Fig. 317.—Digestive canal of Carabus monilis: h, œsophagus; i, gizzard or proventriculus; k, “stomach,” with its cœca (r); p, urinary tubes; q, their point of insertion; m, n, colon, with cœcal glands; s, anal glands; a, b, c, a gastric cœcum; a, b, portion of lining of gizzard.—After Newport.
Fig. 318.—Digestive canal of Meloe: sch, œsophagus; Kl, œsophageal valve; mD, mid-intestine; eD, hind-intestine; Ei, eggs; g, sexual opening.—After Graber.
Gehuchten adds that the limit set by the circular projection does not exactly coincide with the opening into the intestine of the urinary tubes and the two annexed glands. He shows by a section (his Fig. 133) that the tubular glands open into the alimentary canal in front of the circular fold. It is the same with the Malpighian tubes. They are not, therefore, he claims, dependences of the terminal intestine, but of the mid-intestine. Beauregard has observed the same thing in the vesicating insects (Meloidæ). The Malpighian tubes, he says, open into the “chylific stomach” before the valvular crown. This arrangement does not seem to be general, because, according to Balbiani, the Malpighian vessels open into the beginning of the intestine in Cryptops. Compare also Minot’s account of the valve in locusts separating the stomach from the intestine, and in front of which the urinary or Malpighian tubes open.
Histology of the mid-intestine.—The walls of the stomach are composed of an internal epithelium, a layer of connective tissue, with two muscular layers, the inner of which is formed of unstriated circular muscular fibres, and the outer of striated longitudinal muscular fibres.
In the cockroach short processes are given off from the free ends of the epithelial cells, as in the intestine of many mammals and other animals. “Between the cells a reticulum is often to be seen, especially where the cells have burst; it extends between and among all the elements of the mucous lining, and probably serves, like the very similar structure met with in mammalian intestines, to absorb and conduct some of the products of digestion.” (Miall and Denny.)
Gehuchten shows that the epithelial lining of the mesenteron (chylific stomach) of the dipterous larva Ptychoptera is composed of two kinds of cells, i.e. secreting or glandular cells and absorbent cells, the former situated at each end of the stomach, and the absorbent cells occupying the middle region. The part played by these cells in digestion will be treated of beyond in the section on digestion. (See p. 327.)
The hind-intestine.—In many insects this is divided into the ileum, or short intestine, and the long intestine. The limit between the intestine and stomach is externally determined by the origin of the urinary tubes, which are outgrowths of the anterior end of the proctodæum. Like the fore-intestine the hind-intestine is lined with a thick muscular layer, and, as Gehuchten states, the passage from the epithelial lining of the stomach (mid-intestine) to the muscular lining of the intestine is abrupt.
Large intestine.—In Ptychoptera, as described by Gehuchten, there are no precise limits between the small and large intestine; the epithelium of the large intestine has a special character, and its constituents present a close resemblance to the absorbed cells of the chylific stomach, being like them large and polygonal. The muscular layer is not continuous, and is formed of longitudinal and circular fibres, the latter being the larger.
The ileum—Though in most insects slender, and therefore called the small intestine, the ileum is in locusts (Fig. 298) and grasshoppers (Anabrus, Fig. 299) as thick as the stomach. In many carnivorous beetles (Dyticus, Fig. 320, il, and Necrophorus) it is very long, but rather slender and short in the Carabidæ and Cicindelidæ, as well as those insects whose food is liquid, such as Diptera. In the Lepidoptera it varies in length, being in Sphinx quite long and bent into seven folds (Fig. 309), while it is very short in the Psocidæ, Chrysomelidæ, and Tenthredinidæ.