NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
24
of the wall resembles that of the preceding portions of the hind-gut, except that
the circular muscle fibres are more numerous. There are six rectal papillæ inset
in the wall. They are small curved, wedge-shaped bodies, projecting into the lumen
of the rectum, their apices being directed towards the anus. They are composed
of a single layer of large cells arranged around a central lumen, down which there
runs a small tracheal twig which enters the papilla from the outside.
The Malpighian Tubes are four in number. Each tube, when dissected out,
is about the same length as the whole alimentary canal. They are arranged in
complicated loops, chiefly in the posterior part of the abdomen, and are closely
bound down to the other organs by tracheal twigs. A long loop passes forwards
in each side of the mid-gut. The tube is of uniform diameter throughout, and
tapers to a blunt point which lies near the rectum. In fresh dissections this end
of the tube has an orange-red colour. The tube is composed of a single layer of
large flattened cells, with large oval nuclei, which are folded round the lumen and
cemented by their edges. They are inserted into small expanded evaginations of
the gut, where the large flattened cells become replaced by small round cells.
The Salivary Apparatus consists of a pair of glands, a duct, and a reservoir.
The glands lie in the thorax, extending to its junction with the abdomen. They
lie beneath the proventriculus, and in either side of the duct of the œsophageal
diverticulum. They are long, simple, and tubular, and dilate gradually from
before backwards, the posterior portions being curved outwards. They are com-
posed of a single layer of cubical cells, and a stout basement membrane. The
lumen generally contains a mass of small granules. Towards their anterior ends
the glands narrow, and the basement membrane is reinforced by a series of coarse,
closely-set chitinous rings, while the cells become flattened. The two ducts which
are thus formed unite just before entering the neck, and the common duct, lying
below the nerve cord, runs to the salivary reservoir. This salivary reservoir is a
small chitinous pouch situated on the lower half of the posterior surface of the
pharynx ; it opens below into the duct in the hypopharynx.
II.—The Respiratory System. (Plate V. Fig. 20.)
The Head.—Two small tracheæ from the anterior thoracic spiracle enter the
head, and at once break up into branches, too minute to be traced. There are
minute tracheæ in the labium and antennæ, which expand at the base of the organs
into small air sacs. The large space above and in front of the brain and pharyn-
geal muscles is entirely filled up with small air sacs, of which as many as twelve
can be counted in some sections of the head. They are closely adherent to the
epicranium and to the brain, and are most formidable obstacles in the dissection of
the head. They have very tough and thick walls, and obviously can have little
25
or no function in the respiration of the insect. They serve as elastic cushions to
protect the cerebral ganglia from the changes of intracranial tension during feed-
ing, and have also a function, as will be seen later, in relation to the mechanism
of feeding.
The Thorax.—The anterior thoracic spiracle is situated at the anterior end of
the membraneous interval between the dorsal and lateral walls of the thorax. The
posterior spiracle lies just in front of the halter. It should be noted that the
spiracles are not orifices in the chitinous wall of the thorax, but are situated on the
membrane joining adjacent sclerites. Each spiracle is a dumbbell-shaped slit,
with a chitinous margin, on each side of which there is a row of short comb-like
spines, to guard the orifice. The tracheæ which arise from them break up at
once into numerous small branches distributed to the muscles and viscera. There
are several small air sacs, resembling those of the head, around the thoracic ganglion.
The Abdomen.—There are seven spiracles, resembling those of the thorax,
situated in the pleural membrane. The first one lies immediately behind the
metathorax, the other being situated rather in front of their respective segments,
and diminish in size from before backwards. The arrangement of the tracheæ
within the abdomen is somewhat complex. The trachea from the first spiracle
, after a short backward course, ends in a small air sac, which leads by a short con-
necting trachea to a second and larger air sac. These air sacs are entirely different
to those of the head. Their walls are composed of a thin white opaque membrane,
which when ruptured spreads out in water like a very thin paraffin section. It
is very difficult to dissolve the air out of these vesicles, and sections of this part of
the abdomen are difficult to prepare. Four small tracheæ arise from the second
air sac. The first runs inwards, and breaks up at once into fine filaments for the
supply of the adjacent fat body; the second trachea communicates with the
second spiracle; the third runs along the dorsal wall, ventral to the heart, com-
municating with its fellow of the opposite side. The fourth and largest of these
tracheæ runs directly backwards to the end of the abdomen as the main lateral
trachea. This gives off six dorsal branches, which communicate with those of
the other side, ventral to the heart, and many small branches to the viscera and
to the ventral wall of the abdomen, and receives communicating branches from the
third to the seventh tracheæ. Each trachea as it passes inwards from its spiracle
gives off a number of small twigs to the surrounding parts; the fourth trachea
supplies most of the mid-gut; and the fifth, which is of considerable size, consti-
tutes the main supply of the ovary, which it enters along its posterior border.
III.—The Muscles of the Head.
The muscles associated with the mouth parts have been elaborately described
and figured by Meinert (11) and Hansen (12), in Tabanus, and some of Meinert's
26
figures are to be found in most text books on Entomology. The arrangement of
the parts in Hæmatopota does not differ materially from that described in
Tabanus, but since the works of the above authors are in Danish, and are not
readily accessible, an account of the parts will be given here. It will be necessary
to go into considerable detail in order to facilitate the subsequent description of
the mechanism of feeding. The muscles were studied by dissection and serial
sections taken at various planes.
Labial Muscles.—The intrinsic muscles acting on the labellæ have already
been described. There is also a pair of small retractor muscles, which arise from
the tubercles at the base of the intracranial tunnels, and are inserted into the
cornua of the labium.
Mandibular Muscles.—There are two adductor muscles attached to the inner
cornua of the mandible. The internal of these, the adductor rectus, arises from the
side of the trabecular membrane and from the intracranial tunnel The oblique
adductor arises from the floor of the head cavity, near the posterior border. The
abductor muscle arises near the oblique adductor, and is inserted in the external
cornu. It has a long tendon.
The Maxillary Muscles are also three in number. The adductor rectus arises
from the inner side of the trabecular membrane, in common with the correspond-
ing mandibular muscle, and the oblique adductor arises above and behind the
oblique mandibular muscle. The two adductors run almost directly in the line of
the maxilla, and are inserted into the lower end of the stipes. The abductor muscle
of the maxilla arises from the gena, external to the anterior orifice of the intra-
cranial tunnel, and runs upwards and backwards to the upper end of the stipes.
The Depressor Labri lies between the two lateral halves of the dilator muscle
of the pharynx. It arises from the internal surface of the clypeus, and after a
short course downwards and backwards is inserted into the tongue-shaped upper
end of the labrum.
The Muscles of the Pharynx. —The dilator of the pharynx is the largest muscle
in the head. It passes from the sides of the clypeus to the anterior plate of the
pharynx, and encloses the depressor labri. It should be noted that the line of
action of this muscle is not directly forwards, but forwards and outwards.
The R e t r a c t o r Mu s c l e s o f t h e p h a r y n x a r i s e f r o m t h e e p i c r a n i u m b e t w e e n t h e
eyes, just above the large pigment spots. They are of small size, and have long
tendons, which are inserted into the cornua of the pharynx.
Hansen describes in Tabanus a "laxator" muscle of the pharynx, passing
from the clypeus to the upper end of the pharynx. I have not been able, in Hæma
topota, to distinguish this muscle from the dilator.
27
The Protractor of the Pharynx arises from the inner sides of the trabecular
membrane, and passes upwards and forwards to the cornua of the pharynx. It is
short and stout, and has no tendon.
Muscles of the Œsophagus.—The Dilator muscle corresponds to that of the
pharynx. It is broad and fleshy, and lies in a vertical plane, being attached to the
epicranium in front of the vertex, and to the whole of the upper surface of the
chitinous plate of the first part of the œsophagus.
The Lateral Dilators of the œsophagus arise from the epicranium in the region
of the antennal sockets and from the adjacent parts of the trabecular membrane.
They run inwards and backwards and a little upwards, and are inserted without
tendons into the down-turned edges of the œsophageal plate. These muscles are
called "Compressors" by Hansen.
The Sphincter Muscles are composed of circular and oblique fibres, which
support the membraneous wall of the first part of the œsophagus. There are two
bands, separated by the cornua of the pharynx, which run transversely between
the lateral edges of the chitinous part of the wall, that is, on the inferior surface of
the œsophageal pouch. The posterior bundle contains many oblique fibres, and
is larger than the anterior one. There is also a circular sphincter, attached to the
cornua of the pharynx, which encircles the short duct between the pharyngeal
and œsophageal pouches.
The Retractors of the œsophagus arise from the sides of the occipital foramen,
and are attached to the second part of the œsophagus as it leaves the brain. There
are two small bundles on each side, arising above and below the foramen.
The salivary reservoir is surrounded by a fan-shaped muscle, which is at-
tached to the posterior plate of the pharynx. Hansen also figures a mesial muscle
lying between the adjacent borders of the lateral halves of this muscle.
IV.—The Mechanism of Feeding.
The sucking of blood is accomplished, after the insertion of the proboscis, by
the alternate dilatation and contraction of the pharyngeal and œsophageal pouches,
and the whole structure of the head is adapted to this function. The anatomy
of the parts, considered with due regard to general mechanical principles, and to
the observed habits of the living fly, provides an explanation of the exact mecha-
nism. The process may be considered in three stages, the insertion of the piercing
parts, the sucking of the blood, and the passage of the blood onwards into the
gut.
The insertion of the piercing parts is accomplished, not by a movement of the
parts themselves, but by a forward thrust of the whole body of the fly. Since the
E 2
28
proboscis has only a slight inclination forwards from the perpendicular, it is neces-
sary, in order to obtain the greatest mechanical advantage, that the proboscis
should be brought more into line with the long axis of the body. This is accom-
plished by bending of the neck, and by the elevation of the fly on its hind legs;
this manœuvre brings the convex end of the prothorax in contact with the con-
cavity in the posterior surface of the head. While in this position the fly, taking
a firm hold in the skin of the host by means of the large claws on the metatarsi,
gives a series of short jerks forwards. The wound is made by the mandibles and
maxillæ, both of which have firm attachment to the chitinous wall of the head.
The epipharynx and hypopharynx take little or no part in this process, since the
pharynx, with which they are continuous above, has only membraneous and mus-
cular connections with the rest of the head, and there is no rigid line through which
the pressure can act. Moreover, the epipharynx has a blunted tip, and the slender
hypopharynx falls a little short of the other mouth parts.
The mandibles and maxillæ can perform lateral and to and fro movements,
by which the wound is enlarged and kept open. The nature of the movement
of their muscles is co-related in a remarkable manner with the nature of the
armature, in each case. By the alternate contraction of its adductor and abductor
muscles, the mandible rotates on its fixed point, the ginglymus, through a short
arc, and at each contraction of the adductor muscles the saw-like edge is brought
into play.
By similar alternate contractions of its muscles the maxilla is alternately bent
and straightened, the movement taking place at the joint between the cardo and
stipes, and in this way the rasps on the end of the blade are forced up and down
in the wound. During the making of the wound, the pharynx, carrying with it
the labrum-epipharynx and the hypopharynx, is retracted by contraction of the
retractor muscles attached to its posterior cornua; this is facilitated by the retrac-
tor muscles of the labrum, which pulls upwards and forwards on that part of the
upper end of the labrum which is not fused with the epipharynx. When the
wound is made, the pharynx is thrust down by the contraction of the protractor
muscles.
The second part of the feeding process is the dilation of the pharynx. This
is accomplished by the contraction of the powerful dilator muscles, which pull the
anterior plate forwards, and reduce its concavity.
At first sight it is a little difficult to understand why the pharynx is not drawn
forwards as a whole, since there is no muscle or firm chitinous attachment to directly
oppose the dilator muscles. This is prevented by the combined action of the
protractors and retractors of the pharynx. When these two muscles act together,
the vertical element in the force of each will be neutralised by the other, so that
29
the resultant pull, the protractor muscle being the larger, will be backwards. The
lower end of the pharynx is meanwhile steadied by the clypeus, against which the
labrum-epipharynx rests.
During the dilatation of the pharynx the sphincter muscles of the œsophagus
are contracted when the pharynx is full, they relax, and the blood is drawn from
the pharynx to the sacular part of the œsophagus by the contraction of dilator
muscles of the œsophagus. The sphincter muscles then again contract, and so
drive the blood backwards into the gut, the second part of the œsophagus being
held taut by its retractor muscles. The pharynx now dilates again, and the
process is repeated until the insect has had a full meal.
The intracranial tunnels have an important relation to the mechanism of
feeding. They are nearly parallel to the dilators of the pharynx, which lie, in fact
in the chamber formed by the trabecular plates arising from their upper surfaces.
The chitin in the region of the clypeus is very thin, and were it not that the
intracranial tunnels support it and distribute the strain on to the thicker parts
at the base of the skull, would be unable to resist the powerful dilators.
It will be noticed that the pharynx has no sphincter muscle to bring the two
plates into apposition when the blood flows to the cesophagus, and that the muscles
which close the œsophagus are very much smaller than the dilators. The collapse
of these chambers is usually attributed, in the mosquito, to the elastic recoil of the
plates, elasticity being assumed as a property of this variety of chitin. Whether
this be the case or not, it is highly probable that the air sacs in the head play an
important part in the mechanism. When the pharynx is filled with blood, the
sphincter between it and the œsophagus being contracted, the intracranial pres-
sure must necessarily be increased. This will result in a compression of the air
in the sacs, and, when the sphincters are again relaxed, the positive pressure will
be used up in bringing the plates once more into contact. Increased pressure is to
a certain extent allowed for by the palatal membrane.
The mechanism of the salivary apparatus is simple. The saliva is forced from
the glands into the reservoir by the contraction of the thoracic muscles in which
they lie, and is ejected from the reservoir by the contraction of its fan-shaped
muscle.
V.—The Reproductive System.
This consists of paired ovaries and accessory glands, an oviduct, a genitial
bursa, and three spermathecæ.
The Ovaries lie in the last three or four segments unlike the mosquitos',
they do not, even when the ova are ripe, produce any considerable distension of
30
the abdomen. They are white pear-shaped bodies, in which the separate ova can
be readily distinguished. The apex of each is produced forwards as a delicate
filament, which can be traced to the fat body on the dorsal wall of the abdomen
in the second segment. The ovary is invested by a few scattered muscle fibres,
continuous with the oviduct and with the apical filament. The rich tracheal
supply enters along the rounded posterior border. Each ovary consists of 54
short ovarian tubes, each of which contains a single large oval ovum. Above the
ovum there is a small conical cap of undifferentiated cells, the apex of which runs
forwards to join the apical filament. Below the ova, the tubes enter a common
duct, which runs in the outer and posterior borders of the organ, and which is
continuous with the oviduct. This duct is well seen in the transverse sections of
the ovary, in which it appears as a narrow slit, extending about one-fourth the
circumference of the organ. The oviducts are short, and unite to form a common
duct, opening into the genital bursa. They are composed of muscle fibres arranged
in circular and longitudinal layers. The fibres are attached behind to the chitin-
ous frame work of the genital bursa; above they are continuous with the thin
wall of the common duct of the ovarian tubes.
The Accessory glands lie between the ovaries, extending as far forwards as the
midgut. They are long and tubular, and open into the genital bursa by short
constricted necks. In fresh dissections they are a dirty yellow colour, owing to the
granular secretion which they contain. The walls of the gland are thrown into
four or five longitudinal folds. They are composed of short cylindrical cells, on a
thin basement membrane.
The Spermathecœ are three long chitinous tubes, bent sharply on themselves
about the middle, so that the proximal and distal halves lie parallel to one another.
They open by a very short common duct into the gential bursa. The blind end of
the tube is dilated and oval, and is bound down to the genital bursa by a few
fine filaments. The chitinous wall of the tube is thickest in the distal half; from
the bend it thins gradually and can only be distinguished with some difficulty
towards the base. It is striated transversely, and appears to consist of a large
number of circular fibres of chitin. It is covered externally by a layer of small
round cells, external to which, in the proxinal part of the tube, where the chitin is
thin, there are a few bundles of muscle fibres. At the base of each spermatheca,
just above the point at which the three unite to form a common duct, there is a
remarkable funnel like structure developed in the wall of the tube. This consists
of a central hollow cylinder of thick chitin, slightly curved, and tapering to a
point below. This is enclosed in a series of thin transparent rings of chitin, some-
what loosely arranged, and apparently spiral. They project a considerable distance
from the surface of the cylinder, and support the layer of round cells. In front of
the tube, and slightly separated from it, there is a short but wide flange of chitin,
31
shaped like a short funnel. The whole structure closely resembles a short, hollow
and slightly bent screw. (Plate VII, Fig. 34.)
The Genital Bursa is the continuation of the common oviduct, and into it
open the accessory glands and the spermathecæ. Its wall is supported by an in-
complete framework of delicate chitin, consisting of one transverse and two lateral
plates. The lateral plates are oblong, and concave upwards, and lie mainly in the
horizontal plane, but are inclined towards one another. The transverse plate lies
in front of them, and is connected with the lateral plate on each side by a thick
rib of chitin, which is continuous with its thickened anterior border, and with a
ridge near the inner border of the lateral plate. The anterior plate is also concave,
and the three together form a support for the lower wall of the bursa.
The posterior end of the bursa is partly closed in by two thick curved ridges
of chitin, which arise from the lateral plates, and converge backwards and inwards
towards one another. Each of these ridges has a row of peculiar spines (Plate
XII, Fig. 26), which project inwards and so further close in the aperture. This
arrangement constitutes a sort of valve, and suggests an explanation of the ex-
traordinary scarcity of the males of the species late in the season. On anatomical
grounds it is unlikely that the males will be able to disengage unharmed after copu-
lation, and it may be that as in the case of the bee (19), copulation is fatal to the
male fly.
VI.—The Nervous System.
This consists of a compound ganglion in the head, thoracic and abdominal
ganglia, and their connectives and nerves. It shows a moderate degree of con-
centration. The optic lobes, cerebral ganglia, and the sub-œsophageal ganglia,
are fused into one mass, which lies in the posterior part of the head, and envelopes
the second part of the œsophagus. It is shaped like a short stout cylinder, the
transverse diameter being about four times the width. The anterior surface is
indented as approximately equal fourths, indicating the lines of fusion of the optic
lobes and central bodies; the posterior surface is similarly indented in thirds, the
nerve cord which passes through the neck arising, by two branches from the
middle third. The antennal nerves arise from the outer sides of the central lobes,
and pass directly forwards to the antennæ, between the air sacs. Three small
nerves arise on each side from the inferior surface of the central body, and break
up above to be distributed to the adjacent muscles. One small filament can be
traced in serial sections to the labium. The optic nerves arise from the whole
outer surfaces of the optic lobe on each side, in a conical bundle, the fibres of which
spread out forwards and inwards over the wide area occupied by the eyes.
The envelopment of the œsophagus by the brain is well demonstrated in serial
sections parallel to the proboscis. The first part of the œsophagus lies at a slightly