NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
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In
the middle, and on each side there is another similar in shape but only about
one-fourth the size, and on the inner side, internal to this, there is a small slip
which may be taken as a rudimentary tooth. As in lineata, only the distal
half of each tooth is free from the membrane ; at the extreme tip there is a
series of rather coarse serrations. The serrated edge must be directed upwards
when the teeth are everted. The proximal ends of the teeth are distinctly bifid.
They are not directly attached to the tooth plate.
Between the teeth there are certain structures (Plate II, fig. 9) simple in
this species, but assuming an extraordinary complexity in the more differentiated
forms, which may be termed generally the interdental armature. They occupy
the position of the Y-shaped strands of chitin which support the terminations
of the pseudotracheæ in lineata, though they, like the teeth, are not attached to
the tooth plate directly. They consist in this species of small leaf-like plates
of chitin, arranged in pairs, one pair between each two adjacent teeth. The
two blades of each pair are turned slightly inwards towards one another, so as to
enclose between them the end of the pseudotracheal channel. The blades are
about the same length as the teeth, and arise from the membrane by short
stalks; the extreme tip and the edge most distant from the pseudotracheæ are
thicker than the rest of the blade.
There are twelve pseudotracheal channels, arranged in the usual manner,
except that the anterior and posterior collecting channels are each only con-
tinuous with two pseudotracheæ, the remainder terminating directly at the
prestomum. There is a slight difference in the method of termination; the
rings, instead of having inwardly directed projections, are split into two halves,
and each half is turned inwards towards the tooth plate, one being in advance
of the other so that they appear to alternate.
The discal sclerite shows a further advance in specialisation both in the
degree of separation of the two constituent parts and in the thickness of the
chitin. The axial apophysis is a stout conical piece of very deeply pigmented
chitin, about twice as long as broad, and produced distally to a sharp point ; it
lies between the anterior ends of the labellar rods, and is fused with them in
the posterior third of its extent. The apex of the apophysis is situated on a
level with the bases of the pair of teeth nearest the middle line, so that when
the teeth are turned outwards its point must project beyond them. The
labellar rods consist of two portions differing greatly in the thickness of the
chitin ; the posterior portions are stout wedge-shaped rods bent slightly out-
wards from one another and terminating in blunt and rather thinner ends;
they therefore closely resemble the corresponding parts of the sclerite in the
last species. The expanded distal ends of these rods extend much further on
the outer than on the inner side, so that the rods are triangular, with two nearly
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equal sides and one long one externally. The distal side of this triangle
is produced forwards and inwards into a thinner sheet of chitin, roughly
quadrilateral in shape, which is fused internally with the axial apophysis,
and to the distal margin of which the tooth plate is attached; this flattened
sheet corresponds to the distal curved portion of the sclerite in lineata and to
the horse-shoe shaped arch in Musca (the extreme tip excepted).
The tooth plate, as seen in the flattened preparation, is a thin sickle-
shaped slip of yellow chitin arising from the distal margin of the labellar rod
on each side. The anterior portion, corresponding to the handle of the sickle,
occupies about one-third of the length, and is directed straight forwards, those
of the two sides being approximately parallel to one another, and situated on
either side of the axial apophysis. They are attached to the labellar rods
at their proximal ends, the rest of their periphery being connected only with
the membrane forming the inner wall of the labella. The middle third of the
plate is not distinguishable from the margin of the rod, but the posterior third
extends forward and outward to about the level of the tip of the axial apophy-
sis. In this way the tooth plate of each side forms a hollow cup into
which the bases of the teeth fit, though in this case without any chitinous
attachment.
The labial gutter in this species appears for the first time as a definite
isolated structure, very different from its inconspicuous homologue in Musca.
It is a thick and heavily pigmented rod, running the whole length of the
labium, and composed for the anterior two-thirds of its length of much thicker
chitin than any other part. The lateral margins of the gutter are separated
from the middle portion by a narrow area of thin chitin ; they do not reach
quite to the upper end of the labium, but merge with the base of the hypo-
pharynx and the adjacent membrane, which connects them with the mentum.
The median groove, so far as can be judged from a specimen mounted on its
side, is of considerable depth; like the lateral portions, it is thinnest at the
upper end.
On the posterior surface of the gutter there is a median backwardly
projecting ridge, commencing about the middle of the gutter and increasing
in width as it passes upwards, but at the same time becoming thinner, until
it is lost in the neighbourhood of the base of the hypopharynx. This is the
"keel ", corresponding to the structure in Stomoxys described by Stephens and
Newstead. It projects backwards into the cavity of the labium, dividing it
into two lateral halves, and is developed to provide additional attachment for
an important group of muscles. Its significance will be shown in connection
with the next species, in which it has been more closely studied, and it will be
sufficient to note here its occurrence and extent.
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The distal end of the gutter, where it articulates with the discal sclerite,
is apparently moulded to form a joint. On the posterior surface there is a
deep notch, caused by the extension of the lateral rods beyond the median
portion of the gutter ; the extremities of the sides of this notch are indented,
and project forwards in their most anterior parts. There is no continuous
chitinous union between the end of the gutter and the discal sclerite, and it
looks as if the two were articulated by this Simple joint to permit of free
movement ; under a high power the slightly projecting internal angles of the
posterior portions of the labellar rods can be seen to be distinctly moulded
so as to fit into the notch on the end of the labial gutter. The wide separation
between them as shown in the drawing is of course an artifact due to the
splitting of the sclerite by pressure.
The ment
um
shows some modification in this species as compared with the
preceding. It is deeper, narrower, more capacious at its proximal end, and
more pointed distally. The arms of the fork of the men
tum
are not so long,
and the distinction between the two joints of the fork cannot be made out in
my specimen. The furca is somewhat rounder and stouter, and is distinctly
thickened in its middle portion, where it lies between the arms of the fork.
Philæmato
myia insignis.
The structure of the proboscis of this fly has already been fully described
in a previous memoir, and it will only be necessary to recapitulate such points
as have an importance from the present point of view. It is the most highly
specialised of the genus, and we may note in passing that it is, so far as is
known at present, also much the most common and widely distributed one. In
Madras it is quite the most common biting fly found on cattle, and an idea of
its prevalence may be gathered from the fact that one can frequently count
twenty or thirty specimens on a single cow. It would be almost justifiable
to infer that whereas the two previously considered members of the genus have
not advanced sufficiently far to adapt themselves with success to an exclu-
sively blood-sucking habit, and have at the same time become less fitted to
obtain food in the manner usual in Muscids, this species has succeeded in es-
tablishing itself, and has become predominant.
In external appearance this fly differs no more from Musca than do the
other species. Its proboscis is as completely retractile and is blunted at the
tip, and the only suggestive thing to be noticed about it is its attitude when
feeding. Instead of moving about the skin of its host, it settles down in a
crouching attitude and remains on the same spot till obviously gorged with
blood, all its legs bent and its head depressed, in an attitude very like that as-
sumed by Stomoxys. This species appears to take no food other than blood, and
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it is somewhat remarkable that the pseudotrachael membrane should be retained
in an apparently functional condition, c
o-existing with a biting apparatus which,
as the anatomy of the parts and the observed habits of the insect both go to show,
is an extremely efficient one. It is possible that the membrane is used in the
same way as that of Musca, when the fly sucks up the blood from the wound.
The changes from the Muscid type are all found in the labium and the
labella. In the rostrum all that can be distinguished is the relatively rather
shorter length, and some elongation of the buccal cavity, the sclerite compos
-iug the walls of which is more pointed than in Musca, and curves over a little
more laterally. The teeth are reduced in number and increased in size and
strength, and are fitted more closely to the end of the discal sclerite. The dis-
cal sclerite itself shows complete differentiation into its two parts, and a definite
change of form, the object of which is to render the joint between it and the
end of the labial gutter more compact, and to fix the bases of the teeth more
firmly. The method of attachment of the teeth should be particularly noted.
Their bases are not attached to the sclerite, but lie in a cup formed by the pro-
jecting flange termed the tooth plate, the bases of the two smaller teeth at either
end of the set on each side being a little in advance of those of the larger
teeth, so that although the teeth are unequal in size their apices are in approxi-
mately the same line, while their bases form a concave line corresponding to
the shape of the cup in which they fit. Owing to the alteration in the shape of
the sclerite the teeth are attached to its distal end, and are directed downwards
when the labellar rods are in the same line as the labium, whereas in Musca the
teeth are directed downwards when the discal sclerite is perpendicular to the
labium. As a consequence of this the axial apophysis now points in the same
direction as the teeth when they are not everted; when they are spread out it
forms the most distal part of the proboscis, and it is possible that it is used as
an accessory tooth, or perhaps as a fixing point when the teeth cut by eversion.
The interdental armature, inconspicuous in the other two species, reaches a
high stage of development in the serrated blades, while the pseudotracheal
membrane is distinctly smaller than one would expect from the size of the fly ;
the rod-like hairs make their first appearance in this species. The labium shows
a high development of the labial gutter, and changes, such as the narrowing
at the lower end of the mentum, the consolidation of the joint between it and
the furca, and the thickening of the latter, which indicate an approach to the
Stomoxys type of piercing proboscis.
Summary of the Genus Philæmatomyia and its relations to Musca.
We have in the genus Philæmatomyia a series of three flies which
mark different stages in the adaptation of the Muscid type of proboscis to a F