LITERATURE ON THE ORGANS OF DIGESTION
Treviranus, G. R. Resultate einiger Untersuchungen über den inneren Bau der Insekten. (Verdauungsorgane von Cimex rufipes.) (Annal. d. Wetterau. Gesells., 1809, i, pp. 169–177, 1 Taf.)
Ramdohr, C. A. Abhandlungen über die Verdauungswerkzeuge der Insekten. 1811, vii, pp. 221, 30 Taf.
Dutrochet, R. J. H. Mémoire sur les métamorphoses du canal alimentaire dans les insectes. (Journal de Physique, 1818, lxxxvi, pp. 130–135, 189–204; Meckel’s Archiv, 1818, iv, pp. 285–293.)
Suckow, F. W. L. Verdauungsorgane der Insekten. (Heusinger’s Zeitschr. f. organ. Physik., 1828, iii, pp. 1–89.)
Doyère, L. Note sur le tube digestif des Cigales. (Ann. Sc. nat. Zool., 1839, 2 Sér., xi, pp. 81–85.)
Grube, A. E. Fehlt den Wespen- oder Hornissenlarven ein After oder nicht? 1 Taf. (Müller’s Archiv für Physiol., 1849, pp. 47–74.)
Sirodot. Recherches sur les sécrétions chez les insectes. (Ann. Sc. nat. Zool., 1858, 4 Sér., x, pp. 141–189, 251–328, 12 Pls.)
Milne-Edwards, H. Leçons sur la physiologie et l’anatomie comparée, v, 1859, pp. 498–536, 581–638.
Dufour, L. Recherches anatomiques sur les Carabiques et sur plusieurs autres insectes Coléoptères. Appareil digestif. (Ann. Sc. nat., ii, 1824, pp. 462–482, 2 Pls.; iii, 1824, pp. 215–242, 5 Pls., pp. 476–491, 3 Pls.; iv, 1824, pp. 103–125, 4 Pls.; iv, 1825, pp. 265–283.)
—— Recherches anatomiques sur l’Hippobosque des chevaux. (Ann. Sc. nat., 1825, vi, pp. 299–322, 1 Pl.)
—— Description et figure de l’appareil digestif de l’Anobium striatum. (Ibid., xiv, 1828, pp. 219–222, 1 Pl.)
—— Recherches anatomiques sur les Labidoures. Appareil de la digestion. (Ibid., xiii, 1828, pp. 348–354, 2 Pls.)
—— Recherches anatomiques et considerations entomologiques sur quelques insectes Coléoptères, compris dans les familles des Dermestins, des Byrrhiens, des Acanthopodes et des Leptodactyles. Appareil digestif. (Ibid., Sér. 2, Zool., i, 1834, pp. 67–76, 2 Pls.)
—— Résumé des recherches anatomiques et physiologiques sur les Hémiptères. (Ibid., pp. 232–239.)
—— Mémoire sur les métamorphoses et l’anatomie de la Pyrochroa coccinea. Appareil digestif. (Ibid., Sér. 2, Zool., xiii, 1840, pp. 328–330, 334–337, 2 Pls.)
—— Histoire comparative des metamorphoses et de l’anatomie des Cetonia aurata et Dorcus parallelepipedus. Appareil digestif. (Ibid., Sér. 2, Zool., 1824, xviii, pp. 174–176, 2 Pls.)
—— Anatomie générale des Diptères. Appareil digestif. (Ibid., Sér. 3, Zool., i, 1814, pp. 248, 249.)
—— Histoire des métamorphoses et de l’anatomie du Piophila petasionis. Appareil digestif. (Ibid., Sér. 3, Zool., i, 1844, pp. 372–377, 1 Pl.)
—— Études anatomiques et physiologiques sur les insectes Diptères de la famille des Pupipares. Appareil digestif. (Ibid., Sér. 3, Zool., iii, 1845, pp. 67–73, 1 Pl.)
—— Recherches sur l’anatomie et l’histoire naturelle de l’Osmylus maculatus. Appareil digestif. (Ibid., Sér. 3, Zool., ix, 1848, pp. 346–349, 1 Pl.)
—— Études anatomiques et physiologiques, et observations sur les larves des Libellules. Appareil digestif. (Ibid., Sér. 3, Zool., xvii, 1852, pp. 101–108, 1 Pl.)
—— Recherches anatomiques sur les Hyménoptères de la famille des Urocerates. Appareil digestif. (Ibid., Sér. 4, Zool., i, 1854, pp. 212–216, 1 Pl.)
—— Fragments d’anatomie entomologique. Sur l’appareil digestif du Nemoptera lusitanica. (Ibid., Sér. 4, viii, 1857, pp. 6–9, 1 Pl.)
—— Recherches anatomique et considerations entomologiques sur les Hémiptères du genre Leptopus. Appareil digestif. (Ibid., Sér. 4, Zool., 1858, x, pp. 352–356, 1 Pl.)
—— Recherches anatomiques sur l’Ascalaphus meridionalis. Appareil digestif. (Ibid., Sér. 4, xiii, 1860, pp. 200–202, 1 Pl.)
Leydig, F. Zur Anatomie von Coccus hesperidum. (Zeitschr. f. wissens. Zool., v, 1853, pp. 1–12, 1 Taf.)
Lubbock, J. On the digestive and nervous System of Coccus hesperidum. (Proc. Roy. Soc., ix, 1886, pp. 480–486; also Ann. Mag. Nat. Hist., 1859, Ser. 3, iii, pp. 306–311.)
Scheiber, S. H. Vergleichende Anatomie und Physiologie der Œstriden-Larven. V. Das chylo- und uropœtische System. (Sitzber. d. k. Akad. d. Wissens. Wien. Math.-naturwiss. Cl., 1862, xlv, pp. 39–64, 1 Taf.)
Gerstaecker, A. Bronn’s Klassen und Ordnungen des Tierreichs. V. Gliederfüssler. (Ernährungsorgane, pp. 87–105.)
Graber, V. Zur naheren Kenntnis des Proventriculus und der Appendices ventriculares bei den Grillen und Laubheuschrecken. (Sitzber. d. k. Akad. d. Wissensch. Wien. Mathem.-naturwiss. Cl., lix, 1869, pp. 29–46, 3 Taf.)
—— Ueber die Ernährungsorgane der Insekten und nächstverwandten Gliederfüssler. (Mitteil. d. naturwiss. Vereins für Steiermark. Graz, 1871, ii, pp. 181, 182.)
—— Verdauungssystem des Prachtkäfers. (Ibid., Graz, 1875.)
—— Die Insekten., i, 1877. (Verdauungsapparat, pp. 308–328.)
Wilde, K. F. Untersuchungen über den Kaumagen der Orthopteren. (Archiv f. Naturgesch., xliii Jahrg., 1877, pp. 135–172, 3 Taf.)
Simroth, H. Ueber den Darmkanal der Larven von Osmoderma eremita mit seinen Anhängen. (Giebel’s Zeitschr. f. d. ges. Naturwiss., 1878, li, pp. 493–518, 3 Taf.)
Müller, H. Ueber die angebliche Afterlösigkeit der Bienenlarven. (Zool. Anzeiger, 1881, pp. 530, 531.)
Schiemenz, Paulus. Ueber das Herkommen des Futtersaftes und die Speicheldrüsen der Bienen, nebst einem Anhänge über das Riechorgan. (Zeitschr. f. wissens. Zool., xxxviii, 1883, pp. 71–135, 3 Taf.)
Rovelli, G. Alcune ricerche sul tubo digerente degli Atteri, Ortotteri e Pseudo-Neurotteri. (Como, 1884, p. 15.)
Beauregard, H. Structure de l’appareil digestif des Insectes de la tribu des Vésicants. (Compt. rend. Acad. Paris, 1884, xcix, pp. 1083–1086.)
—— Recherches sur les Insectes vésicants., 1 Part, Anatomie. (Journ. Anat. Phys. Paris, 1885, xxi Année, pp. 483–524, 4 Pls.; 1886, xxii Année, pp. 85–108, 242–284, 5 Pls.)
—— Les Insectes vésicants, Paris, 1890, Chap. III, Appareil digestif, pp. 63–99; (Phénomènes digestifs, pp. 161–170; Pls. 6–9.)
Wertheimer, L. Sur la structure du tube digestif de l’Oryctes nasicornis. (Compt. rend. Soc. Biol. Paris, 1887, Sér. 8, iv, pp. 531, 532.)
Kowalevsky, A. Beitrage zur Kenntniss der nachembryonal Entwicklung der Musciden. (Zeitschr. f. wissens. Zool., xlv, 1887, pp. 542–594, 5 Taf.)
Schneider, A. Ueber den Darm der Arthropoden, besonders der Insekten. (Zool. Anzeiger, 1887, x Jahrg., pp. 139, 140.)
—— Ueber den Darmkanal der Arthropoden. (Zool. Beiträge von A. Schneider, ii, 1887, pp. 82–96, 3 Taf.)
Fritze, A. Ueber den Darmkanal der Ephemeriden. (Berichte der Naturforsch.-Gesellsch. zu Freiburg i. Br., 1888, iv, pp. 59–82, 2 Taf.)
Emery, C. Ueber den sogenannten Kaumagen einiger Ameisen. (Zeitschr. f. wissens. Zool., 1888, xlvi, pp. 378–412, 3 Taf.)
Meinert, F. Contribution à l’anatomie des Fourmilions. (Overs. Danske Vidensk. Selsk. Forhandl. Kjöbenhavn, 1889, pp. 43–66, 2 Pls.)
Mingazzini, P. Richerche sul canale digerente dei Lamellicorni fitofage (Larve e Insetti perfetti). (Mitteil. Zool. Station zu Neapel, ix, 1889–1891, pp. 1–112, 266–304, 7 Pls.)
Fernald, Henry T. Rectal glands in Coleoptera. (Amer. Naturalist, xxiv, pp. 100, 101, Jan., 1890.)
Visart, O. Digestive canal of Orthoptera. (Atti Soc. Toscana Scient. Natur., vii, 1891, pp. 277–285.)
Eberli, J. Untersuchungen an Verdauungstrakten von Gryllotalpa vulgaris. (Vierteljahresschr. d. Naturforsch. Gesells. Zurich, 1892, Sep., p. 46, Fig.)
Holmgren, Emil. Histologiska studier öfver några lepidopterlarvers digestionskanal och en del af deras Körtelartade bildningar. (Ent. Tidskr. Årg. xiii, pp. 129–170, 1892, 6 Pls.)
Ris, F. Untersuchung über die Gestalt des Kaumagens bei den Libellen und ihren Larven. (Zool. Jahrb. Abth. Syst., ix, 1896, pp. 596–624, 13 Figs.)
See also the works of Straus-Dürckheim, Newport, Mark, Witlaczil, Vayssière, Landois, Jordan, Oudemans, Berlese, List, Grassi, Verson, Miall and Denny, Leidy, Cheshire, Kowalevsky, Gehuchten, Locy, etc.
b. Digestion in insects
For the most complete and reliable investigation of the process of digestion, we are indebted to Plateau, whose results we give, besides the conclusions of later authors:
In mandibulate or biting insects, the food is conducted through the œsophagus by means of the muscular coating of this part of the digestive canal. Suctorial insects draw in their liquid food by the contractions followed by the dilatations of the mid-intestine (chylific stomach). Dragon-flies, Orthoptera, and Lepidoptera swallow some air with their food.
Where the salivary glands are present, the neutral alkaline fluid secreted by them has the same property as the salivary fluid of vertebrates of rapidly transforming starchy foods into soluble and assimilable glucose. In such forms as have no salivary glands, their place is almost always supplied by an epithelial lining of the œsophagus, or, as in the Hydrophilidæ, a fluid is secreted which has the same function as the true salivary fluid.
Nagel states that the saliva of the larva of Dyticus is powerfully digestive, and has a marked poisonous action, killing other insects, and even tadpoles of twice the size of the attacking larva, very rapidly. The larvæ not only suck the blood of their victims, but absorb the proteid substances. Drops of salivary juice seem to paralyze the victim, and to ferment the proteids. The secretion is neutral, the digestion tryptic. Similar extra-oral digestion seems to occur in larvæ of ant-lions, etc. (Biol. Centralbl., xvi, 1896, pp. 51–57, 103–112; Journ. Roy. Micr. Soc., 1896, p. 184.)
In carnivorous insects and in Orthoptera, the œsophagus dilates into a crop (ingluvies) ended by a narrow, valvular apparatus (or gizzard of authors). The food, more or less divided by the jaws, accumulates in the crop, which is very distensible; and, when the food is penetrated by the neutral or alkaline liquid, there undergoes an evident digestive action resulting, in carnivorous insects, in the transformation of albuminoid substances into soluble and assimilable matter analogous to peptones, and, in herbivorous insects, an abundant production of sugar from starch. This digestion in the crop, a food-reservoir, is very slow, and, until it is ended, the rest of the digestive canal remains empty.
“Any decided acidity found in the crop is due to the injection of acid food; but a very faint acidity may occur, which results from the presence in the crop of a fluid secreted by the cæcal diverticula of the mesenteron.” (Miall and Denny.)
When digestion in the crop is accomplished, the matters are subjected to an energetic pressure of the walls through peristaltic contractions, and then, guided by the furrows and chitinous teeth, pass along or gradually filter through the valvular apparatus or proventriculus, whose function is that of a strainer.
At the beginning of the “chyle-stomach” (mesenteron) of Orthoptera are glandular cæca which secrete a feebly acid fluid. This fluid emulsifies fats, and converts albuminoids into peptones. It passes forwards into the crop, and there acts upon the food.
In the mesenteron (mid-intestine) the food is acted upon by an alkaline or neutral fluid, never acid, either secreted, as in Orthoptera, by local special glands, or by a multitude of minute glandular cæca, as in many Coleoptera, or by a simple epithelial layer. It has no analogy with the gastric juices of vertebrates; its function differs in insects of different groups; in carnivorous Coleoptera it actively emulsionizes greasy matters; in the Hydrophilidæ it continues the process of transformation of starch into glucose, begun in the œsophagus. In the Scarabæidæ, it also produces glucose, but this action is local, not occurring elsewhere; in caterpillars, it causes a production of glucose, and transforms the albuminoids into soluble and assimilable bodies analogous to peptones, and also emulsionizes greasy matters. Finally, in the herbivorous Orthoptera there does not seem to be any formation of sugar in the stomach itself, the production of glucose being confined to the crop (jabot).
When digestion in the crop is finished, the proventriculus relaxes, and the contents of the crop, now in a semi-fluid condition, guided by the furrows and teeth, passes into the mesenteron, which is without a chitinous lining, and is thus fitted for absorption.
The contents of the mid-intestine (chylific stomach) then slowly and gradually pass into the intestine, the first anterior portion of which, usually long and slender, is the seat of an active absorption. The epithelial lining observed in certain insects seems, however, to indicate that secondary digestion takes place in this section. The reaction of the contents is neutral or alkaline.
The second and larger division of the intestine only acts as a stercoral reservoir. (The voluminous cæcum occurring in Dyticidæ, Nepa, and Ranatra, whether full or empty, never contains gas, and it is not, as some have supposed, a swimming-bladder.) The liquid product secreted by the Malpighian tubes accumulates in this division, and, under certain circumstances, very large calculi are often formed. In his subsequent paper on the digestion of the cockroach, Plateau states that in the intestine are united the residue of the work of digestion and the secretion of the urinary or Malpighian tubes, this secretion being purely urinary.
These organs are exclusively depuratory and urinary, freeing the body from waste products of the organic elements. The liquid they secrete contains urea (?), uric acid and abundant urates, hippuric acid (?), chloride of sodium, phosphates, carbonate of lime, oxalate of lime in quantity, leucine, and coloring-matters.
The products of the rectal or anal glands vary much in different groups, but they take no part in digestion, nor are they depuratory in their nature.
Insects have nothing resembling chylific substances.[[51]] The products of digestion, dissolved salts, peptones, sugar in solution, emulsionized greasy matters, pass through the relatively delicate walls of the digestive canal by osmose, and mingle outside of the canal with the blood.
Whatever substances remain undigested are expelled with the excrements; such are the chitin of the integuments of insects, vegetable cellulose, and chlorophyll, which is detected by the microspectroscope all along the digestive canal of phytophagous insects.
In his experiments in feeding the larvæ of Musca with lacmus, Kowalevsky found that the œsophagus, food-reservoir, and proventriculus, with its cæcal appendages, always remained blue, and had an alkaline reaction; the mid-intestine, also, in its anterior portion, remained blue, but a portion of its posterior half became deep red, and also exhibited a strong reaction. The hind-intestine, however, always remained blue, and also had an alkaline reaction. (Biol. Centralbl., ix, 1889, p. 46.)
The mechanism of secretion.—Gehuchten describes the process of secretion in insects, the following extract being taken from his researches on the digestive apparatus of the larva of Ptychoptera. The products of secretion poured into the alimentary canal are more or less fluid; for this reason, it is impossible to say when an epithelial cell at rest contains these products. For the secreting nature of these cells is only apparent at the moment when they are ready for excretion; then the cellular membrane swells out, and a part of the protoplasmic body projects into the intestinal cavity.
Before going farther, the terms secretion and excretion should, he says, be defined. With Ranvier, he believes that the elaboration in the protoplasm of a definite fluid substance is, par excellence, the secretory act, while the removal of this substance is the act of excretion.
Fig. 322.—Different phases of the mechanism of secretion and of excretion.—After Gehuchten.
A glandular cell of the chylific stomach, when at rest, is always furnished with a striated “platform,” or flat surface, or face, on the side facing the cavity of the stomach, and the free edge of the platform, or plateau, is provided with filaments projecting into the digestive cavity (Fig. 322, f). These glandular cells, when active, differ much in appearance. In a great number, the platform (plateau) has disappeared, and is replaced by a simple, regular membrane. During the process of secretion, a finely granular mass, in direct continuity with the protoplasm, swells, and raises the membrane over the entire breadth of the cell, causing it to project into the intestinal cavity (Fig. 322, A, B). These vesicles, or drops of the secretion, whether free or still attached by a web to the cells, are clear and transparent in the living insect, but granular in the portions of the digestive canal fixed for cutting into sections. Gehuchten then asks: “How does a cell gorged with the products of secretion empty itself?” Both Ranvier and also Heidenhain believe that one and the same glandular cell may secrete and excrete several times without undergoing destruction, but their researches made on salivary glands have not answered the question. Gehuchten explains the process thus: when the epithelial cell begins to secrete, the clear fluid elaborated in the protoplasm of the cell increases the intracellular tension, until, finally, the fluid breaks through certain weak places in the swollen basal membrane of the platform, and then easily passes through the closely crowded filaments, and projects out into the intestinal cavity as a pear-shaped vesicle of a liquid rich in albumens at first attached to the free face of the cell, but finally becoming free, as at Fig. 322, A, B.
When the elaboration of the substance to be secreted is more active, the mechanism of the secretion is modified. The basal membrane of the platform may then be raised at several places at once; instead of a single vesicle projecting into the intestinal cavity, each cell may present a great number more or less voluminous. If all remain small and rapidly detach themselves from the glandular cell, the filaments of the platform are simply separated from each other at different points of the free face, as in Fig. 322, C. On the other hand, when the different vesicles of a single cell become larger, the filaments of the platform are compressed and crowded against each other in the spaces between the vesicles remaining free, and the undisturbed portions of the platform appear homogeneous (Fig. 322, D). After the excretion of the secretory products by this process of strangulation, the cell then assumes the aspect of a glandular cell at rest, and may begin again to form a new secretion.
To sum up: The process of excretion may occur in two ways:
1. Where the membrane ruptures and the substances secreted are sent directly out into the digestive cavity. 2. Where the vesicles become free by strangulation, floating in the glandular or intestinal cavity, and ending by rupturing and coming into contact with the neighboring vesicles or with the food.
Absorbent cells.—Besides the glandular or secreting cells in Ptychoptera, there is between the two regions of the chyle-stomach lined with these cells a region about a centimetre long composed of absorbent cells. The absorbent cells are very large, polygonal, and contain a large nucleus, in which is a striated convoluted chromatic cord.
The food on entering the chyle-stomach is brought into contact with the products secreted in the proventriculus, in the first part of the chyle-stomach, and in the tubular glands. These products of secretion act on the food, extracting from them useful substances which they render soluble. These substances, after having been absorbed by the absorbent cells in the middle region of the stomach, undergo special modifications, and are transformed into solid products, which are situated at the bottom of these cells. Afterwards the alimentary substances freed from a portion of their useful substances are again placed in contact with the products of secretion in the distal part of the chylific ventricle, and reach the terminal part of the intestine.
“The products of secretion,” adds Gehuchten, “diverted into the intestinal canal do not come into immediate contact with the alimentary substances; they are separated from it by a continuous, structureless, quite thick membrane (the peritrophic membrane), which directly envelops the cylinder of food matters, extending from the orifice of the œsophageal valvule to the end of the intestine. Between this membrane and the free face of the epithelial lining there exists a circular space, into which are thrown and accumulate the excreted substances. The latter then cannot directly mingle with the aliments; but when they are liquid they undoubtedly pass through this membrane by osmose, and thus come into contact with the nutritive substances. It is the same with the products of absorption. The absorption of soluble products of the intestinal cavity is not then so simple a phenomenon as it was at first thought to be, since these products are nowhere brought into immediate contact with the absorbent cells” (pp. 90, 91).
The most recent authority, Cuénot, states that absorption of the products of digestion takes place entirely in the mid-intestine, and in its cæca when these are present. The mid-intestine exercises a selective action on the constituents of the food comparable to the action of the vertebrate liver.