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.)