This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(835). When bruised in the gizzard, the food passes on into the proper stomach, which is generally a long intestiniform organ (fig. 159, d d), extending from the crop or gizzard to the point where the biliary vessels discharge themselves into the intestine. The size and shape of this organ will vary, of course, with the nature of the food. Thus, in the Butterfly, which scarcely eats at all, or sparingly sips the honey from the flowers, it is very minute (fig. 160, b); but in insects which live upon coarse and indigestible materials, it is proportionately elongated and capacious.
(836). The stomach generally ends in the small intestine (fig. 159, e; fig. 160, i); but this is occasionally entirely wanting, so that the stomach seems to terminate immediately in the colon or large intestine, which is the terminal portion of the alimentary canal: when much developed, the small intestine is sometimes divided by a constriction into two parts, to which the names of duodenum and ilium have been applied by entomological writers. The colon (fig. 159, f; fig. 160, k) is separated from the small intestine by a distinct valve; and in connexion with its commencement a wide blind sacculus or caecum is often met with.
Fig. 160. Alimentary canal of a Butterfly: a a, proboscis; p, mouth; m m, pharynx; sp, s p, salivary glands; o, oesophagus; v v, crop, communicating by a canal (c,f) with the stomach proper (b, z); i, small intestine; k, large intestine; g g g, hepatic vessels; h, n, their terminations in the vicinity of the pylorus.
(837). We may now notice the secerning organs that pour fluids into different parts of the digestive apparatus, beginning with those which open into the oesophagus in the vicinity of the mouth, and examining them in the order of their occurrence as we proceed backwards.
(838). The first are the salivary vessels, which terminate in the neighbourhood of the mouth itself, into which they seem to pour a secretion analogous to saliva. These glands are principally met with in suctorial insects, but not unfrequently among the mandibulate orders. Their form varies; but they are generally simple slender tubes, that float loosely among the juices of the body, from which they separate the salivary fluid. There are, for the most part, only two of these organs (fig. 160, s s); but in fleas (Puleoc) and bugs (Cimex) there are four, and in a water-bug (Nepa) there are six such vessels, all of which open into the cavity of the mouth. The fluid supplied by the salivary glands is usually merely intended to facilitate deglutition; but there are cases in which the saliva is excessively acrid and irritating, acting as a kind of poison when infused into a puncture made by the mouth: this is especially remarkable in many bugs and gnats, and is the chief cause of the pain and inflammation frequently occasioned by their bite.
(839). Besides the proper salivary vessels, there are other glands, or rather caeca, which open into the stomach itself, occasionally covering that organ over its entire surface, as is the case in some water-beetles (Hydrophilus): these, no doubt, secrete a fluid subservient to digestion; but whether of a peculiar description, or allied to saliva in its properties, is unknown.
(840). The third kind of auxiliary vessels connected with the intestinal canal of insects are supposed to furnish a secretion analogous to the bile of other animals, and consequently to represent the liver. These bile-vessels (fig. 159, hh; fig. 160, g g) are generally four, six, or eight in number, but occasionally much more numerous; they are usually of great length, but exceedingly slender, and wind around the intestine in all directions. When unravelled, they are found to terminate in the neighbourhood of the pylorus (fig. 160, h, n), close to the commencement of the intestine, at which point the secretion produced by them is mixed with the food after it has undergone the process of digestion.
(841). Appended to the termination of the alimentary tube, close to its anal extremity, other vessels are met with in some insects, that are looked upon by authors as being allied in function to the kidneys of higher animals; but apparently this opinion rests upon very doubtful grounds. They indubitably furnish some secretion, the use of which is perhaps connected with defecation; but that it is of the same character as the fluid separated by the renal organs of Vertebrata may well be called in question, as no such parts are distinctly recognizable until we arrive at much more elevated forms of life than the insects we are now considering. There is, however, another reason for rejecting the opinion that these accessory vessels secrete urine; and that is, that they are only met with in a few beetles and some species of Orthoptera - a circumstance that alone would be sufficient to disprove such a supposition.
(842). In the vertebrate animals, as the reader is well aware, the nutritious products of digestion are taken up by a system of absorbing vessels that ramify extensively over the coats of the intestine; and the nutriment is thus conveyed into the mass of the circulating fluid by ducts appropriated specially to this office: in animals of less perfect structure than these, such as the Mollusca, the veins themselves absorb the nutritive materials. But in insects, in which we find neither absorbents nor veins, a different arrangement is necessary, and, in the little creatures before us, nutrition appears to be carried on by the simple transudation of the chyle through the coats of the intestine; so that it escapes into the general cavity of the abdomen, where, as we shall see when we examine the arrangement of their circulating organs, it is immediately mixed up with the blood. This transudation has indeed been actually witnessed by Ramdohr and Rengger*, and even analysed by the last-mentioned physiologist, who found it to consist almost entirely of albumen.