they are
intermediate in size, and. the inner edge of the blade is bent rather abruptly
outwards, so that the teeth are directed inwards and forwards, and. the point
of the blade a blunt one.
In Phlebotomus the serrations are a little coarser than those of Tabanus,
and are more widely separated ; the blade is flattened. Newstead states that
it is the outer edge which bears the serrations. I have not had. the opportunity
of making a sufficiently large number of preparations to express an opinion on
the point, nor have I been able to consult Grassi's paper on this fly, though,
thanks to the kindness of Dr. Ashmorth, I have obtained a copy of his draw-
ings. In these I note that he represents, in a cross section of the proboscis,
that the mandibles lie superimposed upon one another between the labrum-
epipharynx and the hypopharynx, a most remarkable position for them to
occupy, for they render it impossible for these two structures to form a closed
food canal. Newstead, who offers no remark on this point, gives a drawing
of the labrum-epipliarynx and hypopharynx in apposition with one another,
without showing the mandibles between them. Great difficulty arises in
decided points such as these on account of the small size of the parts and the
impossibility of separating the component parts of the proboscis without
injuring some of them. In many of my preparations of Ceratapogon the ser-
rated edge of the mandible appears to be external, and is in fact external to
the edge of the labrum-epipharynx, but this is due to the two mandibles
having crossed one another, and what appears to be the external edge of the
mandible of the left side is really the internal edge of the one on the right. As
already explained the direction of movement of the mandible is inwards and
outwards in the lateral plane, and. although there is no proof that they nor-
mally pass beyond the middle line, there is no reason, anatomical or otherwise,
why they should not do so.
The Culicidæ show a certain amount of variation in the mandibles and it
looks as if they were tending to disappear in that family. On account of the
great elongation of the parts in the mosquito the cutting portion of the blade
is limited to the distal end, which is slightly expanded. The serrations in
Anopheles (figure 5) are excessively minute, and can only just be made out
10
with a magnification of a thousand diameters, and in Culex it is difficult to be
sure that they exist at all, nor is it possible in either case to say whether the
serrations are on the outer or the inner side. In the male mosquito the whole
appendage is missing. The muscles at the base of the mandible are not
recognisible in this form, and one is justified in assuming that the appendage
is of much less importance in the mosquito than in the other families. It
would be interesting to know if the gradual disappearance of the mandible
goes on pari passu with other changes in the different genera, and if there are
any blood-sucking species in which the female is without mandibles.
The maxillæ are even more admirably adapted for the purpose of making
a wound than the mandibles. The most remarkable feature about these ap-
pendages is the manner in which the primitive arrangement of the joints is.
maintained. The maxilla of a primitive insect, such as the cockroach, consists
of five parts, the lacinia, galea, and palp, external organs, named from within
outwards, and the stipes and cardo which are internal. Of these the palp
persists and retains its sensory character throughout the Diptera; the galea
becomes the cutting blade in the Nematocerous biting flies and the lacinia
remains only as a small peg shaped projection at the base of the blade on the
inner side. The stipes appears in. these forms as a backward continuation of
the blade, and passes through the interior of the head cavity in its lower part,.
external to and below the first part of the sucking apparatus, to a point almost
as far back as the occipital foramen. Here it is attached to the wall of the-
head cavity by the interposition of the cardo, which is always very small and
in the case of the smaller and more delicate flies may be absent. The maxilla
is capable of movement on the joints between the stipes and the cardo, and
between the cardo and the wall of the head capsule, into a small notch in
which the cardo is fitted.
This arrangement is best studied in the large Tabanidæ. Here the card°,
is easily seen as a wedge-shaped rod articulated in a small notch at the edge of
the space occupied by the palatal membrane, and the method of action can be
readily understood by an examination of the muscles which are attached to the
stipes and cardo. As in the case of the mandible, there are two sets, termed.
by Meinert the adductor and abductor ; the former arise from the internal
surface of the head capsule near the occipital foramen, and run directly for-
wards in line with the stipes ; the latter run in the opposite direction, arising-
from the anterior wall of the head capsule in the region of the gena, and are
inserted into the cardo. The two sets are therefore antagonistic, and act on
the joint between the stipes and the cardo, and on that between the cardo and
the wall of the head, in opposite directions. The adductors would be more
accurately described as retractors, and the abductors as protectors, for the
11
action of these muscles must be to alternately protract and retract the stipes,
and with it the cutting blade.
The armature of the maxillæ presents both a remarkable uniformity in
all the flies included in the group, and a remarkable adaptation to the method
of action of the muscles. In all cases the type is that of a rasp, on which most
of the teeth are directed backwards, so that it cuts mainly when it is retracted.
The cutting edge may be on the inner or the outer side, or both edges may be
armed ; the blade may be straight or a little curved.
Before going further the individual variations in the maxillæ may be
noted. In the Tabanidæ (figure 10) the blade is stout and roughly quadri-
lateral, and is armed all round the distal end with strong spines which are
only slightly raised from the surface of the blade ; these spines are continued
a considerable distance down the internal border, but only for a short distance
externally. On the external border there is a row of extremely fine and
closely set hair-like spines, the distal ones of which are enlarged basally to
form regular spines like those on a rose tree. The blade is almost straight
from apex to base, and is continued directly into the stipes. In Simulium
(figure 1) the blade is flattened and triangular, and has down each side a single
row of spines similar to those of Tabanus, but smaller. In Ceracapogon
(figure 8) the blade is flattened and at the base is thicker on the external side
than internally. Towards the apex the inner side becomes attenuated to a
very thin and almost invisible edge, and the blade itself is twisted transversely,
so the terminal portion of the internal edge is directed outwards. This
terminal portion is armed with a row of extremely minute backwardly directed
spines, set on the fine internal edge and somewhat widely separated from one
another. In Phlebotomus (figure 12) the blade is similarly flattened and bent
transversely,* but in this case both edges bear teeth. On one side there are
six, all directed backwards and extending to the extreme tip, and on the other
a row of very much smaller ones, not extending to the distal end, and directed.
upwards. In mosquitoes the blade is flattened and very much elongated.
The inner side of the blade is thickened to a prominent ridge from the base and.
upwards, and the external edge is very thin and sharp, so that on cross-section
the blade has a triangular outline. In cleared preparations the distal portion
of the blade is usually bent outwards, but this is probably an artifact due to
the different amounts of shrinkage in the thick and the thin edges. The blade
is produced to a fine point, and in some cases, especially in the smaller
forms such as Anopheles maculipennis, figured by Nuttall and Shipley and in
*The transverse bend may be an artifact.
c
12
Culex fatigans, is slightly expanded. On this portion there is a single row of
minute thorn-like teeth, all projecting backwards. In a large species of Job-
lotia (figure 3) I have seen a much more complex arrangement. The blade is
the same as in the smaller forms, but at the tip it is armed on both sides. On
the external side there are five comparatively large teeth, similar to those of
the smaller mosquitoes. The distal one of these is situated some little distance
from the pointed tip of the blade, and in the interval there are two much more
delicate teeth with very sharp points, which point in the opposite direction
to those above them, that is, towards the tip of the blade. On the internal
edge there is a single row of short but stout teeth, seven in number, resembling
those of Tabanus, and extending around the edge on to the two surfaces of
the blade.
The stipes is readily seen in all these flies in cleared preparations of the
head, as a stout rod running across the floor of the head cavity. It has been
described in Tabanus, Anopheles, and Phlebotomus, in the latter two as the
" apodeme " of the maxilla, and it presents very much the same appearance
in Simulium and Ceretapagon. The cardo is less easy to distinguish on account
of its small size, and I have not succeeded in distinguishing it in small species.
In Culex concolor and in Joblotia it can be made out as a distinct piece at the
base of the stipes. Its existence is not of course necessary for the mechanism
of the maxilla, as it would suffice if the stipes were connected to the wall of
the head capsule by the fibrous tissue, and I am inclined to think that this is
the case in many forms, for one notices that the stipes remains adherent to the
chitinuous wall in cleared preparations.
The muscles which act on these joints to produce the protraction and
retraction are arranged in the same manner throughout, though it is a matter
of some difficulty to make out their precise insertions in the smaller species.
They are always to be found in transverse sections of the head, and I have
found them in this manner in Ceratapogon after failing to dissect them.
There is no doubt of the existence of a relatively large bundle of muscle fibres
surrounding the stipes in all case, though one cannot always differentiate
between the adductors and the abductors. The arrangement depicted by
Nuttall and Shipley in Anopheles maculipennis is found also in Joblotia and
Culex concolor.
The mode of action of the maxilla will now be evident. The extreme tip
of the blade is pushed into the skin by the protractor muscles, and meets with
very little resistance on account of the fineness of the point and the upward
direction of the spines. It is then withdrawn by the retractor muscles, and
as this takes place the rasps catch against the tissues and lacerate them.
13
Protraction and retraction follow one another rapidly, and each time the barbs are
drawn upwards through the wound they enlarge it, so that at the next protrac-
TEXT FIGURE 1.
Diagram of the mandibles and maxillæ of the orthorraphic biting flies.
md., mandible.
ab., abductor muscle of the mandible, ad., its adductor.
mx. g., the galea, (the cutting portion) of the
maxilla.
la., the lacinia.
p., the palp.
st., the stipes.
cd., the cardo.
pr., the protector muscle.
rt., the retractor.
tion the tip of the maxilla can penetrate, still against little or no resistance,.
further into the wound. When a certain depth is reached, the teeth on the
opposite side of the blade, in the cases of Joblotia, Tabanus and Phlebotomus,
also take part in the enlarging of the wound, acting, however, during the
protraction of the blade, for the points of the barbs are directed towards the tip,
and do not meet with resistance during refraction. The mandibles and
maxillæ, acting together, the former as a saw and the latter as a file, bore a
14
hole in the skin of the host, and as the hole is deepened the piercing mouth
parts are lowered into it either by the movement of the head on the neck or by
the flexion of the middle and hind pairs of legs, which results in approximating
the body of the fly to the skin of the host. The retraction movement is a more
powerful one than the protraction, and we find as a matter of fact, in the forms
in which the muscles have been studied carefully, that the retractor muscles
are of greater bulk than the protractors, as for instance in Tabanus and
Anopheles.
The process is beautifully demonstrated in the case of Joblotia. In this fly
the extreme tip is armed on the inner side only with small but stout teeth
which do not project from the surface to any great extent, and the first three
teeth of this row will be inserted before any resistance is met with in the pro-
traction of the blade. At the level of the third tooth of this set the first of the
two sharply pointed teeth on the outer side come into play, and at and beyond
this depth both protraction and retraction result in enlarging the wound.
When the puncture is so deep that the whole series of seven teeth on the
outside and the two distal ones on the inside are introduced into the skin, the
larger and blunter teeth on the outer edge come into play, and these are so
shaped that they will cut in both directions, having two edges, of which,
however, the one projecting backwards is the sharpest and strongest. In
Simulium the case is much simpler than this, for the armature of the two edges
is almost identical, and as all the barbs point upward the movement of retrac-
tion is the only one which is efficient in making the puncture. In Tabanus
the resemblance to a file is very striking, for almost the whole surface of the
distal half the blade is beset with powerful spines.
The analogy of the file must not, however, be carried too far. Except
perhaps in the case of Tabanus and Simulium, the blade of the maxilla is not
a rigid structure of the same thickness throughout, but is thinned out from the
base upwards, and has its teeth set on the thinnest and most flexible portion.
This is especially the case in Ceratapogan and Phlebotoms. In the case of
Joblotia and Anopheles, the thickened internal border is continued to the tip
of the blade, but the teeth are on the thin external edge. It might be urged,
especially with regard to the mosquito, that the elongated stylets are too slender
to bear the strain of this method of action, but the bending stress is not, when
one considers the matter carefully, a very great one, On account of the
direction of the barbs the greatest part of the resistance of the tissues has to be
overcome while the blade is being withdrawn, and it will therefore tend to
straighten rather than to bend the blade. The slight flexibility of the blade is
in fact an advantage, as it will enable the point to travel to a certain extent
15
in the direction of least resistance, and if in doing so the blade becomes bent,
it will straighten itself on the return, the teeth meanwhile cutting their way
through the resistant tissue. The extent of movement is of course a very short
one, and is limited to the difference in length between the whole maxilla when
the stipes and cardo are in the same straight line and when the stipes and blade
form one side of a triangle and the cardo the base. In the most extreme case
the excursion of the tip of the blade could not be greater than twice the length
of the cardo, supposing the joint between it and the stipes were capable of mov-
ing through 180˚ (Figure 16). The joint between the proximal end of the
cardo and the epicranial wall must of course move with the joint distal
to it.
It is at first sight a little difficult to believe that these minute cutting
organs, so small that even in the largest horse flies they are almost invisible to
the naked eye, even though they are provided with such a perfectly adapted
armature and muculature, should be able to pierce the thick hide of the larger
mammals. And yet they not only do so efficiently, but the larger flies make
such a formidable wound that the blood continues to flow after the proboscis is
withdrawn, and here, where such species as Tabanus straitus and albamedius
are common, it is by no means rare to see the legs of cattle splashed with blood
as if from a bad scratch, as a result of the attack of these flies. The wound
made is in fact not a punctured one in the surgical sense, but a lacerated one,
and if it happens to involve a capillary of some size the amount of bleeding
may be enormous in comparison with the size of the instrument by which it
was made. If the piercing parts were simply thrust in it is inconceivable that
such minute lancets could draw blood at all.
With regard to the muscles which are concerned in the biting action, it is
by no means di f f i cul t t o bel i eve t hat t hey ar e capabl e of adequat el y per f or mi ng
their function without other aid, when we remember the extraordinary rapidity
of action of which insect muscle is capable. Marey, quoted by Packard, states
that the wing of the fly is capable of making 330 strokes per second, that
of the bee 190. This rapidity, probably unequalled in any other creatures, is
indeed characteristic of insect muscle, and it seems evident that in the move-
ments of the mandibles and maxillæ of these flies we have another and still
more remarkable instance of it, applied in a highly specialised manner. In
spite of the very short range of movement and the minute size of the cutting
weapons, there is no mechanical reason why a comparatively deep wound should
not be made in a very short time, provided the rate of action of the muscles is
sufficiently rapid. The abdomen of a mosquito can be seen to begin to distend
within half a minute or less from the time it settles down to feed, and it is of
course probable that the mandibles and maxillæ continue to act even after the
yr
16
blood has commenced to flow, and are continually employed in deepening and
enlarging the wound as required.
The Sucking Apparatus.
It will be convenient to discuss the sucking apparatus in accordance with
its natural division into three parts, the canal in the proboscis, and the buccal
and. pharyngeal cavities. We may commence with the former, leaving till
later the question of the nomenclature of the two cavities which function as
sucking pumps.
The canal in the proboscis is formed by the apposition of the labrum-
epipharynx and the hypopharynx, the former taking always the larger share
in the circumference of the canal. When the labrum-epipharynx is examined
in cross section it is seen that it consists of two distinct laminæ, which
represent respectively the labrum and the epipharynx. The outer lamina gives
its shape to the combined organ, whatever that may be in the individual case,..
and may be either circular with a gap on the under surface or flattened dorso-
ventrally. The upper end of this lamina is attached to the clypeus, and is
either quite free from the inner lamina or else more loosely attached than at
the distal end. Attached to the internal portion of it, where it reaches the
clypeus, there is a well developed muscle which arises from the inner surface
of the clypeus. The inner lamina, or the epipharynx, is always circular in out-
line, or nearly so, and has, like the outer, a gap on its lower or oral side. The
two laminæ are attached to one another by the margins which bound the gap,
so that they enclose a space in which a small amount of cellular tissue is found,
though there is no muscle in this situation comparable with that found. in the
house fly. The free side of the epipharynx forms the greater part of the food
canal, its circumference varying roughly from two-thirds to four-fifths of a
circle. The hypopharynx also consists of two laminæ, though they are not so
easily distinguished from one another as in the case of the labrum-epipharynx,
and no soft tissue can be seen between them. In form the organ is always flat
and wide enough to fill up the interval between the lateral borders of the groove
in the epipharynx, being in most cases almost as wide as the latter. The epi-
pharynx is always directly continuous with the upper or anterior plate of the
first sucking chamber and in a similar manner the upper lamina of the hypo-
pharynx is continuous with the lower plate. The two laminæ are, in fact,
simply outgrowths from the stomodæum and are homologous with those of other
families of insects. The lower lamina of the hypopharynx arises on the other
17
hand from the labium, and is therefore quite distinct in its origin from the
upper, since it comes from an appendage of the exo-skeleton, and not from the
stomodæum.
The most interesting feature of the food canal in the proboscis is the rela-
tion of its width to the size of the particles which have to pass along it.
Unfortunately it is not possible to make exact measurements of the diameter of
the canal in the normal condition of the parts, and any manipulation intro-
duces such a large possibility of error that it is perhaps better to avoid exact
figures altogether. In sections prepared by either the combined paraffin and
celloidin method or by simple celloidin there is always a good deal of shrink-
ing, and in attempting to measure the width of the canal in fresh or cleared
preparations one is met with the difficulty of defining the exact edge of the
inner lamina when it is seen in optical section. The width of the channel is
only to a limited extent correlated to the size of the fly, and varies from
approximately twenty-five microns in the larger Tabanidæ to fifteen or less in
Ceratapogon and in mosquitoes ; that is to say, it measures from once to twice
the diameter of a human red blood corpuscle, it being of course understood that
all measurements are made at the narrowest part of the distal end of the canal
as in many cases there is a gradual widening of the channel from the tip up-
wards. There are two obvious results of the small size of the channel ; in the
first place, objects definitely larger than the measurements given will not
ordinarily be ingested by these biting flies, a point of some importance to the
parasitologist, and which would well repay further investigation, especially with
regard to the transmission of filaria by mosquitoes ; secondly, if such an object,
larger than the canal could conveniently accommodate, were to gain access
to the canal, or to become impacted at the opening, which is the narrowest
part, there would be considerable danger that the fly would be unable to get
rid of it and would consequently die. Now one would think that in such
a lacerated wound as that which must be made by the saw and file action
of the mandibles and maxillæ there must of necessity be lumps of tissue much
larger than can be ingested, and yet it is inherently improbable that nature
would allow such an accident to occur. The only conceivable method by which
such an impacted object could be got rid of would be by regurgitation, which,
since it implies a reversed peristalsis, is a most unlikely event. We must
therefore enquire as to what are the possibilities of a defensive mechanism to
prevent the ingress into the canal of particles too large to pass down its
lumen.
I have already expressed the opinion that the labrum-epipharynx and the
hypopharynx can take no part in the making of the wound in the skin, on
account of the nature of their attachment to the wall of the head capsule