NationalLibraryOfScotland/medical-history-of-british-india huggingface dataset:
chambers appear to be always separated by a narrower channel, which may
be surrounded by a sphincter muscle, so that they work alternately, the
posterior chamber drawing the blood from the anterior one, and passing it
onwards into the alimentary canal. It is with regard to the respective names
of these two chambers that the confusion exists, and this has arisen chiefly
through the differences in degree of development of the two in different
families, and to attempts to follow too closely the terms employed in general
anatomy. Meinert, in his description of Tabanus, termed the first of
these, which in that family is rather more prominent than the second, the
pharynx, and then divided the succeeding part into the first and second parts
of the œsophagus, the first part being a large and dilatable chamber, the
second a simple tube passing through the brain to the neck. In the mos-
quito it is the second chamber which is the most conspicuous, and this is
universally known as the pharynx, at least at the present time, subsequent
to the description of Christophers. Nuttall and Shipley term the first portion
the buccal cavity and the second the pharynx in the mosquito, and I think
it would be of advantage to retain these terms throughout the group. The
word œsophagus is chiefly used in zoological nomenclature for a simple
channel, and there is no special advantage in retaining it to signify a part
of the sucking apparatus. This nomenclature has the advantage that it
is equally applicable to the Cyclorraphic Diptera, in which there are also
two parts to the apparatus, only one of which, the upper one, is a func-
tional pump. It must be remembered, however, that that pharynx in Tabanus
and in the mosquito, the two most divergent members of the group, possess
this difference, that they are on opposite sides of the brain, the pharynx
of the former—the first part of the œsophagus according to Meinert—being
anterior, and that of the mosquito posterior. In what follows, then, the
first chamber will be termed the buccal cavity, corresponding to the pharynx
of Meinert and Dimmock, and the second the pharynx, corresponding to
the first part of the œsophagus of the same authors, and homologous with
the fulcrum in the house fly as described by Kraepelin, and the pharynx
of Lowne. The simple tube which connects the pharynx with the alimen-
tary canal in the thorax may be termed the œsophagus. The duct which
unites the buccal cavity and the pharynx hardly needs a special name.
It is to be understood that this nomenclature has no precise embryolo-
gical significance, and that the pharynx in one fly is not necessarily strictly
homologous with that of another. We may imagine that in a very primi-
tive insect of this type the whole of the outer wall of the stomodæum would
be united with the wall of the head capsule by strands of muscle fibre, so
that it formed one continuous dilatable chamber, in which a sucking action
27
could be produced by the peristaltic contraction of the muscles, the ante-
rior one contracting first, and then relaxing, so that a wave of dilatation
TEXT FIGURE 2.
Diagram of the sucking apparatus of the orthoriaphic biting flies. lbr., labrum. ep., epipharynx. cl.,
clypeus.
hy., hy'., the anterior and posterior laminæ of the hypopharynx. sl.d., the salivary duct. li., labium. lbl.
the labella. sl. r., the salivary reservoir.
b. c., the buccal cavity. ph., the pharynx. dl. m., the dilator muscles, passing from the wall of the head
capsule to the sucking chambers. oe., the œsophagus, entering he neck.
passed along the chamber, carrying the food with it onwards to the diges-
tive portion of the canal. In course of time, as the various genera were
differentiated from one another, certain portions of the muscle disappeared,
and the chamber became divided up, until in the present forms two main
dilatations were left, a small portion of the stomodæum in the head remaining E
28
as a simple duct. The condition found in the different families depends
simply on the particular portions of the primitive chamber which have
retained the original character, and it is quite possible that these are not
always the same. One is not perhaps justified in speculating further on
this subject in view of the little that is known of the larval characters of
these flies, but it is interesting to note in passing that there are points which
suggest that the pharynx of the mosquito is developed from a part of the
stomodæum much posterior to that from which the pharynx of Tabanus
arises, although the chamber here referred to was regarded as the first part
of the œsophagus by Meinert. In the first place, the second chamber of
the mosquito is posterior to the brain, while the corresponding chamber in
the Tabanidœ is anterior. In Tabanus we find also that there is a well-
defined pair of muscles, the retractors of the œsophagus, which run from
the posterior wall of the head capsule in the neighbourhood of the occipital
foramen to the œsophagus, and though their function is probably correctly
indicated by their name, they represent a portion of the once continuous
band which united the whole of the wall of the canal to the head capsule.
They correspond in position, with regard to the brain, to the large and power-
ful dilator muscles of the pharynx of the mosquito, and it certainly looks
as if the part of the canal to which they are attached is homologous with
the pharynx of the mosquito.
The Mechanism of the Sucking Pumps.
In the numerous descriptions that have been published of the pharynx
and its muscles, no account seems to have been taken of the necessity that
exists for some method by which the distended chamber can be closed again,
and the plates once more brought in contact with one another in readiness
for the next contraction. It is generally assumed that the recoil of the plates
takes place by virtue of their elasticity, an assumption that does not seem
to be justified by the facts of the case as far as they are known. Chitin is
certainly to some extent flexible when it is in thin sheets, and the chitinous
mouth parts when bent will as a rule return to their original shape, but it
by no means follows that the elasticity of the chitinous plates of the sucking
chambers is sufficient to bring about their return, for it must be remembered
that such a degree of elasticity would have to be overcome by the dilator
muscles when the chamber were expanded. It is of course as difficult to prove that
the plates are not elastic as it is easy to assume that they are, and I will not go
further than to say that the assumption is an unnecessary one. Let us consider
what must happen when the chamber is dilated. In the first place, if the head
29
were entirely separated from the body, the total contents would be increased,
and either some of the cranial contents would have to be compressed or the
capacity of the head increased by distension ; the difference in size between the
cavity of the pharynx of the mosquito when it is empty and when it is full
is considerable, when compared with the total size of the head. The only
compressible substance in the insect head is the air in the air sacs, and the
only substances which can be displaced from the head by simple pressure are
the air in the air sacs and the blood in the hæmatocœle of the head. If these
are displaced by the expansion of the sucking chamber they will pass through
the neck and increase the total contents of the body. There are therefore two
possible ways by which the chambers can be emptied after dilatation, viz., by
blood pressure or air pressure. Of the two the latter is the more likely, both
on account of the structural peculiarities of the head and by reason of the ana-
logy of the method by which the retractile proboscis of Musca is known to be
thrust out. The head of the fly, and particularly of these blood-sucking flies,
contains a remarkably large amount of air, contained in air sacs with relatively
thick walls—very thick in the case of the Tabanidœ. The air in these sacs is in
communication with the rest of the tracheal system by means of tracheæ
which pass through the neck, and so with the thoracic spiracles. That the
insect is capable of distending the air sacs of the head by means of respiratory
movements we know from the case of Musca, so thoroughly studied by
Kraepelin, and I think that the dilatation of these sacs offers a far more
rational explanation of the means by which the plates of the pharynx and the
buccal cavity are brought together, than the assumption that the chitin has of
itself sufficient elacticity to accomplish it.
The Intracranial Tunnels.
In all the flies of this group there exists a pair of hollow tunnels which
pass through the lower part of the head from front to back, and open on to the
anterior and posterior surfaces. They have been described and figured in
Anopheles by Nuttall and Shipley, and by myself in Hœmatopota. Similar
tunnels occur also in Chironomus (Miall and Hammond) and in Asilus.
They act as supporting buttresses to counteract the tendency of the dilator
muscles of the sucking apparatus to pull the walls of the cranial cavity toge-
ther, and, as I have pointed out in connection with those of Hœmatopota, the
fact that they are hollow is in accordance with the mechanical principle that
a hollow cylinder is stronger, weight for weight, than a solid rod. They also act
as an additional surface for muscle insertion, and several muscle bundles can be
traced to their posterior ends. The anterior end of the tunnel opens in all
30
cases just below and at the outer side of the antenna, and the posterior a little
in front of the occipital foramen. The interesting point about these structures
is their constant occurrence and their connection with the sucking apparatus.
The Oesophageal Diverticula.
It does not appear to be fully recognised that the presence of an œsopha-
geal diverticulum is a constant feature in Diptera, and that the form in which
it occurs in the mosquito is merely a modification of a structure well known
not only in this order but throughout the whole class Insecta. A good deal of
the confusion is possibly clue to the loose way in which many names, more or
less applicable, are used for the different forms in which the structure occurs.
Many of them are open to objection ; probably it would conduce to clearer ideas
on the subject if the term "crop" were adopted for all, though it is of course
rather too late to attempt to change the term in such common use in connec-
tion with mosquitoes. The words "œsophageal diverticulum" are, however,
somewhat misleading. The real relations of the two parts are best seen in a
longitudinal section of the anterior part of the thorax of a Tabanid, or in a
series of transverse sections. The duct of the diverculum passes in a straight
line backwards through the neck, lying between the salivary glands ; in front
it is directly continuous with the œsophagus, and there is nothing to indicate
where the one ends and the other begins, as they are precisely similar in the
structure, and form one continuous duct extending from the intracerebral
part of the œsophagus in the head to the small sac-like diverticulum in the
second or third segment of the abdomen. At a point a little distance be-
hind the inlet of the thorax the proventriculus is seen lying below the duct
and between the salivary glands, and it has an aperture on its upper surface
by means of which it opens into the lower surface of the duct. The junction
between the two is therefore at a right angle to the line of the gut, and it is,
in the Tabanidœ, surrounded by a few circular muscle fibres which may func-
tion as a sphincter muscle. It is clear that in view of this it is literally
incorrect to speak of the sac as a diverticulum of the œsophagus. The term
" sucking stomach" is open to the even more serious objection that it is not a
stomach, in the sense of an organ in which digestion takes place, and that it is
not a sucking organ. On the whole the term crop is the most applicable of
any of he words in use, as it indicates the function fairly closely, and has the
advantage of emphasising the homology between the apparently different
structures in the mosquito and the house fly.
The crop is primarily the chamber into which the blood first flows from
the pharynx, and is intended, no doubt, as a reservoir to contain the food until
the digestive portion of the midgut is ready for it. The different degrees of
31
development which it attains are dependent on the amount of food the midgut
can deal with at one time. In a full-fed mosquito killed immediately after
feeding one finds the three divisions of the crop all filled with blood, and very
little in the midgut, while in a horse fly the crop is empty and the midgut
distended with blood. In the former case we may regard the crop as a true
reservoir, as was pointed out by Nuttall and Shipley, while in the latter the
function appears to be merely to pass on the blood slowly from the pharynx to
the midgut. The wall of the crop is composed of a very fine basement mem-
brane and an interlacing network of delicate muscle fibres, and the degree of
the development of the muscle gives one support to the observations made on
the fly in dissections. I n the Tabanidæ the muscle fibres are very well deve
-loped, forming a network which can easily be seen in unstained preparations,
and the sac is thus able to pass on at once the whole of the blood received by it
to the gut, contractions of the wall of the sac following relaxation and empty-
ing of the pharynx. I have dissected for various purposes several hundreds of
Tabanids of various species, and at varying times after feeding, and have
never found fresh blood in the sac in any case. In flies killed immediately
after feeding one often sees the wall of the sac displaying rather rapid
peristaltic movements, such as would result in driving the blood out of
the sac and up the tube into the proventriculus. In flies killed some time
after feeding the sac is either empty or contains a few granules of blood pig-
ment. In the Tabanidœ the crop is therefore to be regarded as functionally a
part of the sucking apparatus, its task being to receive the blood from the suck-
ing pharynx and to expel it again into the midgut, while in the mosquitoes it
is a true reservoir, retaining the blood at the time of feeding and passing it on to
the gut as it is required later, the contractions not necessarily taking place
immediately after the blood is received and not being correlated with the
contractions of the pharynx.
32
References.
ANNETT, H. E. DUTTON, J. E., AND ELLIOT, J. H.—Report of the Liverpool Expedition to
Nigeria, Part II. Thompson Yates
Laboratories Reports, Vol. IV, Part I,
1901.
CRAGG, F W.—The Structure of Hæmatopota pluvialis, Meigen. Scientific Memoirs of the
Officers of the Medical and Sanitary Departments of the Government of
India, New Series, No. 55. Calcutta, 1912.
CHRISTOPHERS, S. R.—The Anatomy and Histology of the Adult Female Mosquito. Red
Mal. Comm. Royal Soc., IV. London, 1901.
DIMMOCK, G.—The Anatomy of the Mouth Parts and of the Sucking apparatus of some
Diptera. Boston, 1881.
GRASSI, B.—Ricerche Sui Flebotomi. Memorie della Societa Italiana delle Scienze, ser 3
tom. XIV. Rome, 1907.
HANSEN, H. J.—Fabrica Oris Dipterorum. Naturbist Tidsskrift, 1883.
KRAEPELIN, K.—Zur Anatomie and Physiologie des Russels von Musca. Zeit. f. Wissens-
chaftliche Zoologie, Vol. 39, 1883.
LOWNE, B. T.—The Anatomy, Physiology and Development of the Blow-Fly. London, 1890-
1895, 3 Vols.
MEINERT, F.—Fleurnes Munddele. Trophi Dipterorum. Kjobonharn, 188].
MIALL, L. C., AND DENNY, A.—The Structure and Life History of the Cockroach. London,
1886.
MIALL, L. C., AND HAMMOND, A. R.—The Harlequin Fly. Oxford, 1900.
NEWSTEAD, R.—The Papertaci Flies (Phlebotomus) of the Maltese Islands. Bull. Entom.
Research, Vol. II, Part I. 1911.
NUTTALL, G. H. F., AND SHIPLEY, A. E.—The Structure and Biology of Anopheles. Journal
of Hygiene, 1902.
PACKARD, A. S.—A Textbook of Entomology. London, 1898.
SCHAUDINN, F.—Generations und Wirtswechel bei Trypanosoma und Spirochate. Arb. Kais
Gesund. B. XX, 1904.
STEPHENS, J. W. W., AND NEWSTEAD, R.—The Anatomy of the Proboscis of Biting Flies. I.
Glossina, Memoir XVIII, Liverpool School of
Tropical Medicine, 1906. II. Stomoxys, Ann. of
Trop. Med. and Paras, Vol. I. No. 2, 1907.
33
Description of Plate.
The figures were drawn with the aid of a camera lucida.
Figures 1 to 15 are on the same scale as that to the right of figure 1 and figures 17 to
19 on the same scale as that at the right lower corner of the plate. The divisions in the scales
in each case represent hundredths of a millemetre.
Figure 1. The edge of the maxilla of Simulium.
Figure 2. The edge of the mandible of Simulium.
Figure 3. Ditto Ceratapogon.
Figure 4. Ditto Tabanus.
Figure 5. Ditto Anopheles.
Figure 6. The distal end of the labrum-epipharynx of Ceratapogon.
Figure 7. Ditto hypopharynx do.
Figure 8. Ditto Maxilla do.
Figure 9. Ditto hypopharynx of Joblotia.
Figure 10. The edge of the maxilla of Tabanus, showing some of the teeth.
Figure 11. Three red cells and a large mononuclear cell of human blood drawn for comparison
of size.
Figure 12. The distal end of the maxilla of Phlebotomus.
Figure 13. The distal end of the maxilla of Joblotia. The curvature is probably an artifact,
vide page 11.
Figure 14. The distal end of the hypopharynx of Simulium indicum.
Figure 15. Ditto
Labrum-epipharynx
do.
Figure 16. To indicate the extent and direction of the movement of the maxilla in Tabanus
Diagramatic.
Figure 17. The distal end of the labrum-epipharynx of Joblotia.
Figure 18. Ditto hypopharynx of Hæmatopota.
Figure 19. Ditto labrum-epipharynx do.
Photo.-Engraved & printed at the Offices of the Survey of India, Calcutta, 1912
F. W. C DELET,